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SPDX-License-Identifier: CC-BY-SA-2.0-UK +.. highlight:: shell + +*************************************************************** +Basic Usage (with examples) for each of the Yocto Tracing Tools +*************************************************************** + +| + +This chapter presents basic usage examples for each of the tracing +tools. + +perf +==== + +The 'perf' tool is the profiling and tracing tool that comes bundled +with the Linux kernel. + +Don't let the fact that it's part of the kernel fool you into thinking +that it's only for tracing and profiling the kernel - you can indeed use +it to trace and profile just the kernel, but you can also use it to +profile specific applications separately (with or without kernel +context), and you can also use it to trace and profile the kernel and +all applications on the system simultaneously to gain a system-wide view +of what's going on. + +In many ways, perf aims to be a superset of all the tracing and +profiling tools available in Linux today, including all the other tools +covered in this HOWTO. The past couple of years have seen perf subsume a +lot of the functionality of those other tools and, at the same time, +those other tools have removed large portions of their previous +functionality and replaced it with calls to the equivalent functionality +now implemented by the perf subsystem. Extrapolation suggests that at +some point those other tools will simply become completely redundant and +go away; until then, we'll cover those other tools in these pages and in +many cases show how the same things can be accomplished in perf and the +other tools when it seems useful to do so. + +The coverage below details some of the most common ways you'll likely +want to apply the tool; full documentation can be found either within +the tool itself or in the man pages at +`perf(1) <http://linux.die.net/man/1/perf>`__. + +Perf Setup +---------- + +For this section, we'll assume you've already performed the basic setup +outlined in the ":ref:`profile-manual/intro:General Setup`" section. + +In particular, you'll get the most mileage out of perf if you profile an +image built with the following in your ``local.conf`` file: :: + + INHIBIT_PACKAGE_STRIP = "1" + +perf runs on the target system for the most part. You can archive +profile data and copy it to the host for analysis, but for the rest of +this document we assume you've ssh'ed to the host and will be running +the perf commands on the target. + +Basic Perf Usage +---------------- + +The perf tool is pretty much self-documenting. To remind yourself of the +available commands, simply type 'perf', which will show you basic usage +along with the available perf subcommands: :: + + root@crownbay:~# perf + + usage: perf [--version] [--help] COMMAND [ARGS] + + The most commonly used perf commands are: + annotate Read perf.data (created by perf record) and display annotated code + archive Create archive with object files with build-ids found in perf.data file + bench General framework for benchmark suites + buildid-cache Manage build-id cache. + buildid-list List the buildids in a perf.data file + diff Read two perf.data files and display the differential profile + evlist List the event names in a perf.data file + inject Filter to augment the events stream with additional information + kmem Tool to trace/measure kernel memory(slab) properties + kvm Tool to trace/measure kvm guest os + list List all symbolic event types + lock Analyze lock events + probe Define new dynamic tracepoints + record Run a command and record its profile into perf.data + report Read perf.data (created by perf record) and display the profile + sched Tool to trace/measure scheduler properties (latencies) + script Read perf.data (created by perf record) and display trace output + stat Run a command and gather performance counter statistics + test Runs sanity tests. + timechart Tool to visualize total system behavior during a workload + top System profiling tool. + + See 'perf help COMMAND' for more information on a specific command. + + +Using perf to do Basic Profiling +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +As a simple test case, we'll profile the 'wget' of a fairly large file, +which is a minimally interesting case because it has both file and +network I/O aspects, and at least in the case of standard Yocto images, +it's implemented as part of busybox, so the methods we use to analyze it +can be used in a very similar way to the whole host of supported busybox +applets in Yocto. :: + + root@crownbay:~# rm linux-2.6.19.2.tar.bz2; \ + wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2 + +The quickest and easiest way to get some basic overall data about what's +going on for a particular workload is to profile it using 'perf stat'. +'perf stat' basically profiles using a few default counters and displays +the summed counts at the end of the run: :: + + root@crownbay:~# perf stat wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2 + Connecting to downloads.yoctoproject.org (140.211.169.59:80) + linux-2.6.19.2.tar.b 100% |***************************************************| 41727k 0:00:00 ETA + + Performance counter stats for 'wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2': + + 4597.223902 task-clock # 0.077 CPUs utilized + 23568 context-switches # 0.005 M/sec + 68 CPU-migrations # 0.015 K/sec + 241 page-faults # 0.052 K/sec + 3045817293 cycles # 0.663 GHz + <not supported> stalled-cycles-frontend + <not supported> stalled-cycles-backend + 858909167 instructions # 0.28 insns per cycle + 165441165 branches # 35.987 M/sec + 19550329 branch-misses # 11.82% of all branches + + 59.836627620 seconds time elapsed + +Many times such a simple-minded test doesn't yield much of +interest, but sometimes it does (see Real-world Yocto bug (slow +loop-mounted write speed)). + +Also, note that 'perf stat' isn't restricted to a fixed set of counters +- basically any event listed in the output of 'perf list' can be tallied +by 'perf stat'. For example, suppose we wanted to see a summary of all +the events related to kernel memory allocation/freeing along with cache +hits and misses: :: + + root@crownbay:~# perf stat -e kmem:* -e cache-references -e cache-misses wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2 + Connecting to downloads.yoctoproject.org (140.211.169.59:80) + linux-2.6.19.2.tar.b 100% |***************************************************| 41727k 0:00:00 ETA + + Performance counter stats for 'wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2': + + 5566 kmem:kmalloc + 125517 kmem:kmem_cache_alloc + 0 kmem:kmalloc_node + 0 kmem:kmem_cache_alloc_node + 34401 kmem:kfree + 69920 kmem:kmem_cache_free + 133 kmem:mm_page_free + 41 kmem:mm_page_free_batched + 11502 kmem:mm_page_alloc + 11375 kmem:mm_page_alloc_zone_locked + 0 kmem:mm_page_pcpu_drain + 0 kmem:mm_page_alloc_extfrag + 66848602 cache-references + 2917740 cache-misses # 4.365 % of all cache refs + + 44.831023415 seconds time elapsed + +So 'perf stat' gives us a nice easy +way to get a quick overview of what might be happening for a set of +events, but normally we'd need a little more detail in order to +understand what's going on in a way that we can act on in a useful way. + +To dive down into a next level of detail, we can use 'perf record'/'perf +report' which will collect profiling data and present it to use using an +interactive text-based UI (or simply as text if we specify --stdio to +'perf report'). + +As our first attempt at profiling this workload, we'll simply run 'perf +record', handing it the workload we want to profile (everything after +'perf record' and any perf options we hand it - here none - will be +executed in a new shell). perf collects samples until the process exits +and records them in a file named 'perf.data' in the current working +directory. :: + + root@crownbay:~# perf record wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2 + + Connecting to downloads.yoctoproject.org (140.211.169.59:80) + linux-2.6.19.2.tar.b 100% |************************************************| 41727k 0:00:00 ETA + [ perf record: Woken up 1 times to write data ] + [ perf record: Captured and wrote 0.176 MB perf.data (~7700 samples) ] + +To see the results in a +'text-based UI' (tui), simply run 'perf report', which will read the +perf.data file in the current working directory and display the results +in an interactive UI: :: + + root@crownbay:~# perf report + +.. image:: figures/perf-wget-flat-stripped.png + :align: center + +The above screenshot displays a 'flat' profile, one entry for each +'bucket' corresponding to the functions that were profiled during the +profiling run, ordered from the most popular to the least (perf has +options to sort in various orders and keys as well as display entries +only above a certain threshold and so on - see the perf documentation +for details). Note that this includes both userspace functions (entries +containing a [.]) and kernel functions accounted to the process (entries +containing a [k]). (perf has command-line modifiers that can be used to +restrict the profiling to kernel or userspace, among others). + +Notice also that the above report shows an entry for 'busybox', which is +the executable that implements 'wget' in Yocto, but that instead of a +useful function name in that entry, it displays a not-so-friendly hex +value instead. The steps below will show how to fix that problem. + +Before we do that, however, let's try running a different profile, one +which shows something a little more interesting. The only difference +between the new profile and the previous one is that we'll add the -g +option, which will record not just the address of a sampled function, +but the entire callchain to the sampled function as well: :: + + root@crownbay:~# perf record -g wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2 + Connecting to downloads.yoctoproject.org (140.211.169.59:80) + linux-2.6.19.2.tar.b 100% |************************************************| 41727k 0:00:00 ETA + [ perf record: Woken up 3 times to write data ] + [ perf record: Captured and wrote 0.652 MB perf.data (~28476 samples) ] + + + root@crownbay:~# perf report + +.. image:: figures/perf-wget-g-copy-to-user-expanded-stripped.png + :align: center + +Using the callgraph view, we can actually see not only which functions +took the most time, but we can also see a summary of how those functions +were called and learn something about how the program interacts with the +kernel in the process. + +Notice that each entry in the above screenshot now contains a '+' on the +left-hand side. This means that we can expand the entry and drill down +into the callchains that feed into that entry. Pressing 'enter' on any +one of them will expand the callchain (you can also press 'E' to expand +them all at the same time or 'C' to collapse them all). + +In the screenshot above, we've toggled the ``__copy_to_user_ll()`` entry +and several subnodes all the way down. This lets us see which callchains +contributed to the profiled ``__copy_to_user_ll()`` function which +contributed 1.77% to the total profile. + +As a bit of background explanation for these callchains, think about +what happens at a high level when you run wget to get a file out on the +network. Basically what happens is that the data comes into the kernel +via the network connection (socket) and is passed to the userspace +program 'wget' (which is actually a part of busybox, but that's not +important for now), which takes the buffers the kernel passes to it and +writes it to a disk file to save it. + +The part of this process that we're looking at in the above call stacks +is the part where the kernel passes the data it's read from the socket +down to wget i.e. a copy-to-user. + +Notice also that here there's also a case where the hex value is +displayed in the callstack, here in the expanded ``sys_clock_gettime()`` +function. Later we'll see it resolve to a userspace function call in +busybox. + +.. image:: figures/perf-wget-g-copy-from-user-expanded-stripped.png + :align: center + +The above screenshot shows the other half of the journey for the data - +from the wget program's userspace buffers to disk. To get the buffers to +disk, the wget program issues a ``write(2)``, which does a ``copy-from-user`` to +the kernel, which then takes care via some circuitous path (probably +also present somewhere in the profile data), to get it safely to disk. + +Now that we've seen the basic layout of the profile data and the basics +of how to extract useful information out of it, let's get back to the +task at hand and see if we can get some basic idea about where the time +is spent in the program we're profiling, wget. Remember that wget is +actually implemented as an applet in busybox, so while the process name +is 'wget', the executable we're actually interested in is busybox. So +let's expand the first entry containing busybox: + +.. image:: figures/perf-wget-busybox-expanded-stripped.png + :align: center + +Again, before we expanded we saw that the function was labeled with a +hex value instead of a symbol as with most of the kernel entries. +Expanding the busybox entry doesn't make it any better. + +The problem is that perf can't find the symbol information for the +busybox binary, which is actually stripped out by the Yocto build +system. + +One way around that is to put the following in your ``local.conf`` file +when you build the image: :: + + INHIBIT_PACKAGE_STRIP = "1" + +However, we already have an image with the binaries stripped, so +what can we do to get perf to resolve the symbols? Basically we need to +install the debuginfo for the busybox package. + +To generate the debug info for the packages in the image, we can add +``dbg-pkgs`` to :term:`EXTRA_IMAGE_FEATURES` in ``local.conf``. For example: :: + + EXTRA_IMAGE_FEATURES = "debug-tweaks tools-profile dbg-pkgs" + +Additionally, in order to generate the type of debuginfo that perf +understands, we also need to set +:term:`PACKAGE_DEBUG_SPLIT_STYLE` +in the ``local.conf`` file: :: + + PACKAGE_DEBUG_SPLIT_STYLE = 'debug-file-directory' + +Once we've done that, we can install the +debuginfo for busybox. The debug packages once built can be found in +``build/tmp/deploy/rpm/*`` on the host system. Find the busybox-dbg-...rpm +file and copy it to the target. For example: :: + + [trz@empanada core2]$ scp /home/trz/yocto/crownbay-tracing-dbg/build/tmp/deploy/rpm/core2_32/busybox-dbg-1.20.2-r2.core2_32.rpm root@192.168.1.31: + busybox-dbg-1.20.2-r2.core2_32.rpm 100% 1826KB 1.8MB/s 00:01 + +Now install the debug rpm on the target: :: + + root@crownbay:~# rpm -i busybox-dbg-1.20.2-r2.core2_32.rpm + +Now that the debuginfo is installed, we see that the busybox entries now display +their functions symbolically: + +.. image:: figures/perf-wget-busybox-debuginfo.png + :align: center + +If we expand one of the entries and press 'enter' on a leaf node, we're +presented with a menu of actions we can take to get more information +related to that entry: + +.. image:: figures/perf-wget-busybox-dso-zoom-menu.png + :align: center + +One of these actions allows us to show a view that displays a +busybox-centric view of the profiled functions (in this case we've also +expanded all the nodes using the 'E' key): + +.. image:: figures/perf-wget-busybox-dso-zoom.png + :align: center + +Finally, we can see that now that the busybox debuginfo is installed, +the previously unresolved symbol in the ``sys_clock_gettime()`` entry +mentioned previously is now resolved, and shows that the +sys_clock_gettime system call that was the source of 6.75% of the +copy-to-user overhead was initiated by the ``handle_input()`` busybox +function: + +.. image:: figures/perf-wget-g-copy-to-user-expanded-debuginfo.png + :align: center + +At the lowest level of detail, we can dive down to the assembly level +and see which instructions caused the most overhead in a function. +Pressing 'enter' on the 'udhcpc_main' function, we're again presented +with a menu: + +.. image:: figures/perf-wget-busybox-annotate-menu.png + :align: center + +Selecting 'Annotate udhcpc_main', we get a detailed listing of +percentages by instruction for the udhcpc_main function. From the +display, we can see that over 50% of the time spent in this function is +taken up by a couple tests and the move of a constant (1) to a register: + +.. image:: figures/perf-wget-busybox-annotate-udhcpc.png + :align: center + +As a segue into tracing, let's try another profile using a different +counter, something other than the default 'cycles'. + +The tracing and profiling infrastructure in Linux has become unified in +a way that allows us to use the same tool with a completely different +set of counters, not just the standard hardware counters that +traditional tools have had to restrict themselves to (of course the +traditional tools can also make use of the expanded possibilities now +available to them, and in some cases have, as mentioned previously). + +We can get a list of the available events that can be used to profile a +workload via 'perf list': :: + + root@crownbay:~# perf list + + List of pre-defined events (to be used in -e): + cpu-cycles OR cycles [Hardware event] + stalled-cycles-frontend OR idle-cycles-frontend [Hardware event] + stalled-cycles-backend OR idle-cycles-backend [Hardware event] + instructions [Hardware event] + cache-references [Hardware event] + cache-misses [Hardware event] + branch-instructions OR branches [Hardware event] + branch-misses [Hardware event] + bus-cycles [Hardware event] + ref-cycles [Hardware event] + + cpu-clock [Software event] + task-clock [Software event] + page-faults OR faults [Software event] + minor-faults [Software event] + major-faults [Software event] + context-switches OR cs [Software event] + cpu-migrations OR migrations [Software event] + alignment-faults [Software event] + emulation-faults [Software event] + + L1-dcache-loads [Hardware cache event] + L1-dcache-load-misses [Hardware cache event] + L1-dcache-prefetch-misses [Hardware cache event] + L1-icache-loads [Hardware cache event] + L1-icache-load-misses [Hardware cache event] + . + . + . + rNNN [Raw hardware event descriptor] + cpu/t1=v1[,t2=v2,t3 ...]/modifier [Raw hardware event descriptor] + (see 'perf list --help' on how to encode it) + + mem:<addr>[:access] [Hardware breakpoint] + + sunrpc:rpc_call_status [Tracepoint event] + sunrpc:rpc_bind_status [Tracepoint event] + sunrpc:rpc_connect_status [Tracepoint event] + sunrpc:rpc_task_begin [Tracepoint event] + skb:kfree_skb [Tracepoint event] + skb:consume_skb [Tracepoint event] + skb:skb_copy_datagram_iovec [Tracepoint event] + net:net_dev_xmit [Tracepoint event] + net:net_dev_queue [Tracepoint event] + net:netif_receive_skb [Tracepoint event] + net:netif_rx [Tracepoint event] + napi:napi_poll [Tracepoint event] + sock:sock_rcvqueue_full [Tracepoint event] + sock:sock_exceed_buf_limit [Tracepoint event] + udp:udp_fail_queue_rcv_skb [Tracepoint event] + hda:hda_send_cmd [Tracepoint event] + hda:hda_get_response [Tracepoint event] + hda:hda_bus_reset [Tracepoint event] + scsi:scsi_dispatch_cmd_start [Tracepoint event] + scsi:scsi_dispatch_cmd_error [Tracepoint event] + scsi:scsi_eh_wakeup [Tracepoint event] + drm:drm_vblank_event [Tracepoint event] + drm:drm_vblank_event_queued [Tracepoint event] + drm:drm_vblank_event_delivered [Tracepoint event] + random:mix_pool_bytes [Tracepoint event] + random:mix_pool_bytes_nolock [Tracepoint event] + random:credit_entropy_bits [Tracepoint event] + gpio:gpio_direction [Tracepoint event] + gpio:gpio_value [Tracepoint event] + block:block_rq_abort [Tracepoint event] + block:block_rq_requeue [Tracepoint event] + block:block_rq_issue [Tracepoint event] + block:block_bio_bounce [Tracepoint event] + block:block_bio_complete [Tracepoint event] + block:block_bio_backmerge [Tracepoint event] + . + . + writeback:writeback_wake_thread [Tracepoint event] + writeback:writeback_wake_forker_thread [Tracepoint event] + writeback:writeback_bdi_register [Tracepoint event] + . + . + writeback:writeback_single_inode_requeue [Tracepoint event] + writeback:writeback_single_inode [Tracepoint event] + kmem:kmalloc [Tracepoint event] + kmem:kmem_cache_alloc [Tracepoint event] + kmem:mm_page_alloc [Tracepoint event] + kmem:mm_page_alloc_zone_locked [Tracepoint event] + kmem:mm_page_pcpu_drain [Tracepoint event] + kmem:mm_page_alloc_extfrag [Tracepoint event] + vmscan:mm_vmscan_kswapd_sleep [Tracepoint event] + vmscan:mm_vmscan_kswapd_wake [Tracepoint event] + vmscan:mm_vmscan_wakeup_kswapd [Tracepoint event] + vmscan:mm_vmscan_direct_reclaim_begin [Tracepoint event] + . + . + module:module_get [Tracepoint event] + module:module_put [Tracepoint event] + module:module_request [Tracepoint event] + sched:sched_kthread_stop [Tracepoint event] + sched:sched_wakeup [Tracepoint event] + sched:sched_wakeup_new [Tracepoint event] + sched:sched_process_fork [Tracepoint event] + sched:sched_process_exec [Tracepoint event] + sched:sched_stat_runtime [Tracepoint event] + rcu:rcu_utilization [Tracepoint event] + workqueue:workqueue_queue_work [Tracepoint event] + workqueue:workqueue_execute_end [Tracepoint event] + signal:signal_generate [Tracepoint event] + signal:signal_deliver [Tracepoint event] + timer:timer_init [Tracepoint event] + timer:timer_start [Tracepoint event] + timer:hrtimer_cancel [Tracepoint event] + timer:itimer_state [Tracepoint event] + timer:itimer_expire [Tracepoint event] + irq:irq_handler_entry [Tracepoint event] + irq:irq_handler_exit [Tracepoint event] + irq:softirq_entry [Tracepoint event] + irq:softirq_exit [Tracepoint event] + irq:softirq_raise [Tracepoint event] + printk:console [Tracepoint event] + task:task_newtask [Tracepoint event] + task:task_rename [Tracepoint event] + syscalls:sys_enter_socketcall [Tracepoint event] + syscalls:sys_exit_socketcall [Tracepoint event] + . + . + . + syscalls:sys_enter_unshare [Tracepoint event] + syscalls:sys_exit_unshare [Tracepoint event] + raw_syscalls:sys_enter [Tracepoint event] + raw_syscalls:sys_exit [Tracepoint event] + +.. admonition:: Tying it Together + + These are exactly the same set of events defined by the trace event + subsystem and exposed by ftrace/tracecmd/kernelshark as files in + /sys/kernel/debug/tracing/events, by SystemTap as + kernel.trace("tracepoint_name") and (partially) accessed by LTTng. + +Only a subset of these would be of interest to us when looking at this +workload, so let's choose the most likely subsystems (identified by the +string before the colon in the Tracepoint events) and do a 'perf stat' +run using only those wildcarded subsystems: :: + + root@crownbay:~# perf stat -e skb:* -e net:* -e napi:* -e sched:* -e workqueue:* -e irq:* -e syscalls:* wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2 + Performance counter stats for 'wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2': + + 23323 skb:kfree_skb + 0 skb:consume_skb + 49897 skb:skb_copy_datagram_iovec + 6217 net:net_dev_xmit + 6217 net:net_dev_queue + 7962 net:netif_receive_skb + 2 net:netif_rx + 8340 napi:napi_poll + 0 sched:sched_kthread_stop + 0 sched:sched_kthread_stop_ret + 3749 sched:sched_wakeup + 0 sched:sched_wakeup_new + 0 sched:sched_switch + 29 sched:sched_migrate_task + 0 sched:sched_process_free + 1 sched:sched_process_exit + 0 sched:sched_wait_task + 0 sched:sched_process_wait + 0 sched:sched_process_fork + 1 sched:sched_process_exec + 0 sched:sched_stat_wait + 2106519415641 sched:sched_stat_sleep + 0 sched:sched_stat_iowait + 147453613 sched:sched_stat_blocked + 12903026955 sched:sched_stat_runtime + 0 sched:sched_pi_setprio + 3574 workqueue:workqueue_queue_work + 3574 workqueue:workqueue_activate_work + 0 workqueue:workqueue_execute_start + 0 workqueue:workqueue_execute_end + 16631 irq:irq_handler_entry + 16631 irq:irq_handler_exit + 28521 irq:softirq_entry + 28521 irq:softirq_exit + 28728 irq:softirq_raise + 1 syscalls:sys_enter_sendmmsg + 1 syscalls:sys_exit_sendmmsg + 0 syscalls:sys_enter_recvmmsg + 0 syscalls:sys_exit_recvmmsg + 14 syscalls:sys_enter_socketcall + 14 syscalls:sys_exit_socketcall + . + . + . + 16965 syscalls:sys_enter_read + 16965 syscalls:sys_exit_read + 12854 syscalls:sys_enter_write + 12854 syscalls:sys_exit_write + . + . + . + + 58.029710972 seconds time elapsed + + + +Let's pick one of these tracepoints +and tell perf to do a profile using it as the sampling event: :: + + root@crownbay:~# perf record -g -e sched:sched_wakeup wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2 + +.. image:: figures/sched-wakeup-profile.png + :align: center + +The screenshot above shows the results of running a profile using +sched:sched_switch tracepoint, which shows the relative costs of various +paths to sched_wakeup (note that sched_wakeup is the name of the +tracepoint - it's actually defined just inside ttwu_do_wakeup(), which +accounts for the function name actually displayed in the profile: + +.. code-block:: c + + /* + * Mark the task runnable and perform wakeup-preemption. + */ + static void + ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags) + { + trace_sched_wakeup(p, true); + . + . + . + } + +A couple of the more interesting +callchains are expanded and displayed above, basically some network +receive paths that presumably end up waking up wget (busybox) when +network data is ready. + +Note that because tracepoints are normally used for tracing, the default +sampling period for tracepoints is 1 i.e. for tracepoints perf will +sample on every event occurrence (this can be changed using the -c +option). This is in contrast to hardware counters such as for example +the default 'cycles' hardware counter used for normal profiling, where +sampling periods are much higher (in the thousands) because profiling +should have as low an overhead as possible and sampling on every cycle +would be prohibitively expensive. + +Using perf to do Basic Tracing +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Profiling is a great tool for solving many problems or for getting a +high-level view of what's going on with a workload or across the system. +It is however by definition an approximation, as suggested by the most +prominent word associated with it, 'sampling'. On the one hand, it +allows a representative picture of what's going on in the system to be +cheaply taken, but on the other hand, that cheapness limits its utility +when that data suggests a need to 'dive down' more deeply to discover +what's really going on. In such cases, the only way to see what's really +going on is to be able to look at (or summarize more intelligently) the +individual steps that go into the higher-level behavior exposed by the +coarse-grained profiling data. + +As a concrete example, we can trace all the events we think might be +applicable to our workload: :: + + root@crownbay:~# perf record -g -e skb:* -e net:* -e napi:* -e sched:sched_switch -e sched:sched_wakeup -e irq:* + -e syscalls:sys_enter_read -e syscalls:sys_exit_read -e syscalls:sys_enter_write -e syscalls:sys_exit_write + wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2 + +We can look at the raw trace output using 'perf script' with no +arguments: :: + + root@crownbay:~# perf script + + perf 1262 [000] 11624.857082: sys_exit_read: 0x0 + perf 1262 [000] 11624.857193: sched_wakeup: comm=migration/0 pid=6 prio=0 success=1 target_cpu=000 + wget 1262 [001] 11624.858021: softirq_raise: vec=1 [action=TIMER] + wget 1262 [001] 11624.858074: softirq_entry: vec=1 [action=TIMER] + wget 1262 [001] 11624.858081: softirq_exit: vec=1 [action=TIMER] + wget 1262 [001] 11624.858166: sys_enter_read: fd: 0x0003, buf: 0xbf82c940, count: 0x0200 + wget 1262 [001] 11624.858177: sys_exit_read: 0x200 + wget 1262 [001] 11624.858878: kfree_skb: skbaddr=0xeb248d80 protocol=0 location=0xc15a5308 + wget 1262 [001] 11624.858945: kfree_skb: skbaddr=0xeb248000 protocol=0 location=0xc15a5308 + wget 1262 [001] 11624.859020: softirq_raise: vec=1 [action=TIMER] + wget 1262 [001] 11624.859076: softirq_entry: vec=1 [action=TIMER] + wget 1262 [001] 11624.859083: softirq_exit: vec=1 [action=TIMER] + wget 1262 [001] 11624.859167: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400 + wget 1262 [001] 11624.859192: sys_exit_read: 0x1d7 + wget 1262 [001] 11624.859228: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400 + wget 1262 [001] 11624.859233: sys_exit_read: 0x0 + wget 1262 [001] 11624.859573: sys_enter_read: fd: 0x0003, buf: 0xbf82c580, count: 0x0200 + wget 1262 [001] 11624.859584: sys_exit_read: 0x200 + wget 1262 [001] 11624.859864: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400 + wget 1262 [001] 11624.859888: sys_exit_read: 0x400 + wget 1262 [001] 11624.859935: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400 + wget 1262 [001] 11624.859944: sys_exit_read: 0x400 + +This gives us a detailed timestamped sequence of events that occurred within the +workload with respect to those events. + +In many ways, profiling can be viewed as a subset of tracing - +theoretically, if you have a set of trace events that's sufficient to +capture all the important aspects of a workload, you can derive any of +the results or views that a profiling run can. + +Another aspect of traditional profiling is that while powerful in many +ways, it's limited by the granularity of the underlying data. Profiling +tools offer various ways of sorting and presenting the sample data, +which make it much more useful and amenable to user experimentation, but +in the end it can't be used in an open-ended way to extract data that +just isn't present as a consequence of the fact that conceptually, most +of it has been thrown away. + +Full-blown detailed tracing data does however offer the opportunity to +manipulate and present the information collected during a tracing run in +an infinite variety of ways. + +Another way to look at it is that there are only so many ways that the +'primitive' counters can be used on their own to generate interesting +output; to get anything more complicated than simple counts requires +some amount of additional logic, which is typically very specific to the +problem at hand. For example, if we wanted to make use of a 'counter' +that maps to the value of the time difference between when a process was +scheduled to run on a processor and the time it actually ran, we +wouldn't expect such a counter to exist on its own, but we could derive +one called say 'wakeup_latency' and use it to extract a useful view of +that metric from trace data. Likewise, we really can't figure out from +standard profiling tools how much data every process on the system reads +and writes, along with how many of those reads and writes fail +completely. If we have sufficient trace data, however, we could with the +right tools easily extract and present that information, but we'd need +something other than pre-canned profiling tools to do that. + +Luckily, there is a general-purpose way to handle such needs, called +'programming languages'. Making programming languages easily available +to apply to such problems given the specific format of data is called a +'programming language binding' for that data and language. Perf supports +two programming language bindings, one for Python and one for Perl. + +.. admonition:: Tying it Together + + Language bindings for manipulating and aggregating trace data are of + course not a new idea. One of the first projects to do this was IBM's + DProbes dpcc compiler, an ANSI C compiler which targeted a low-level + assembly language running on an in-kernel interpreter on the target + system. This is exactly analogous to what Sun's DTrace did, except + that DTrace invented its own language for the purpose. Systemtap, + heavily inspired by DTrace, also created its own one-off language, + but rather than running the product on an in-kernel interpreter, + created an elaborate compiler-based machinery to translate its + language into kernel modules written in C. + +Now that we have the trace data in perf.data, we can use 'perf script +-g' to generate a skeleton script with handlers for the read/write +entry/exit events we recorded: :: + + root@crownbay:~# perf script -g python + generated Python script: perf-script.py + +The skeleton script simply creates a python function for each event type in the +perf.data file. The body of each function simply prints the event name along +with its parameters. For example: + +.. code-block:: python + + def net__netif_rx(event_name, context, common_cpu, + common_secs, common_nsecs, common_pid, common_comm, + skbaddr, len, name): + print_header(event_name, common_cpu, common_secs, common_nsecs, + common_pid, common_comm) + + print "skbaddr=%u, len=%u, name=%s\n" % (skbaddr, len, name), + +We can run that script directly to print all of the events contained in the +perf.data file: :: + + root@crownbay:~# perf script -s perf-script.py + + in trace_begin + syscalls__sys_exit_read 0 11624.857082795 1262 perf nr=3, ret=0 + sched__sched_wakeup 0 11624.857193498 1262 perf comm=migration/0, pid=6, prio=0, success=1, target_cpu=0 + irq__softirq_raise 1 11624.858021635 1262 wget vec=TIMER + irq__softirq_entry 1 11624.858074075 1262 wget vec=TIMER + irq__softirq_exit 1 11624.858081389 1262 wget vec=TIMER + syscalls__sys_enter_read 1 11624.858166434 1262 wget nr=3, fd=3, buf=3213019456, count=512 + syscalls__sys_exit_read 1 11624.858177924 1262 wget nr=3, ret=512 + skb__kfree_skb 1 11624.858878188 1262 wget skbaddr=3945041280, location=3243922184, protocol=0 + skb__kfree_skb 1 11624.858945608 1262 wget skbaddr=3945037824, location=3243922184, protocol=0 + irq__softirq_raise 1 11624.859020942 1262 wget vec=TIMER + irq__softirq_entry 1 11624.859076935 1262 wget vec=TIMER + irq__softirq_exit 1 11624.859083469 1262 wget vec=TIMER + syscalls__sys_enter_read 1 11624.859167565 1262 wget nr=3, fd=3, buf=3077701632, count=1024 + syscalls__sys_exit_read 1 11624.859192533 1262 wget nr=3, ret=471 + syscalls__sys_enter_read 1 11624.859228072 1262 wget nr=3, fd=3, buf=3077701632, count=1024 + syscalls__sys_exit_read 1 11624.859233707 1262 wget nr=3, ret=0 + syscalls__sys_enter_read 1 11624.859573008 1262 wget nr=3, fd=3, buf=3213018496, count=512 + syscalls__sys_exit_read 1 11624.859584818 1262 wget nr=3, ret=512 + syscalls__sys_enter_read 1 11624.859864562 1262 wget nr=3, fd=3, buf=3077701632, count=1024 + syscalls__sys_exit_read 1 11624.859888770 1262 wget nr=3, ret=1024 + syscalls__sys_enter_read 1 11624.859935140 1262 wget nr=3, fd=3, buf=3077701632, count=1024 + syscalls__sys_exit_read 1 11624.859944032 1262 wget nr=3, ret=1024 + +That in itself isn't very useful; after all, we can accomplish pretty much the +same thing by simply running 'perf script' without arguments in the same +directory as the perf.data file. + +We can however replace the print statements in the generated function +bodies with whatever we want, and thereby make it infinitely more +useful. + +As a simple example, let's just replace the print statements in the +function bodies with a simple function that does nothing but increment a +per-event count. When the program is run against a perf.data file, each +time a particular event is encountered, a tally is incremented for that +event. For example: + +.. code-block:: python + + def net__netif_rx(event_name, context, common_cpu, + common_secs, common_nsecs, common_pid, common_comm, + skbaddr, len, name): + inc_counts(event_name) + +Each event handler function in the generated code +is modified to do this. For convenience, we define a common function +called inc_counts() that each handler calls; inc_counts() simply tallies +a count for each event using the 'counts' hash, which is a specialized +hash function that does Perl-like autovivification, a capability that's +extremely useful for kinds of multi-level aggregation commonly used in +processing traces (see perf's documentation on the Python language +binding for details): + +.. code-block:: python + + counts = autodict() + + def inc_counts(event_name): + try: + counts[event_name] += 1 + except TypeError: + counts[event_name] = 1 + +Finally, at the end of the trace processing run, we want to print the +result of all the per-event tallies. For that, we use the special +'trace_end()' function: + +.. code-block:: python + + def trace_end(): + for event_name, count in counts.iteritems(): + print "%-40s %10s\n" % (event_name, count) + +The end result is a summary of all the events recorded in the trace: :: + + skb__skb_copy_datagram_iovec 13148 + irq__softirq_entry 4796 + irq__irq_handler_exit 3805 + irq__softirq_exit 4795 + syscalls__sys_enter_write 8990 + net__net_dev_xmit 652 + skb__kfree_skb 4047 + sched__sched_wakeup 1155 + irq__irq_handler_entry 3804 + irq__softirq_raise 4799 + net__net_dev_queue 652 + syscalls__sys_enter_read 17599 + net__netif_receive_skb 1743 + syscalls__sys_exit_read 17598 + net__netif_rx 2 + napi__napi_poll 1877 + syscalls__sys_exit_write 8990 + +Note that this is +pretty much exactly the same information we get from 'perf stat', which +goes a little way to support the idea mentioned previously that given +the right kind of trace data, higher-level profiling-type summaries can +be derived from it. + +Documentation on using the `'perf script' python +binding <http://linux.die.net/man/1/perf-script-python>`__. + +System-Wide Tracing and Profiling +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The examples so far have focused on tracing a particular program or +workload - in other words, every profiling run has specified the program +to profile in the command-line e.g. 'perf record wget ...'. + +It's also possible, and more interesting in many cases, to run a +system-wide profile or trace while running the workload in a separate +shell. + +To do system-wide profiling or tracing, you typically use the -a flag to +'perf record'. + +To demonstrate this, open up one window and start the profile using the +-a flag (press Ctrl-C to stop tracing): :: + + root@crownbay:~# perf record -g -a + ^C[ perf record: Woken up 6 times to write data ] + [ perf record: Captured and wrote 1.400 MB perf.data (~61172 samples) ] + +In another window, run the wget test: :: + + root@crownbay:~# wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2 + Connecting to downloads.yoctoproject.org (140.211.169.59:80) + linux-2.6.19.2.tar.b 100% \|*******************************\| 41727k 0:00:00 ETA + +Here we see entries not only for our wget load, but for +other processes running on the system as well: + +.. image:: figures/perf-systemwide.png + :align: center + +In the snapshot above, we can see callchains that originate in libc, and +a callchain from Xorg that demonstrates that we're using a proprietary X +driver in userspace (notice the presence of 'PVR' and some other +unresolvable symbols in the expanded Xorg callchain). + +Note also that we have both kernel and userspace entries in the above +snapshot. We can also tell perf to focus on userspace but providing a +modifier, in this case 'u', to the 'cycles' hardware counter when we +record a profile: :: + + root@crownbay:~# perf record -g -a -e cycles:u + ^C[ perf record: Woken up 2 times to write data ] + [ perf record: Captured and wrote 0.376 MB perf.data (~16443 samples) ] + +.. image:: figures/perf-report-cycles-u.png + :align: center + +Notice in the screenshot above, we see only userspace entries ([.]) + +Finally, we can press 'enter' on a leaf node and select the 'Zoom into +DSO' menu item to show only entries associated with a specific DSO. In +the screenshot below, we've zoomed into the 'libc' DSO which shows all +the entries associated with the libc-xxx.so DSO. + +.. image:: figures/perf-systemwide-libc.png + :align: center + +We can also use the system-wide -a switch to do system-wide tracing. +Here we'll trace a couple of scheduler events: :: + + root@crownbay:~# perf record -a -e sched:sched_switch -e sched:sched_wakeup + ^C[ perf record: Woken up 38 times to write data ] + [ perf record: Captured and wrote 9.780 MB perf.data (~427299 samples) ] + +We can look at the raw output using 'perf script' with no arguments: :: + + root@crownbay:~# perf script + + perf 1383 [001] 6171.460045: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001 + perf 1383 [001] 6171.460066: sched_switch: prev_comm=perf prev_pid=1383 prev_prio=120 prev_state=R+ ==> next_comm=kworker/1:1 next_pid=21 next_prio=120 + kworker/1:1 21 [001] 6171.460093: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=1383 next_prio=120 + swapper 0 [000] 6171.468063: sched_wakeup: comm=kworker/0:3 pid=1209 prio=120 success=1 target_cpu=000 + swapper 0 [000] 6171.468107: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120 + kworker/0:3 1209 [000] 6171.468143: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120 + perf 1383 [001] 6171.470039: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001 + perf 1383 [001] 6171.470058: sched_switch: prev_comm=perf prev_pid=1383 prev_prio=120 prev_state=R+ ==> next_comm=kworker/1:1 next_pid=21 next_prio=120 + kworker/1:1 21 [001] 6171.470082: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=1383 next_prio=120 + perf 1383 [001] 6171.480035: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001 + +Filtering +^^^^^^^^^ + +Notice that there are a lot of events that don't really have anything to +do with what we're interested in, namely events that schedule 'perf' +itself in and out or that wake perf up. We can get rid of those by using +the '--filter' option - for each event we specify using -e, we can add a +--filter after that to filter out trace events that contain fields with +specific values: :: + + root@crownbay:~# perf record -a -e sched:sched_switch --filter 'next_comm != perf && prev_comm != perf' -e sched:sched_wakeup --filter 'comm != perf' + ^C[ perf record: Woken up 38 times to write data ] + [ perf record: Captured and wrote 9.688 MB perf.data (~423279 samples) ] + + + root@crownbay:~# perf script + + swapper 0 [000] 7932.162180: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120 + kworker/0:3 1209 [000] 7932.162236: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120 + perf 1407 [001] 7932.170048: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001 + perf 1407 [001] 7932.180044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001 + perf 1407 [001] 7932.190038: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001 + perf 1407 [001] 7932.200044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001 + perf 1407 [001] 7932.210044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001 + perf 1407 [001] 7932.220044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001 + swapper 0 [001] 7932.230111: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001 + swapper 0 [001] 7932.230146: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/1:1 next_pid=21 next_prio=120 + kworker/1:1 21 [001] 7932.230205: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=swapper/1 next_pid=0 next_prio=120 + swapper 0 [000] 7932.326109: sched_wakeup: comm=kworker/0:3 pid=1209 prio=120 success=1 target_cpu=000 + swapper 0 [000] 7932.326171: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120 + kworker/0:3 1209 [000] 7932.326214: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120 + +In this case, we've filtered out all events that have +'perf' in their 'comm' or 'comm_prev' or 'comm_next' fields. Notice that +there are still events recorded for perf, but notice that those events +don't have values of 'perf' for the filtered fields. To completely +filter out anything from perf will require a bit more work, but for the +purpose of demonstrating how to use filters, it's close enough. + +.. admonition:: Tying it Together + + These are exactly the same set of event filters defined by the trace + event subsystem. See the ftrace/tracecmd/kernelshark section for more + discussion about these event filters. + +.. admonition:: Tying it Together + + These event filters are implemented by a special-purpose + pseudo-interpreter in the kernel and are an integral and + indispensable part of the perf design as it relates to tracing. + kernel-based event filters provide a mechanism to precisely throttle + the event stream that appears in user space, where it makes sense to + provide bindings to real programming languages for postprocessing the + event stream. This architecture allows for the intelligent and + flexible partitioning of processing between the kernel and user + space. Contrast this with other tools such as SystemTap, which does + all of its processing in the kernel and as such requires a special + project-defined language in order to accommodate that design, or + LTTng, where everything is sent to userspace and as such requires a + super-efficient kernel-to-userspace transport mechanism in order to + function properly. While perf certainly can benefit from for instance + advances in the design of the transport, it doesn't fundamentally + depend on them. Basically, if you find that your perf tracing + application is causing buffer I/O overruns, it probably means that + you aren't taking enough advantage of the kernel filtering engine. + +Using Dynamic Tracepoints +~~~~~~~~~~~~~~~~~~~~~~~~~ + +perf isn't restricted to the fixed set of static tracepoints listed by +'perf list'. Users can also add their own 'dynamic' tracepoints anywhere +in the kernel. For instance, suppose we want to define our own +tracepoint on do_fork(). We can do that using the 'perf probe' perf +subcommand: :: + + root@crownbay:~# perf probe do_fork + Added new event: + probe:do_fork (on do_fork) + + You can now use it in all perf tools, such as: + + perf record -e probe:do_fork -aR sleep 1 + +Adding a new tracepoint via +'perf probe' results in an event with all the expected files and format +in /sys/kernel/debug/tracing/events, just the same as for static +tracepoints (as discussed in more detail in the trace events subsystem +section: :: + + root@crownbay:/sys/kernel/debug/tracing/events/probe/do_fork# ls -al + drwxr-xr-x 2 root root 0 Oct 28 11:42 . + drwxr-xr-x 3 root root 0 Oct 28 11:42 .. + -rw-r--r-- 1 root root 0 Oct 28 11:42 enable + -rw-r--r-- 1 root root 0 Oct 28 11:42 filter + -r--r--r-- 1 root root 0 Oct 28 11:42 format + -r--r--r-- 1 root root 0 Oct 28 11:42 id + + root@crownbay:/sys/kernel/debug/tracing/events/probe/do_fork# cat format + name: do_fork + ID: 944 + format: + field:unsigned short common_type; offset:0; size:2; signed:0; + field:unsigned char common_flags; offset:2; size:1; signed:0; + field:unsigned char common_preempt_count; offset:3; size:1; signed:0; + field:int common_pid; offset:4; size:4; signed:1; + field:int common_padding; offset:8; size:4; signed:1; + + field:unsigned long __probe_ip; offset:12; size:4; signed:0; + + print fmt: "(%lx)", REC->__probe_ip + +We can list all dynamic tracepoints currently in +existence: :: + + root@crownbay:~# perf probe -l + probe:do_fork (on do_fork) + probe:schedule (on schedule) + +Let's record system-wide ('sleep 30' is a +trick for recording system-wide but basically do nothing and then wake +up after 30 seconds): :: + + root@crownbay:~# perf record -g -a -e probe:do_fork sleep 30 + [ perf record: Woken up 1 times to write data ] + [ perf record: Captured and wrote 0.087 MB perf.data (~3812 samples) ] + +Using 'perf script' we can see each do_fork event that fired: :: + + root@crownbay:~# perf script + + # ======== + # captured on: Sun Oct 28 11:55:18 2012 + # hostname : crownbay + # os release : 3.4.11-yocto-standard + # perf version : 3.4.11 + # arch : i686 + # nrcpus online : 2 + # nrcpus avail : 2 + # cpudesc : Intel(R) Atom(TM) CPU E660 @ 1.30GHz + # cpuid : GenuineIntel,6,38,1 + # total memory : 1017184 kB + # cmdline : /usr/bin/perf record -g -a -e probe:do_fork sleep 30 + # event : name = probe:do_fork, type = 2, config = 0x3b0, config1 = 0x0, config2 = 0x0, excl_usr = 0, excl_kern + = 0, id = { 5, 6 } + # HEADER_CPU_TOPOLOGY info available, use -I to display + # ======== + # + matchbox-deskto 1197 [001] 34211.378318: do_fork: (c1028460) + matchbox-deskto 1295 [001] 34211.380388: do_fork: (c1028460) + pcmanfm 1296 [000] 34211.632350: do_fork: (c1028460) + pcmanfm 1296 [000] 34211.639917: do_fork: (c1028460) + matchbox-deskto 1197 [001] 34217.541603: do_fork: (c1028460) + matchbox-deskto 1299 [001] 34217.543584: do_fork: (c1028460) + gthumb 1300 [001] 34217.697451: do_fork: (c1028460) + gthumb 1300 [001] 34219.085734: do_fork: (c1028460) + gthumb 1300 [000] 34219.121351: do_fork: (c1028460) + gthumb 1300 [001] 34219.264551: do_fork: (c1028460) + pcmanfm 1296 [000] 34219.590380: do_fork: (c1028460) + matchbox-deskto 1197 [001] 34224.955965: do_fork: (c1028460) + matchbox-deskto 1306 [001] 34224.957972: do_fork: (c1028460) + matchbox-termin 1307 [000] 34225.038214: do_fork: (c1028460) + matchbox-termin 1307 [001] 34225.044218: do_fork: (c1028460) + matchbox-termin 1307 [000] 34225.046442: do_fork: (c1028460) + matchbox-deskto 1197 [001] 34237.112138: do_fork: (c1028460) + matchbox-deskto 1311 [001] 34237.114106: do_fork: (c1028460) + gaku 1312 [000] 34237.202388: do_fork: (c1028460) + +And using 'perf report' on the same file, we can see the +callgraphs from starting a few programs during those 30 seconds: + +.. image:: figures/perf-probe-do_fork-profile.png + :align: center + +.. admonition:: Tying it Together + + The trace events subsystem accommodate static and dynamic tracepoints + in exactly the same way - there's no difference as far as the + infrastructure is concerned. See the ftrace section for more details + on the trace event subsystem. + +.. admonition:: Tying it Together + + Dynamic tracepoints are implemented under the covers by kprobes and + uprobes. kprobes and uprobes are also used by and in fact are the + main focus of SystemTap. + +Perf Documentation +------------------ + +Online versions of the man pages for the commands discussed in this +section can be found here: + +- The `'perf stat' manpage <http://linux.die.net/man/1/perf-stat>`__. + +- The `'perf record' + manpage <http://linux.die.net/man/1/perf-record>`__. + +- The `'perf report' + manpage <http://linux.die.net/man/1/perf-report>`__. + +- The `'perf probe' manpage <http://linux.die.net/man/1/perf-probe>`__. + +- The `'perf script' + manpage <http://linux.die.net/man/1/perf-script>`__. + +- Documentation on using the `'perf script' python + binding <http://linux.die.net/man/1/perf-script-python>`__. + +- The top-level `perf(1) manpage <http://linux.die.net/man/1/perf>`__. + +Normally, you should be able to invoke the man pages via perf itself +e.g. 'perf help' or 'perf help record'. + +However, by default Yocto doesn't install man pages, but perf invokes +the man pages for most help functionality. This is a bug and is being +addressed by a Yocto bug: `Bug 3388 - perf: enable man pages for basic +'help' +functionality <https://bugzilla.yoctoproject.org/show_bug.cgi?id=3388>`__. + +The man pages in text form, along with some other files, such as a set +of examples, can be found in the 'perf' directory of the kernel tree: :: + + tools/perf/Documentation + +There's also a nice perf tutorial on the perf +wiki that goes into more detail than we do here in certain areas: `Perf +Tutorial <https://perf.wiki.kernel.org/index.php/Tutorial>`__ + +ftrace +====== + +'ftrace' literally refers to the 'ftrace function tracer' but in reality +this encompasses a number of related tracers along with the +infrastructure that they all make use of. + +ftrace Setup +------------ + +For this section, we'll assume you've already performed the basic setup +outlined in the ":ref:`profile-manual/intro:General Setup`" section. + +ftrace, trace-cmd, and kernelshark run on the target system, and are +ready to go out-of-the-box - no additional setup is necessary. For the +rest of this section we assume you've ssh'ed to the host and will be +running ftrace on the target. kernelshark is a GUI application and if +you use the '-X' option to ssh you can have the kernelshark GUI run on +the target but display remotely on the host if you want. + +Basic ftrace usage +------------------ + +'ftrace' essentially refers to everything included in the /tracing +directory of the mounted debugfs filesystem (Yocto follows the standard +convention and mounts it at /sys/kernel/debug). Here's a listing of all +the files found in /sys/kernel/debug/tracing on a Yocto system: :: + + root@sugarbay:/sys/kernel/debug/tracing# ls + README kprobe_events trace + available_events kprobe_profile trace_clock + available_filter_functions options trace_marker + available_tracers per_cpu trace_options + buffer_size_kb printk_formats trace_pipe + buffer_total_size_kb saved_cmdlines tracing_cpumask + current_tracer set_event tracing_enabled + dyn_ftrace_total_info set_ftrace_filter tracing_on + enabled_functions set_ftrace_notrace tracing_thresh + events set_ftrace_pid + free_buffer set_graph_function + +The files listed above are used for various purposes +- some relate directly to the tracers themselves, others are used to set +tracing options, and yet others actually contain the tracing output when +a tracer is in effect. Some of the functions can be guessed from their +names, others need explanation; in any case, we'll cover some of the +files we see here below but for an explanation of the others, please see +the ftrace documentation. + +We'll start by looking at some of the available built-in tracers. + +cat'ing the 'available_tracers' file lists the set of available tracers: :: + + root@sugarbay:/sys/kernel/debug/tracing# cat available_tracers + blk function_graph function nop + +The 'current_tracer' file contains the tracer currently in effect: :: + + root@sugarbay:/sys/kernel/debug/tracing# cat current_tracer + nop + +The above listing of current_tracer shows that the +'nop' tracer is in effect, which is just another way of saying that +there's actually no tracer currently in effect. + +echo'ing one of the available_tracers into current_tracer makes the +specified tracer the current tracer: :: + + root@sugarbay:/sys/kernel/debug/tracing# echo function > current_tracer + root@sugarbay:/sys/kernel/debug/tracing# cat current_tracer + function + +The above sets the current tracer to be the 'function tracer'. This tracer +traces every function call in the kernel and makes it available as the +contents of the 'trace' file. Reading the 'trace' file lists the +currently buffered function calls that have been traced by the function +tracer: :: + + root@sugarbay:/sys/kernel/debug/tracing# cat trace | less + + # tracer: function + # + # entries-in-buffer/entries-written: 310629/766471 #P:8 + # + # _-----=> irqs-off + # / _----=> need-resched + # | / _---=> hardirq/softirq + # || / _--=> preempt-depth + # ||| / delay + # TASK-PID CPU# |||| TIMESTAMP FUNCTION + # | | | |||| | | + <idle>-0 [004] d..1 470.867169: ktime_get_real <-intel_idle + <idle>-0 [004] d..1 470.867170: getnstimeofday <-ktime_get_real + <idle>-0 [004] d..1 470.867171: ns_to_timeval <-intel_idle + <idle>-0 [004] d..1 470.867171: ns_to_timespec <-ns_to_timeval + <idle>-0 [004] d..1 470.867172: smp_apic_timer_interrupt <-apic_timer_interrupt + <idle>-0 [004] d..1 470.867172: native_apic_mem_write <-smp_apic_timer_interrupt + <idle>-0 [004] d..1 470.867172: irq_enter <-smp_apic_timer_interrupt + <idle>-0 [004] d..1 470.867172: rcu_irq_enter <-irq_enter + <idle>-0 [004] d..1 470.867173: rcu_idle_exit_common.isra.33 <-rcu_irq_enter + <idle>-0 [004] d..1 470.867173: local_bh_disable <-irq_enter + <idle>-0 [004] d..1 470.867173: add_preempt_count <-local_bh_disable + <idle>-0 [004] d.s1 470.867174: tick_check_idle <-irq_enter + <idle>-0 [004] d.s1 470.867174: tick_check_oneshot_broadcast <-tick_check_idle + <idle>-0 [004] d.s1 470.867174: ktime_get <-tick_check_idle + <idle>-0 [004] d.s1 470.867174: tick_nohz_stop_idle <-tick_check_idle + <idle>-0 [004] d.s1 470.867175: update_ts_time_stats <-tick_nohz_stop_idle + <idle>-0 [004] d.s1 470.867175: nr_iowait_cpu <-update_ts_time_stats + <idle>-0 [004] d.s1 470.867175: tick_do_update_jiffies64 <-tick_check_idle + <idle>-0 [004] d.s1 470.867175: _raw_spin_lock <-tick_do_update_jiffies64 + <idle>-0 [004] d.s1 470.867176: add_preempt_count <-_raw_spin_lock + <idle>-0 [004] d.s2 470.867176: do_timer <-tick_do_update_jiffies64 + <idle>-0 [004] d.s2 470.867176: _raw_spin_lock <-do_timer + <idle>-0 [004] d.s2 470.867176: add_preempt_count <-_raw_spin_lock + <idle>-0 [004] d.s3 470.867177: ntp_tick_length <-do_timer + <idle>-0 [004] d.s3 470.867177: _raw_spin_lock_irqsave <-ntp_tick_length + . + . + . + +Each line in the trace above shows what was happening in the kernel on a given +cpu, to the level of detail of function calls. Each entry shows the function +called, followed by its caller (after the arrow). + +The function tracer gives you an extremely detailed idea of what the +kernel was doing at the point in time the trace was taken, and is a +great way to learn about how the kernel code works in a dynamic sense. + +.. admonition:: Tying it Together + + The ftrace function tracer is also available from within perf, as the + ftrace:function tracepoint. + +It is a little more difficult to follow the call chains than it needs to +be - luckily there's a variant of the function tracer that displays the +callchains explicitly, called the 'function_graph' tracer: :: + + root@sugarbay:/sys/kernel/debug/tracing# echo function_graph > current_tracer + root@sugarbay:/sys/kernel/debug/tracing# cat trace | less + + tracer: function_graph + + CPU DURATION FUNCTION CALLS + | | | | | | | + 7) 0.046 us | pick_next_task_fair(); + 7) 0.043 us | pick_next_task_stop(); + 7) 0.042 us | pick_next_task_rt(); + 7) 0.032 us | pick_next_task_fair(); + 7) 0.030 us | pick_next_task_idle(); + 7) | _raw_spin_unlock_irq() { + 7) 0.033 us | sub_preempt_count(); + 7) 0.258 us | } + 7) 0.032 us | sub_preempt_count(); + 7) + 13.341 us | } /* __schedule */ + 7) 0.095 us | } /* sub_preempt_count */ + 7) | schedule() { + 7) | __schedule() { + 7) 0.060 us | add_preempt_count(); + 7) 0.044 us | rcu_note_context_switch(); + 7) | _raw_spin_lock_irq() { + 7) 0.033 us | add_preempt_count(); + 7) 0.247 us | } + 7) | idle_balance() { + 7) | _raw_spin_unlock() { + 7) 0.031 us | sub_preempt_count(); + 7) 0.246 us | } + 7) | update_shares() { + 7) 0.030 us | __rcu_read_lock(); + 7) 0.029 us | __rcu_read_unlock(); + 7) 0.484 us | } + 7) 0.030 us | __rcu_read_lock(); + 7) | load_balance() { + 7) | find_busiest_group() { + 7) 0.031 us | idle_cpu(); + 7) 0.029 us | idle_cpu(); + 7) 0.035 us | idle_cpu(); + 7) 0.906 us | } + 7) 1.141 us | } + 7) 0.022 us | msecs_to_jiffies(); + 7) | load_balance() { + 7) | find_busiest_group() { + 7) 0.031 us | idle_cpu(); + . + . + . + 4) 0.062 us | msecs_to_jiffies(); + 4) 0.062 us | __rcu_read_unlock(); + 4) | _raw_spin_lock() { + 4) 0.073 us | add_preempt_count(); + 4) 0.562 us | } + 4) + 17.452 us | } + 4) 0.108 us | put_prev_task_fair(); + 4) 0.102 us | pick_next_task_fair(); + 4) 0.084 us | pick_next_task_stop(); + 4) 0.075 us | pick_next_task_rt(); + 4) 0.062 us | pick_next_task_fair(); + 4) 0.066 us | pick_next_task_idle(); + ------------------------------------------ + 4) kworker-74 => <idle>-0 + ------------------------------------------ + + 4) | finish_task_switch() { + 4) | _raw_spin_unlock_irq() { + 4) 0.100 us | sub_preempt_count(); + 4) 0.582 us | } + 4) 1.105 us | } + 4) 0.088 us | sub_preempt_count(); + 4) ! 100.066 us | } + . + . + . + 3) | sys_ioctl() { + 3) 0.083 us | fget_light(); + 3) | security_file_ioctl() { + 3) 0.066 us | cap_file_ioctl(); + 3) 0.562 us | } + 3) | do_vfs_ioctl() { + 3) | drm_ioctl() { + 3) 0.075 us | drm_ut_debug_printk(); + 3) | i915_gem_pwrite_ioctl() { + 3) | i915_mutex_lock_interruptible() { + 3) 0.070 us | mutex_lock_interruptible(); + 3) 0.570 us | } + 3) | drm_gem_object_lookup() { + 3) | _raw_spin_lock() { + 3) 0.080 us | add_preempt_count(); + 3) 0.620 us | } + 3) | _raw_spin_unlock() { + 3) 0.085 us | sub_preempt_count(); + 3) 0.562 us | } + 3) 2.149 us | } + 3) 0.133 us | i915_gem_object_pin(); + 3) | i915_gem_object_set_to_gtt_domain() { + 3) 0.065 us | i915_gem_object_flush_gpu_write_domain(); + 3) 0.065 us | i915_gem_object_wait_rendering(); + 3) 0.062 us | i915_gem_object_flush_cpu_write_domain(); + 3) 1.612 us | } + 3) | i915_gem_object_put_fence() { + 3) 0.097 us | i915_gem_object_flush_fence.constprop.36(); + 3) 0.645 us | } + 3) 0.070 us | add_preempt_count(); + 3) 0.070 us | sub_preempt_count(); + 3) 0.073 us | i915_gem_object_unpin(); + 3) 0.068 us | mutex_unlock(); + 3) 9.924 us | } + 3) + 11.236 us | } + 3) + 11.770 us | } + 3) + 13.784 us | } + 3) | sys_ioctl() { + +As you can see, the function_graph display is much easier +to follow. Also note that in addition to the function calls and +associated braces, other events such as scheduler events are displayed +in context. In fact, you can freely include any tracepoint available in +the trace events subsystem described in the next section by simply +enabling those events, and they'll appear in context in the function +graph display. Quite a powerful tool for understanding kernel dynamics. + +Also notice that there are various annotations on the left hand side of +the display. For example if the total time it took for a given function +to execute is above a certain threshold, an exclamation point or plus +sign appears on the left hand side. Please see the ftrace documentation +for details on all these fields. + +The 'trace events' Subsystem +---------------------------- + +One especially important directory contained within the +/sys/kernel/debug/tracing directory is the 'events' subdirectory, which +contains representations of every tracepoint in the system. Listing out +the contents of the 'events' subdirectory, we see mainly another set of +subdirectories: :: + + root@sugarbay:/sys/kernel/debug/tracing# cd events + root@sugarbay:/sys/kernel/debug/tracing/events# ls -al + drwxr-xr-x 38 root root 0 Nov 14 23:19 . + drwxr-xr-x 5 root root 0 Nov 14 23:19 .. + drwxr-xr-x 19 root root 0 Nov 14 23:19 block + drwxr-xr-x 32 root root 0 Nov 14 23:19 btrfs + drwxr-xr-x 5 root root 0 Nov 14 23:19 drm + -rw-r--r-- 1 root root 0 Nov 14 23:19 enable + drwxr-xr-x 40 root root 0 Nov 14 23:19 ext3 + drwxr-xr-x 79 root root 0 Nov 14 23:19 ext4 + drwxr-xr-x 14 root root 0 Nov 14 23:19 ftrace + drwxr-xr-x 8 root root 0 Nov 14 23:19 hda + -r--r--r-- 1 root root 0 Nov 14 23:19 header_event + -r--r--r-- 1 root root 0 Nov 14 23:19 header_page + drwxr-xr-x 25 root root 0 Nov 14 23:19 i915 + drwxr-xr-x 7 root root 0 Nov 14 23:19 irq + drwxr-xr-x 12 root root 0 Nov 14 23:19 jbd + drwxr-xr-x 14 root root 0 Nov 14 23:19 jbd2 + drwxr-xr-x 14 root root 0 Nov 14 23:19 kmem + drwxr-xr-x 7 root root 0 Nov 14 23:19 module + drwxr-xr-x 3 root root 0 Nov 14 23:19 napi + drwxr-xr-x 6 root root 0 Nov 14 23:19 net + drwxr-xr-x 3 root root 0 Nov 14 23:19 oom + drwxr-xr-x 12 root root 0 Nov 14 23:19 power + drwxr-xr-x 3 root root 0 Nov 14 23:19 printk + drwxr-xr-x 8 root root 0 Nov 14 23:19 random + drwxr-xr-x 4 root root 0 Nov 14 23:19 raw_syscalls + drwxr-xr-x 3 root root 0 Nov 14 23:19 rcu + drwxr-xr-x 6 root root 0 Nov 14 23:19 rpm + drwxr-xr-x 20 root root 0 Nov 14 23:19 sched + drwxr-xr-x 7 root root 0 Nov 14 23:19 scsi + drwxr-xr-x 4 root root 0 Nov 14 23:19 signal + drwxr-xr-x 5 root root 0 Nov 14 23:19 skb + drwxr-xr-x 4 root root 0 Nov 14 23:19 sock + drwxr-xr-x 10 root root 0 Nov 14 23:19 sunrpc + drwxr-xr-x 538 root root 0 Nov 14 23:19 syscalls + drwxr-xr-x 4 root root 0 Nov 14 23:19 task + drwxr-xr-x 14 root root 0 Nov 14 23:19 timer + drwxr-xr-x 3 root root 0 Nov 14 23:19 udp + drwxr-xr-x 21 root root 0 Nov 14 23:19 vmscan + drwxr-xr-x 3 root root 0 Nov 14 23:19 vsyscall + drwxr-xr-x 6 root root 0 Nov 14 23:19 workqueue + drwxr-xr-x 26 root root 0 Nov 14 23:19 writeback + +Each one of these subdirectories +corresponds to a 'subsystem' and contains yet again more subdirectories, +each one of those finally corresponding to a tracepoint. For example, +here are the contents of the 'kmem' subsystem: :: + + root@sugarbay:/sys/kernel/debug/tracing/events# cd kmem + root@sugarbay:/sys/kernel/debug/tracing/events/kmem# ls -al + drwxr-xr-x 14 root root 0 Nov 14 23:19 . + drwxr-xr-x 38 root root 0 Nov 14 23:19 .. + -rw-r--r-- 1 root root 0 Nov 14 23:19 enable + -rw-r--r-- 1 root root 0 Nov 14 23:19 filter + drwxr-xr-x 2 root root 0 Nov 14 23:19 kfree + drwxr-xr-x 2 root root 0 Nov 14 23:19 kmalloc + drwxr-xr-x 2 root root 0 Nov 14 23:19 kmalloc_node + drwxr-xr-x 2 root root 0 Nov 14 23:19 kmem_cache_alloc + drwxr-xr-x 2 root root 0 Nov 14 23:19 kmem_cache_alloc_node + drwxr-xr-x 2 root root 0 Nov 14 23:19 kmem_cache_free + drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_alloc + drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_alloc_extfrag + drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_alloc_zone_locked + drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_free + drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_free_batched + drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_pcpu_drain + +Let's see what's inside the subdirectory for a +specific tracepoint, in this case the one for kmalloc: :: + + root@sugarbay:/sys/kernel/debug/tracing/events/kmem# cd kmalloc + root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# ls -al + drwxr-xr-x 2 root root 0 Nov 14 23:19 . + drwxr-xr-x 14 root root 0 Nov 14 23:19 .. + -rw-r--r-- 1 root root 0 Nov 14 23:19 enable + -rw-r--r-- 1 root root 0 Nov 14 23:19 filter + -r--r--r-- 1 root root 0 Nov 14 23:19 format + -r--r--r-- 1 root root 0 Nov 14 23:19 id + +The 'format' file for the +tracepoint describes the event in memory, which is used by the various +tracing tools that now make use of these tracepoint to parse the event +and make sense of it, along with a 'print fmt' field that allows tools +like ftrace to display the event as text. Here's what the format of the +kmalloc event looks like: :: + + root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# cat format + name: kmalloc + ID: 313 + format: + field:unsigned short common_type; offset:0; size:2; signed:0; + field:unsigned char common_flags; offset:2; size:1; signed:0; + field:unsigned char common_preempt_count; offset:3; size:1; signed:0; + field:int common_pid; offset:4; size:4; signed:1; + field:int common_padding; offset:8; size:4; signed:1; + + field:unsigned long call_site; offset:16; size:8; signed:0; + field:const void * ptr; offset:24; size:8; signed:0; + field:size_t bytes_req; offset:32; size:8; signed:0; + field:size_t bytes_alloc; offset:40; size:8; signed:0; + field:gfp_t gfp_flags; offset:48; size:4; signed:0; + + print fmt: "call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s", REC->call_site, REC->ptr, REC->bytes_req, REC->bytes_alloc, + (REC->gfp_flags) ? __print_flags(REC->gfp_flags, "|", {(unsigned long)(((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( + gfp_t)0x20000u) | (( gfp_t)0x02u) | (( gfp_t)0x08u)) | (( gfp_t)0x4000u) | (( gfp_t)0x10000u) | (( gfp_t)0x1000u) | (( gfp_t)0x200u) | (( + gfp_t)0x400000u)), "GFP_TRANSHUGE"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( gfp_t)0x20000u) | (( + gfp_t)0x02u) | (( gfp_t)0x08u)), "GFP_HIGHUSER_MOVABLE"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( + gfp_t)0x20000u) | (( gfp_t)0x02u)), "GFP_HIGHUSER"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( + gfp_t)0x20000u)), "GFP_USER"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( gfp_t)0x80000u)), GFP_TEMPORARY"}, + {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u)), "GFP_KERNEL"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u)), + "GFP_NOFS"}, {(unsigned long)((( gfp_t)0x20u)), "GFP_ATOMIC"}, {(unsigned long)((( gfp_t)0x10u)), "GFP_NOIO"}, {(unsigned long)(( + gfp_t)0x20u), "GFP_HIGH"}, {(unsigned long)(( gfp_t)0x10u), "GFP_WAIT"}, {(unsigned long)(( gfp_t)0x40u), "GFP_IO"}, {(unsigned long)(( + gfp_t)0x100u), "GFP_COLD"}, {(unsigned long)(( gfp_t)0x200u), "GFP_NOWARN"}, {(unsigned long)(( gfp_t)0x400u), "GFP_REPEAT"}, {(unsigned + long)(( gfp_t)0x800u), "GFP_NOFAIL"}, {(unsigned long)(( gfp_t)0x1000u), "GFP_NORETRY"}, {(unsigned long)(( gfp_t)0x4000u), "GFP_COMP"}, + {(unsigned long)(( gfp_t)0x8000u), "GFP_ZERO"}, {(unsigned long)(( gfp_t)0x10000u), "GFP_NOMEMALLOC"}, {(unsigned long)(( gfp_t)0x20000u), + "GFP_HARDWALL"}, {(unsigned long)(( gfp_t)0x40000u), "GFP_THISNODE"}, {(unsigned long)(( gfp_t)0x80000u), "GFP_RECLAIMABLE"}, {(unsigned + long)(( gfp_t)0x08u), "GFP_MOVABLE"}, {(unsigned long)(( gfp_t)0), "GFP_NOTRACK"}, {(unsigned long)(( gfp_t)0x400000u), "GFP_NO_KSWAPD"}, + {(unsigned long)(( gfp_t)0x800000u), "GFP_OTHER_NODE"} ) : "GFP_NOWAIT" + +The 'enable' file +in the tracepoint directory is what allows the user (or tools such as +trace-cmd) to actually turn the tracepoint on and off. When enabled, the +corresponding tracepoint will start appearing in the ftrace 'trace' file +described previously. For example, this turns on the kmalloc tracepoint: :: + + root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# echo 1 > enable + +At the moment, we're not interested in the function tracer or +some other tracer that might be in effect, so we first turn it off, but +if we do that, we still need to turn tracing on in order to see the +events in the output buffer: :: + + root@sugarbay:/sys/kernel/debug/tracing# echo nop > current_tracer + root@sugarbay:/sys/kernel/debug/tracing# echo 1 > tracing_on + +Now, if we look at the the 'trace' file, we see nothing +but the kmalloc events we just turned on: :: + + root@sugarbay:/sys/kernel/debug/tracing# cat trace | less + # tracer: nop + # + # entries-in-buffer/entries-written: 1897/1897 #P:8 + # + # _-----=> irqs-off + # / _----=> need-resched + # | / _---=> hardirq/softirq + # || / _--=> preempt-depth + # ||| / delay + # TASK-PID CPU# |||| TIMESTAMP FUNCTION + # | | | |||| | | + dropbear-1465 [000] ...1 18154.620753: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL + <idle>-0 [000] ..s3 18154.621640: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + <idle>-0 [000] ..s3 18154.621656: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + matchbox-termin-1361 [001] ...1 18154.755472: kmalloc: call_site=ffffffff81614050 ptr=ffff88006d5f0e00 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_KERNEL|GFP_REPEAT + Xorg-1264 [002] ...1 18154.755581: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY + Xorg-1264 [002] ...1 18154.755583: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO + Xorg-1264 [002] ...1 18154.755589: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO + matchbox-termin-1361 [001] ...1 18155.354594: kmalloc: call_site=ffffffff81614050 ptr=ffff88006db35400 bytes_req=576 bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT + Xorg-1264 [002] ...1 18155.354703: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY + Xorg-1264 [002] ...1 18155.354705: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO + Xorg-1264 [002] ...1 18155.354711: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO + <idle>-0 [000] ..s3 18155.673319: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + dropbear-1465 [000] ...1 18155.673525: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL + <idle>-0 [000] ..s3 18155.674821: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + <idle>-0 [000] ..s3 18155.793014: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + dropbear-1465 [000] ...1 18155.793219: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL + <idle>-0 [000] ..s3 18155.794147: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + <idle>-0 [000] ..s3 18155.936705: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + dropbear-1465 [000] ...1 18155.936910: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL + <idle>-0 [000] ..s3 18155.937869: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + matchbox-termin-1361 [001] ...1 18155.953667: kmalloc: call_site=ffffffff81614050 ptr=ffff88006d5f2000 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_KERNEL|GFP_REPEAT + Xorg-1264 [002] ...1 18155.953775: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY + Xorg-1264 [002] ...1 18155.953777: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO + Xorg-1264 [002] ...1 18155.953783: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO + <idle>-0 [000] ..s3 18156.176053: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + dropbear-1465 [000] ...1 18156.176257: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL + <idle>-0 [000] ..s3 18156.177717: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + <idle>-0 [000] ..s3 18156.399229: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + dropbear-1465 [000] ...1 18156.399434: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_http://rostedt.homelinux.com/kernelshark/req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL + <idle>-0 [000] ..s3 18156.400660: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC + matchbox-termin-1361 [001] ...1 18156.552800: kmalloc: call_site=ffffffff81614050 ptr=ffff88006db34800 bytes_req=576 bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT + +To again disable the kmalloc event, we need to send 0 to the enable file: :: + + root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# echo 0 > enable + +You can enable any number of events or complete subsystems (by +using the 'enable' file in the subsystem directory) and get an +arbitrarily fine-grained idea of what's going on in the system by +enabling as many of the appropriate tracepoints as applicable. + +A number of the tools described in this HOWTO do just that, including +trace-cmd and kernelshark in the next section. + +.. admonition:: Tying it Together + + These tracepoints and their representation are used not only by + ftrace, but by many of the other tools covered in this document and + they form a central point of integration for the various tracers + available in Linux. They form a central part of the instrumentation + for the following tools: perf, lttng, ftrace, blktrace and SystemTap + +.. admonition:: Tying it Together + + Eventually all the special-purpose tracers currently available in + /sys/kernel/debug/tracing will be removed and replaced with + equivalent tracers based on the 'trace events' subsystem. + +trace-cmd/kernelshark +--------------------- + +trace-cmd is essentially an extensive command-line 'wrapper' interface +that hides the details of all the individual files in +/sys/kernel/debug/tracing, allowing users to specify specific particular +events within the /sys/kernel/debug/tracing/events/ subdirectory and to +collect traces and avoid having to deal with those details directly. + +As yet another layer on top of that, kernelshark provides a GUI that +allows users to start and stop traces and specify sets of events using +an intuitive interface, and view the output as both trace events and as +a per-CPU graphical display. It directly uses 'trace-cmd' as the +plumbing that accomplishes all that underneath the covers (and actually +displays the trace-cmd command it uses, as we'll see). + +To start a trace using kernelshark, first start kernelshark: :: + + root@sugarbay:~# kernelshark + +Then bring up the 'Capture' dialog by +choosing from the kernelshark menu: :: + + Capture | Record + +That will display the following dialog, which allows you to choose one or more +events (or even one or more complete subsystems) to trace: + +.. image:: figures/kernelshark-choose-events.png + :align: center + +Note that these are exactly the same sets of events described in the +previous trace events subsystem section, and in fact is where trace-cmd +gets them for kernelshark. + +In the above screenshot, we've decided to explore the graphics subsystem +a bit and so have chosen to trace all the tracepoints contained within +the 'i915' and 'drm' subsystems. + +After doing that, we can start and stop the trace using the 'Run' and +'Stop' button on the lower right corner of the dialog (the same button +will turn into the 'Stop' button after the trace has started): + +.. image:: figures/kernelshark-output-display.png + :align: center + +Notice that the right-hand pane shows the exact trace-cmd command-line +that's used to run the trace, along with the results of the trace-cmd +run. + +Once the 'Stop' button is pressed, the graphical view magically fills up +with a colorful per-cpu display of the trace data, along with the +detailed event listing below that: + +.. image:: figures/kernelshark-i915-display.png + :align: center + +Here's another example, this time a display resulting from tracing 'all +events': + +.. image:: figures/kernelshark-all.png + :align: center + +The tool is pretty self-explanatory, but for more detailed information +on navigating through the data, see the `kernelshark +website <http://rostedt.homelinux.com/kernelshark/>`__. + +ftrace Documentation +-------------------- + +The documentation for ftrace can be found in the kernel Documentation +directory: :: + + Documentation/trace/ftrace.txt + +The documentation for the trace event subsystem can also be found in the kernel +Documentation directory: :: + + Documentation/trace/events.txt + +There is a nice series of articles on using ftrace and trace-cmd at LWN: + +- `Debugging the kernel using Ftrace - part + 1 <http://lwn.net/Articles/365835/>`__ + +- `Debugging the kernel using Ftrace - part + 2 <http://lwn.net/Articles/366796/>`__ + +- `Secrets of the Ftrace function + tracer <http://lwn.net/Articles/370423/>`__ + +- `trace-cmd: A front-end for + Ftrace <https://lwn.net/Articles/410200/>`__ + +There's more detailed documentation kernelshark usage here: +`KernelShark <http://rostedt.homelinux.com/kernelshark/>`__ + +An amusing yet useful README (a tracing mini-HOWTO) can be found in +``/sys/kernel/debug/tracing/README``. + +systemtap +========= + +SystemTap is a system-wide script-based tracing and profiling tool. + +SystemTap scripts are C-like programs that are executed in the kernel to +gather/print/aggregate data extracted from the context they end up being +invoked under. + +For example, this probe from the `SystemTap +tutorial <http://sourceware.org/systemtap/tutorial/>`__ simply prints a +line every time any process on the system open()s a file. For each line, +it prints the executable name of the program that opened the file, along +with its PID, and the name of the file it opened (or tried to open), +which it extracts from the open syscall's argstr. + +.. code-block:: none + + probe syscall.open + { + printf ("%s(%d) open (%s)\n", execname(), pid(), argstr) + } + + probe timer.ms(4000) # after 4 seconds + { + exit () + } + +Normally, to execute this +probe, you'd simply install systemtap on the system you want to probe, +and directly run the probe on that system e.g. assuming the name of the +file containing the above text is trace_open.stp: :: + + # stap trace_open.stp + +What systemtap does under the covers to run this probe is 1) parse and +convert the probe to an equivalent 'C' form, 2) compile the 'C' form +into a kernel module, 3) insert the module into the kernel, which arms +it, and 4) collect the data generated by the probe and display it to the +user. + +In order to accomplish steps 1 and 2, the 'stap' program needs access to +the kernel build system that produced the kernel that the probed system +is running. In the case of a typical embedded system (the 'target'), the +kernel build system unfortunately isn't typically part of the image +running on the target. It is normally available on the 'host' system +that produced the target image however; in such cases, steps 1 and 2 are +executed on the host system, and steps 3 and 4 are executed on the +target system, using only the systemtap 'runtime'. + +The systemtap support in Yocto assumes that only steps 3 and 4 are run +on the target; it is possible to do everything on the target, but this +section assumes only the typical embedded use-case. + +So basically what you need to do in order to run a systemtap script on +the target is to 1) on the host system, compile the probe into a kernel +module that makes sense to the target, 2) copy the module onto the +target system and 3) insert the module into the target kernel, which +arms it, and 4) collect the data generated by the probe and display it +to the user. + +systemtap Setup +--------------- + +Those are a lot of steps and a lot of details, but fortunately Yocto +includes a script called 'crosstap' that will take care of those +details, allowing you to simply execute a systemtap script on the remote +target, with arguments if necessary. + +In order to do this from a remote host, however, you need to have access +to the build for the image you booted. The 'crosstap' script provides +details on how to do this if you run the script on the host without +having done a build: :: + + $ crosstap root@192.168.1.88 trace_open.stp + + Error: No target kernel build found. + Did you forget to create a local build of your image? + + 'crosstap' requires a local sdk build of the target system + (or a build that includes 'tools-profile') in order to build + kernel modules that can probe the target system. + + Practically speaking, that means you need to do the following: + - If you're running a pre-built image, download the release + and/or BSP tarballs used to build the image. + - If you're working from git sources, just clone the metadata + and BSP layers needed to build the image you'll be booting. + - Make sure you're properly set up to build a new image (see + the BSP README and/or the widely available basic documentation + that discusses how to build images). + - Build an -sdk version of the image e.g.: + $ bitbake core-image-sato-sdk + OR + - Build a non-sdk image but include the profiling tools: + [ edit local.conf and add 'tools-profile' to the end of + the EXTRA_IMAGE_FEATURES variable ] + $ bitbake core-image-sato + + Once you've build the image on the host system, you're ready to + boot it (or the equivalent pre-built image) and use 'crosstap' + to probe it (you need to source the environment as usual first): + + $ source oe-init-build-env + $ cd ~/my/systemtap/scripts + $ crosstap root@192.168.1.xxx myscript.stp + +.. note:: + + SystemTap, which uses 'crosstap', assumes you can establish an ssh + connection to the remote target. Please refer to the crosstap wiki + page for details on verifying ssh connections at + . Also, the ability to ssh into the target system is not enabled by + default in \*-minimal images. + +So essentially what you need to +do is build an SDK image or image with 'tools-profile' as detailed in +the ":ref:`profile-manual/intro:General Setup`" section of this +manual, and boot the resulting target image. + +.. note:: + + If you have a build directory containing multiple machines, you need + to have the MACHINE you're connecting to selected in local.conf, and + the kernel in that machine's build directory must match the kernel on + the booted system exactly, or you'll get the above 'crosstap' message + when you try to invoke a script. + +Running a Script on a Target +---------------------------- + +Once you've done that, you should be able to run a systemtap script on +the target: :: + + $ cd /path/to/yocto + $ source oe-init-build-env + + ### Shell environment set up for builds. ### + + You can now run 'bitbake <target>' + + Common targets are: + core-image-minimal + core-image-sato + meta-toolchain + meta-ide-support + + You can also run generated qemu images with a command like 'runqemu qemux86-64' + +Once you've done that, you can cd to whatever +directory contains your scripts and use 'crosstap' to run the script: :: + + $ cd /path/to/my/systemap/script + $ crosstap root@192.168.7.2 trace_open.stp + +If you get an error connecting to the target e.g.: :: + + $ crosstap root@192.168.7.2 trace_open.stp + error establishing ssh connection on remote 'root@192.168.7.2' + +Try ssh'ing to the target and see what happens: :: + + $ ssh root@192.168.7.2 + +A lot of the time, connection +problems are due specifying a wrong IP address or having a 'host key +verification error'. + +If everything worked as planned, you should see something like this +(enter the password when prompted, or press enter if it's set up to use +no password): + +.. code-block:: none + + $ crosstap root@192.168.7.2 trace_open.stp + root@192.168.7.2's password: + matchbox-termin(1036) open ("/tmp/vte3FS2LW", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE, 0600) + matchbox-termin(1036) open ("/tmp/vteJMC7LW", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE, 0600) + +systemtap Documentation +----------------------- + +The SystemTap language reference can be found here: `SystemTap Language +Reference <http://sourceware.org/systemtap/langref/>`__ + +Links to other SystemTap documents, tutorials, and examples can be found +here: `SystemTap documentation +page <http://sourceware.org/systemtap/documentation.html>`__ + +Sysprof +======= + +Sysprof is a very easy to use system-wide profiler that consists of a +single window with three panes and a few buttons which allow you to +start, stop, and view the profile from one place. + +Sysprof Setup +------------- + +For this section, we'll assume you've already performed the basic setup +outlined in the ":ref:`profile-manual/intro:General Setup`" section. + +Sysprof is a GUI-based application that runs on the target system. For +the rest of this document we assume you've ssh'ed to the host and will +be running Sysprof on the target (you can use the '-X' option to ssh and +have the Sysprof GUI run on the target but display remotely on the host +if you want). + +Basic Sysprof Usage +------------------- + +To start profiling the system, you simply press the 'Start' button. To +stop profiling and to start viewing the profile data in one easy step, +press the 'Profile' button. + +Once you've pressed the profile button, the three panes will fill up +with profiling data: + +.. image:: figures/sysprof-copy-to-user.png + :align: center + +The left pane shows a list of functions and processes. Selecting one of +those expands that function in the right pane, showing all its callees. +Note that this caller-oriented display is essentially the inverse of +perf's default callee-oriented callchain display. + +In the screenshot above, we're focusing on ``__copy_to_user_ll()`` and +looking up the callchain we can see that one of the callers of +``__copy_to_user_ll`` is sys_read() and the complete callpath between them. +Notice that this is essentially a portion of the same information we saw +in the perf display shown in the perf section of this page. + +.. image:: figures/sysprof-copy-from-user.png + :align: center + +Similarly, the above is a snapshot of the Sysprof display of a +copy-from-user callchain. + +Finally, looking at the third Sysprof pane in the lower left, we can see +a list of all the callers of a particular function selected in the top +left pane. In this case, the lower pane is showing all the callers of +``__mark_inode_dirty``: + +.. image:: figures/sysprof-callers.png + :align: center + +Double-clicking on one of those functions will in turn change the focus +to the selected function, and so on. + +.. admonition:: Tying it Together + + If you like sysprof's 'caller-oriented' display, you may be able to + approximate it in other tools as well. For example, 'perf report' has + the -g (--call-graph) option that you can experiment with; one of the + options is 'caller' for an inverted caller-based callgraph display. + +Sysprof Documentation +--------------------- + +There doesn't seem to be any documentation for Sysprof, but maybe that's +because it's pretty self-explanatory. The Sysprof website, however, is +here: `Sysprof, System-wide Performance Profiler for +Linux <http://sysprof.com/>`__ + +LTTng (Linux Trace Toolkit, next generation) +============================================ + +LTTng Setup +----------- + +For this section, we'll assume you've already performed the basic setup +outlined in the ":ref:`profile-manual/intro:General Setup`" section. +LTTng is run on the target system by ssh'ing to it. + +Collecting and Viewing Traces +----------------------------- + +Once you've applied the above commits and built and booted your image +(you need to build the core-image-sato-sdk image or use one of the other +methods described in the ":ref:`profile-manual/intro:General Setup`" section), you're ready to start +tracing. + +Collecting and viewing a trace on the target (inside a shell) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +First, from the host, ssh to the target: :: + + $ ssh -l root 192.168.1.47 + The authenticity of host '192.168.1.47 (192.168.1.47)' can't be established. + RSA key fingerprint is 23:bd:c8:b1:a8:71:52:00:ee:00:4f:64:9e:10:b9:7e. + Are you sure you want to continue connecting (yes/no)? yes + Warning: Permanently added '192.168.1.47' (RSA) to the list of known hosts. + root@192.168.1.47's password: + +Once on the target, use these steps to create a trace: :: + + root@crownbay:~# lttng create + Spawning a session daemon + Session auto-20121015-232120 created. + Traces will be written in /home/root/lttng-traces/auto-20121015-232120 + +Enable the events you want to trace (in this case all kernel events): :: + + root@crownbay:~# lttng enable-event --kernel --all + All kernel events are enabled in channel channel0 + +Start the trace: :: + + root@crownbay:~# lttng start + Tracing started for session auto-20121015-232120 + +And then stop the trace after awhile or after running a particular workload that +you want to trace: :: + + root@crownbay:~# lttng stop + Tracing stopped for session auto-20121015-232120 + +You can now view the trace in text form on the target: :: + + root@crownbay:~# lttng view + [23:21:56.989270399] (+?.?????????) sys_geteuid: { 1 }, { } + [23:21:56.989278081] (+0.000007682) exit_syscall: { 1 }, { ret = 0 } + [23:21:56.989286043] (+0.000007962) sys_pipe: { 1 }, { fildes = 0xB77B9E8C } + [23:21:56.989321802] (+0.000035759) exit_syscall: { 1 }, { ret = 0 } + [23:21:56.989329345] (+0.000007543) sys_mmap_pgoff: { 1 }, { addr = 0x0, len = 10485760, prot = 3, flags = 131362, fd = 4294967295, pgoff = 0 } + [23:21:56.989351694] (+0.000022349) exit_syscall: { 1 }, { ret = -1247805440 } + [23:21:56.989432989] (+0.000081295) sys_clone: { 1 }, { clone_flags = 0x411, newsp = 0xB5EFFFE4, parent_tid = 0xFFFFFFFF, child_tid = 0x0 } + [23:21:56.989477129] (+0.000044140) sched_stat_runtime: { 1 }, { comm = "lttng-consumerd", tid = 1193, runtime = 681660, vruntime = 43367983388 } + [23:21:56.989486697] (+0.000009568) sched_migrate_task: { 1 }, { comm = "lttng-consumerd", tid = 1193, prio = 20, orig_cpu = 1, dest_cpu = 1 } + [23:21:56.989508418] (+0.000021721) hrtimer_init: { 1 }, { hrtimer = 3970832076, clockid = 1, mode = 1 } + [23:21:56.989770462] (+0.000262044) hrtimer_cancel: { 1 }, { hrtimer = 3993865440 } + [23:21:56.989771580] (+0.000001118) hrtimer_cancel: { 0 }, { hrtimer = 3993812192 } + [23:21:56.989776957] (+0.000005377) hrtimer_expire_entry: { 1 }, { hrtimer = 3993865440, now = 79815980007057, function = 3238465232 } + [23:21:56.989778145] (+0.000001188) hrtimer_expire_entry: { 0 }, { hrtimer = 3993812192, now = 79815980008174, function = 3238465232 } + [23:21:56.989791695] (+0.000013550) softirq_raise: { 1 }, { vec = 1 } + [23:21:56.989795396] (+0.000003701) softirq_raise: { 0 }, { vec = 1 } + [23:21:56.989800635] (+0.000005239) softirq_raise: { 0 }, { vec = 9 } + [23:21:56.989807130] (+0.000006495) sched_stat_runtime: { 1 }, { comm = "lttng-consumerd", tid = 1193, runtime = 330710, vruntime = 43368314098 } + [23:21:56.989809993] (+0.000002863) sched_stat_runtime: { 0 }, { comm = "lttng-sessiond", tid = 1181, runtime = 1015313, vruntime = 36976733240 } + [23:21:56.989818514] (+0.000008521) hrtimer_expire_exit: { 0 }, { hrtimer = 3993812192 } + [23:21:56.989819631] (+0.000001117) hrtimer_expire_exit: { 1 }, { hrtimer = 3993865440 } + [23:21:56.989821866] (+0.000002235) hrtimer_start: { 0 }, { hrtimer = 3993812192, function = 3238465232, expires = 79815981000000, softexpires = 79815981000000 } + [23:21:56.989822984] (+0.000001118) hrtimer_start: { 1 }, { hrtimer = 3993865440, function = 3238465232, expires = 79815981000000, softexpires = 79815981000000 } + [23:21:56.989832762] (+0.000009778) softirq_entry: { 1 }, { vec = 1 } + [23:21:56.989833879] (+0.000001117) softirq_entry: { 0 }, { vec = 1 } + [23:21:56.989838069] (+0.000004190) timer_cancel: { 1 }, { timer = 3993871956 } + [23:21:56.989839187] (+0.000001118) timer_cancel: { 0 }, { timer = 3993818708 } + [23:21:56.989841492] (+0.000002305) timer_expire_entry: { 1 }, { timer = 3993871956, now = 79515980, function = 3238277552 } + [23:21:56.989842819] (+0.000001327) timer_expire_entry: { 0 }, { timer = 3993818708, now = 79515980, function = 3238277552 } + [23:21:56.989854831] (+0.000012012) sched_stat_runtime: { 1 }, { comm = "lttng-consumerd", tid = 1193, runtime = 49237, vruntime = 43368363335 } + [23:21:56.989855949] (+0.000001118) sched_stat_runtime: { 0 }, { comm = "lttng-sessiond", tid = 1181, runtime = 45121, vruntime = 36976778361 } + [23:21:56.989861257] (+0.000005308) sched_stat_sleep: { 1 }, { comm = "kworker/1:1", tid = 21, delay = 9451318 } + [23:21:56.989862374] (+0.000001117) sched_stat_sleep: { 0 }, { comm = "kworker/0:0", tid = 4, delay = 9958820 } + [23:21:56.989868241] (+0.000005867) sched_wakeup: { 0 }, { comm = "kworker/0:0", tid = 4, prio = 120, success = 1, target_cpu = 0 } + [23:21:56.989869358] (+0.000001117) sched_wakeup: { 1 }, { comm = "kworker/1:1", tid = 21, prio = 120, success = 1, target_cpu = 1 } + [23:21:56.989877460] (+0.000008102) timer_expire_exit: { 1 }, { timer = 3993871956 } + [23:21:56.989878577] (+0.000001117) timer_expire_exit: { 0 }, { timer = 3993818708 } + . + . + . + +You can now safely destroy the trace +session (note that this doesn't delete the trace - it's still there in +~/lttng-traces): :: + + root@crownbay:~# lttng destroy + Session auto-20121015-232120 destroyed at /home/root + +Note that the trace is saved in a directory of the same name as returned by +'lttng create', under the ~/lttng-traces directory (note that you can change this by +supplying your own name to 'lttng create'): :: + + root@crownbay:~# ls -al ~/lttng-traces + drwxrwx--- 3 root root 1024 Oct 15 23:21 . + drwxr-xr-x 5 root root 1024 Oct 15 23:57 .. + drwxrwx--- 3 root root 1024 Oct 15 23:21 auto-20121015-232120 + +Collecting and viewing a userspace trace on the target (inside a shell) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +For LTTng userspace tracing, you need to have a properly instrumented +userspace program. For this example, we'll use the 'hello' test program +generated by the lttng-ust build. + +The 'hello' test program isn't installed on the rootfs by the lttng-ust +build, so we need to copy it over manually. First cd into the build +directory that contains the hello executable: :: + + $ cd build/tmp/work/core2_32-poky-linux/lttng-ust/2.0.5-r0/git/tests/hello/.libs + +Copy that over to the target machine: :: + + $ scp hello root@192.168.1.20: + +You now have the instrumented lttng 'hello world' test program on the +target, ready to test. + +First, from the host, ssh to the target: :: + + $ ssh -l root 192.168.1.47 + The authenticity of host '192.168.1.47 (192.168.1.47)' can't be established. + RSA key fingerprint is 23:bd:c8:b1:a8:71:52:00:ee:00:4f:64:9e:10:b9:7e. + Are you sure you want to continue connecting (yes/no)? yes + Warning: Permanently added '192.168.1.47' (RSA) to the list of known hosts. + root@192.168.1.47's password: + +Once on the target, use these steps to create a trace: :: + + root@crownbay:~# lttng create + Session auto-20190303-021943 created. + Traces will be written in /home/root/lttng-traces/auto-20190303-021943 + +Enable the events you want to trace (in this case all userspace events): :: + + root@crownbay:~# lttng enable-event --userspace --all + All UST events are enabled in channel channel0 + +Start the trace: :: + + root@crownbay:~# lttng start + Tracing started for session auto-20190303-021943 + +Run the instrumented hello world program: :: + + root@crownbay:~# ./hello + Hello, World! + Tracing... done. + +And then stop the trace after awhile or after running a particular workload +that you want to trace: :: + + root@crownbay:~# lttng stop + Tracing stopped for session auto-20190303-021943 + +You can now view the trace in text form on the target: :: + + root@crownbay:~# lttng view + [02:31:14.906146544] (+?.?????????) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 0, intfield2 = 0x0, longfield = 0, netintfield = 0, netintfieldhex = 0x0, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 } + [02:31:14.906170360] (+0.000023816) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 1, intfield2 = 0x1, longfield = 1, netintfield = 1, netintfieldhex = 0x1, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 } + [02:31:14.906183140] (+0.000012780) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 2, intfield2 = 0x2, longfield = 2, netintfield = 2, netintfieldhex = 0x2, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 } + [02:31:14.906194385] (+0.000011245) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 3, intfield2 = 0x3, longfield = 3, netintfield = 3, netintfieldhex = 0x3, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 } + . + . + . + +You can now safely destroy the trace session (note that this doesn't delete the +trace - it's still there in ~/lttng-traces): :: + + root@crownbay:~# lttng destroy + Session auto-20190303-021943 destroyed at /home/root + +LTTng Documentation +------------------- + +You can find the primary LTTng Documentation on the `LTTng +Documentation <https://lttng.org/docs/>`__ site. The documentation on +this site is appropriate for intermediate to advanced software +developers who are working in a Linux environment and are interested in +efficient software tracing. + +For information on LTTng in general, visit the `LTTng +Project <http://lttng.org/lttng2.0>`__ site. You can find a "Getting +Started" link on this site that takes you to an LTTng Quick Start. + +blktrace +======== + +blktrace is a tool for tracing and reporting low-level disk I/O. +blktrace provides the tracing half of the equation; its output can be +piped into the blkparse program, which renders the data in a +human-readable form and does some basic analysis: + +blktrace Setup +-------------- + +For this section, we'll assume you've already performed the basic setup +outlined in the ":ref:`profile-manual/intro:General Setup`" +section. + +blktrace is an application that runs on the target system. You can run +the entire blktrace and blkparse pipeline on the target, or you can run +blktrace in 'listen' mode on the target and have blktrace and blkparse +collect and analyze the data on the host (see the +":ref:`profile-manual/usage:Using blktrace Remotely`" section +below). For the rest of this section we assume you've ssh'ed to the host and +will be running blkrace on the target. + +Basic blktrace Usage +-------------------- + +To record a trace, simply run the 'blktrace' command, giving it the name +of the block device you want to trace activity on: :: + + root@crownbay:~# blktrace /dev/sdc + +In another shell, execute a workload you want to trace. :: + + root@crownbay:/media/sdc# rm linux-2.6.19.2.tar.bz2; wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2; sync + Connecting to downloads.yoctoproject.org (140.211.169.59:80) + linux-2.6.19.2.tar.b 100% \|*******************************\| 41727k 0:00:00 ETA + +Press Ctrl-C in the blktrace shell to stop the trace. It +will display how many events were logged, along with the per-cpu file +sizes (blktrace records traces in per-cpu kernel buffers and simply +dumps them to userspace for blkparse to merge and sort later). :: + + ^C=== sdc === + CPU 0: 7082 events, 332 KiB data + CPU 1: 1578 events, 74 KiB data + Total: 8660 events (dropped 0), 406 KiB data + +If you examine the files saved to disk, you see multiple files, one per CPU and +with the device name as the first part of the filename: :: + + root@crownbay:~# ls -al + drwxr-xr-x 6 root root 1024 Oct 27 22:39 . + drwxr-sr-x 4 root root 1024 Oct 26 18:24 .. + -rw-r--r-- 1 root root 339938 Oct 27 22:40 sdc.blktrace.0 + -rw-r--r-- 1 root root 75753 Oct 27 22:40 sdc.blktrace.1 + +To view the trace events, simply invoke 'blkparse' in the directory +containing the trace files, giving it the device name that forms the +first part of the filenames: :: + + root@crownbay:~# blkparse sdc + + 8,32 1 1 0.000000000 1225 Q WS 3417048 + 8 [jbd2/sdc-8] + 8,32 1 2 0.000025213 1225 G WS 3417048 + 8 [jbd2/sdc-8] + 8,32 1 3 0.000033384 1225 P N [jbd2/sdc-8] + 8,32 1 4 0.000043301 1225 I WS 3417048 + 8 [jbd2/sdc-8] + 8,32 1 0 0.000057270 0 m N cfq1225 insert_request + 8,32 1 0 0.000064813 0 m N cfq1225 add_to_rr + 8,32 1 5 0.000076336 1225 U N [jbd2/sdc-8] 1 + 8,32 1 0 0.000088559 0 m N cfq workload slice:150 + 8,32 1 0 0.000097359 0 m N cfq1225 set_active wl_prio:0 wl_type:1 + 8,32 1 0 0.000104063 0 m N cfq1225 Not idling. st->count:1 + 8,32 1 0 0.000112584 0 m N cfq1225 fifo= (null) + 8,32 1 0 0.000118730 0 m N cfq1225 dispatch_insert + 8,32 1 0 0.000127390 0 m N cfq1225 dispatched a request + 8,32 1 0 0.000133536 0 m N cfq1225 activate rq, drv=1 + 8,32 1 6 0.000136889 1225 D WS 3417048 + 8 [jbd2/sdc-8] + 8,32 1 7 0.000360381 1225 Q WS 3417056 + 8 [jbd2/sdc-8] + 8,32 1 8 0.000377422 1225 G WS 3417056 + 8 [jbd2/sdc-8] + 8,32 1 9 0.000388876 1225 P N [jbd2/sdc-8] + 8,32 1 10 0.000397886 1225 Q WS 3417064 + 8 [jbd2/sdc-8] + 8,32 1 11 0.000404800 1225 M WS 3417064 + 8 [jbd2/sdc-8] + 8,32 1 12 0.000412343 1225 Q WS 3417072 + 8 [jbd2/sdc-8] + 8,32 1 13 0.000416533 1225 M WS 3417072 + 8 [jbd2/sdc-8] + 8,32 1 14 0.000422121 1225 Q WS 3417080 + 8 [jbd2/sdc-8] + 8,32 1 15 0.000425194 1225 M WS 3417080 + 8 [jbd2/sdc-8] + 8,32 1 16 0.000431968 1225 Q WS 3417088 + 8 [jbd2/sdc-8] + 8,32 1 17 0.000435251 1225 M WS 3417088 + 8 [jbd2/sdc-8] + 8,32 1 18 0.000440279 1225 Q WS 3417096 + 8 [jbd2/sdc-8] + 8,32 1 19 0.000443911 1225 M WS 3417096 + 8 [jbd2/sdc-8] + 8,32 1 20 0.000450336 1225 Q WS 3417104 + 8 [jbd2/sdc-8] + 8,32 1 21 0.000454038 1225 M WS 3417104 + 8 [jbd2/sdc-8] + 8,32 1 22 0.000462070 1225 Q WS 3417112 + 8 [jbd2/sdc-8] + 8,32 1 23 0.000465422 1225 M WS 3417112 + 8 [jbd2/sdc-8] + 8,32 1 24 0.000474222 1225 I WS 3417056 + 64 [jbd2/sdc-8] + 8,32 1 0 0.000483022 0 m N cfq1225 insert_request + 8,32 1 25 0.000489727 1225 U N [jbd2/sdc-8] 1 + 8,32 1 0 0.000498457 0 m N cfq1225 Not idling. st->count:1 + 8,32 1 0 0.000503765 0 m N cfq1225 dispatch_insert + 8,32 1 0 0.000512914 0 m N cfq1225 dispatched a request + 8,32 1 0 0.000518851 0 m N cfq1225 activate rq, drv=2 + . + . + . + 8,32 0 0 58.515006138 0 m N cfq3551 complete rqnoidle 1 + 8,32 0 2024 58.516603269 3 C WS 3156992 + 16 [0] + 8,32 0 0 58.516626736 0 m N cfq3551 complete rqnoidle 1 + 8,32 0 0 58.516634558 0 m N cfq3551 arm_idle: 8 group_idle: 0 + 8,32 0 0 58.516636933 0 m N cfq schedule dispatch + 8,32 1 0 58.516971613 0 m N cfq3551 slice expired t=0 + 8,32 1 0 58.516982089 0 m N cfq3551 sl_used=13 disp=6 charge=13 iops=0 sect=80 + 8,32 1 0 58.516985511 0 m N cfq3551 del_from_rr + 8,32 1 0 58.516990819 0 m N cfq3551 put_queue + + CPU0 (sdc): + Reads Queued: 0, 0KiB Writes Queued: 331, 26,284KiB + Read Dispatches: 0, 0KiB Write Dispatches: 485, 40,484KiB + Reads Requeued: 0 Writes Requeued: 0 + Reads Completed: 0, 0KiB Writes Completed: 511, 41,000KiB + Read Merges: 0, 0KiB Write Merges: 13, 160KiB + Read depth: 0 Write depth: 2 + IO unplugs: 23 Timer unplugs: 0 + CPU1 (sdc): + Reads Queued: 0, 0KiB Writes Queued: 249, 15,800KiB + Read Dispatches: 0, 0KiB Write Dispatches: 42, 1,600KiB + Reads Requeued: 0 Writes Requeued: 0 + Reads Completed: 0, 0KiB Writes Completed: 16, 1,084KiB + Read Merges: 0, 0KiB Write Merges: 40, 276KiB + Read depth: 0 Write depth: 2 + IO unplugs: 30 Timer unplugs: 1 + + Total (sdc): + Reads Queued: 0, 0KiB Writes Queued: 580, 42,084KiB + Read Dispatches: 0, 0KiB Write Dispatches: 527, 42,084KiB + Reads Requeued: 0 Writes Requeued: 0 + Reads Completed: 0, 0KiB Writes Completed: 527, 42,084KiB + Read Merges: 0, 0KiB Write Merges: 53, 436KiB + IO unplugs: 53 Timer unplugs: 1 + + Throughput (R/W): 0KiB/s / 719KiB/s + Events (sdc): 6,592 entries + Skips: 0 forward (0 - 0.0%) + Input file sdc.blktrace.0 added + Input file sdc.blktrace.1 added + +The report shows each event that was +found in the blktrace data, along with a summary of the overall block +I/O traffic during the run. You can look at the +`blkparse <http://linux.die.net/man/1/blkparse>`__ manpage to learn the +meaning of each field displayed in the trace listing. + +Live Mode +~~~~~~~~~ + +blktrace and blkparse are designed from the ground up to be able to +operate together in a 'pipe mode' where the stdout of blktrace can be +fed directly into the stdin of blkparse: :: + + root@crownbay:~# blktrace /dev/sdc -o - | blkparse -i - + +This enables long-lived tracing sessions +to run without writing anything to disk, and allows the user to look for +certain conditions in the trace data in 'real-time' by viewing the trace +output as it scrolls by on the screen or by passing it along to yet +another program in the pipeline such as grep which can be used to +identify and capture conditions of interest. + +There's actually another blktrace command that implements the above +pipeline as a single command, so the user doesn't have to bother typing +in the above command sequence: :: + + root@crownbay:~# btrace /dev/sdc + +Using blktrace Remotely +~~~~~~~~~~~~~~~~~~~~~~~ + +Because blktrace traces block I/O and at the same time normally writes +its trace data to a block device, and in general because it's not really +a great idea to make the device being traced the same as the device the +tracer writes to, blktrace provides a way to trace without perturbing +the traced device at all by providing native support for sending all +trace data over the network. + +To have blktrace operate in this mode, start blktrace on the target +system being traced with the -l option, along with the device to trace: :: + + root@crownbay:~# blktrace -l /dev/sdc + server: waiting for connections... + +On the host system, use the -h option to connect to the target system, +also passing it the device to trace: :: + + $ blktrace -d /dev/sdc -h 192.168.1.43 + blktrace: connecting to 192.168.1.43 + blktrace: connected! + +On the target system, you should see this: :: + + server: connection from 192.168.1.43 + +In another shell, execute a workload you want to trace. :: + + root@crownbay:/media/sdc# rm linux-2.6.19.2.tar.bz2; wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2; sync + Connecting to downloads.yoctoproject.org (140.211.169.59:80) + linux-2.6.19.2.tar.b 100% \|*******************************\| 41727k 0:00:00 ETA + +When it's done, do a Ctrl-C on the host system to stop the +trace: :: + + ^C=== sdc === + CPU 0: 7691 events, 361 KiB data + CPU 1: 4109 events, 193 KiB data + Total: 11800 events (dropped 0), 554 KiB data + +On the target system, you should also see a trace summary for the trace +just ended: :: + + server: end of run for 192.168.1.43:sdc + === sdc === + CPU 0: 7691 events, 361 KiB data + CPU 1: 4109 events, 193 KiB data + Total: 11800 events (dropped 0), 554 KiB data + +The blktrace instance on the host will +save the target output inside a hostname-timestamp directory: :: + + $ ls -al + drwxr-xr-x 10 root root 1024 Oct 28 02:40 . + drwxr-sr-x 4 root root 1024 Oct 26 18:24 .. + drwxr-xr-x 2 root root 1024 Oct 28 02:40 192.168.1.43-2012-10-28-02:40:56 + +cd into that directory to see the output files: :: + + $ ls -l + -rw-r--r-- 1 root root 369193 Oct 28 02:44 sdc.blktrace.0 + -rw-r--r-- 1 root root 197278 Oct 28 02:44 sdc.blktrace.1 + +And run blkparse on the host system using the device name: :: + + $ blkparse sdc + + 8,32 1 1 0.000000000 1263 Q RM 6016 + 8 [ls] + 8,32 1 0 0.000036038 0 m N cfq1263 alloced + 8,32 1 2 0.000039390 1263 G RM 6016 + 8 [ls] + 8,32 1 3 0.000049168 1263 I RM 6016 + 8 [ls] + 8,32 1 0 0.000056152 0 m N cfq1263 insert_request + 8,32 1 0 0.000061600 0 m N cfq1263 add_to_rr + 8,32 1 0 0.000075498 0 m N cfq workload slice:300 + . + . + . + 8,32 0 0 177.266385696 0 m N cfq1267 arm_idle: 8 group_idle: 0 + 8,32 0 0 177.266388140 0 m N cfq schedule dispatch + 8,32 1 0 177.266679239 0 m N cfq1267 slice expired t=0 + 8,32 1 0 177.266689297 0 m N cfq1267 sl_used=9 disp=6 charge=9 iops=0 sect=56 + 8,32 1 0 177.266692649 0 m N cfq1267 del_from_rr + 8,32 1 0 177.266696560 0 m N cfq1267 put_queue + + CPU0 (sdc): + Reads Queued: 0, 0KiB Writes Queued: 270, 21,708KiB + Read Dispatches: 59, 2,628KiB Write Dispatches: 495, 39,964KiB + Reads Requeued: 0 Writes Requeued: 0 + Reads Completed: 90, 2,752KiB Writes Completed: 543, 41,596KiB + Read Merges: 0, 0KiB Write Merges: 9, 344KiB + Read depth: 2 Write depth: 2 + IO unplugs: 20 Timer unplugs: 1 + CPU1 (sdc): + Reads Queued: 688, 2,752KiB Writes Queued: 381, 20,652KiB + Read Dispatches: 31, 124KiB Write Dispatches: 59, 2,396KiB + Reads Requeued: 0 Writes Requeued: 0 + Reads Completed: 0, 0KiB Writes Completed: 11, 764KiB + Read Merges: 598, 2,392KiB Write Merges: 88, 448KiB + Read depth: 2 Write depth: 2 + IO unplugs: 52 Timer unplugs: 0 + + Total (sdc): + Reads Queued: 688, 2,752KiB Writes Queued: 651, 42,360KiB + Read Dispatches: 90, 2,752KiB Write Dispatches: 554, 42,360KiB + Reads Requeued: 0 Writes Requeued: 0 + Reads Completed: 90, 2,752KiB Writes Completed: 554, 42,360KiB + Read Merges: 598, 2,392KiB Write Merges: 97, 792KiB + IO unplugs: 72 Timer unplugs: 1 + + Throughput (R/W): 15KiB/s / 238KiB/s + Events (sdc): 9,301 entries + Skips: 0 forward (0 - 0.0%) + +You should see the trace events and summary just as you would have if you'd run +the same command on the target. + +Tracing Block I/O via 'ftrace' +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +It's also possible to trace block I/O using only +:ref:`profile-manual/usage:The 'trace events' Subsystem`, which +can be useful for casual tracing if you don't want to bother dealing with the +userspace tools. + +To enable tracing for a given device, use /sys/block/xxx/trace/enable, +where xxx is the device name. This for example enables tracing for +/dev/sdc: :: + + root@crownbay:/sys/kernel/debug/tracing# echo 1 > /sys/block/sdc/trace/enable + +Once you've selected the device(s) you want +to trace, selecting the 'blk' tracer will turn the blk tracer on: :: + + root@crownbay:/sys/kernel/debug/tracing# cat available_tracers + blk function_graph function nop + + root@crownbay:/sys/kernel/debug/tracing# echo blk > current_tracer + +Execute the workload you're interested in: :: + + root@crownbay:/sys/kernel/debug/tracing# cat /media/sdc/testfile.txt + +And look at the output (note here that we're using 'trace_pipe' instead of +trace to capture this trace - this allows us to wait around on the pipe +for data to appear): :: + + root@crownbay:/sys/kernel/debug/tracing# cat trace_pipe + cat-3587 [001] d..1 3023.276361: 8,32 Q R 1699848 + 8 [cat] + cat-3587 [001] d..1 3023.276410: 8,32 m N cfq3587 alloced + cat-3587 [001] d..1 3023.276415: 8,32 G R 1699848 + 8 [cat] + cat-3587 [001] d..1 3023.276424: 8,32 P N [cat] + cat-3587 [001] d..2 3023.276432: 8,32 I R 1699848 + 8 [cat] + cat-3587 [001] d..1 3023.276439: 8,32 m N cfq3587 insert_request + cat-3587 [001] d..1 3023.276445: 8,32 m N cfq3587 add_to_rr + cat-3587 [001] d..2 3023.276454: 8,32 U N [cat] 1 + cat-3587 [001] d..1 3023.276464: 8,32 m N cfq workload slice:150 + cat-3587 [001] d..1 3023.276471: 8,32 m N cfq3587 set_active wl_prio:0 wl_type:2 + cat-3587 [001] d..1 3023.276478: 8,32 m N cfq3587 fifo= (null) + cat-3587 [001] d..1 3023.276483: 8,32 m N cfq3587 dispatch_insert + cat-3587 [001] d..1 3023.276490: 8,32 m N cfq3587 dispatched a request + cat-3587 [001] d..1 3023.276497: 8,32 m N cfq3587 activate rq, drv=1 + cat-3587 [001] d..2 3023.276500: 8,32 D R 1699848 + 8 [cat] + +And this turns off tracing for the specified device: :: + + root@crownbay:/sys/kernel/debug/tracing# echo 0 > /sys/block/sdc/trace/enable + +blktrace Documentation +---------------------- + +Online versions of the man pages for the commands discussed in this +section can be found here: + +- http://linux.die.net/man/8/blktrace + +- http://linux.die.net/man/1/blkparse + +- http://linux.die.net/man/8/btrace + +The above manpages, along with manpages for the other blktrace utilities +(btt, blkiomon, etc) can be found in the /doc directory of the blktrace +tools git repo: :: + + $ git clone git://git.kernel.dk/blktrace.git |