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Diffstat (limited to 'Documentation/trace/ftrace.rst')
| -rw-r--r-- | Documentation/trace/ftrace.rst | 121 |
1 files changed, 121 insertions, 0 deletions
diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst index 0131df7f5968..f60079259669 100644 --- a/Documentation/trace/ftrace.rst +++ b/Documentation/trace/ftrace.rst @@ -233,6 +233,12 @@ of ftrace. Here is a list of some of the key files: This interface also allows for commands to be used. See the "Filter commands" section for more details. + As a speed up, since processing strings can't be quite expensive + and requires a check of all functions registered to tracing, instead + an index can be written into this file. A number (starting with "1") + written will instead select the same corresponding at the line position + of the "available_filter_functions" file. + set_ftrace_notrace: This has an effect opposite to that of @@ -759,6 +765,37 @@ Here is the list of current tracers that may be configured. tracers from tracing simply echo "nop" into current_tracer. +Error conditions +---------------- + + For most ftrace commands, failure modes are obvious and communicated + using standard return codes. + + For other more involved commands, extended error information may be + available via the tracing/error_log file. For the commands that + support it, reading the tracing/error_log file after an error will + display more detailed information about what went wrong, if + information is available. The tracing/error_log file is a circular + error log displaying a small number (currently, 8) of ftrace errors + for the last (8) failed commands. + + The extended error information and usage takes the form shown in + this example:: + + # echo xxx > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger + echo: write error: Invalid argument + + # cat /sys/kernel/debug/tracing/error_log + [ 5348.887237] location: error: Couldn't yyy: zzz + Command: xxx + ^ + [ 7517.023364] location: error: Bad rrr: sss + Command: ppp qqq + ^ + + To clear the error log, echo the empty string into it:: + + # echo > /sys/kernel/debug/tracing/error_log Examples of using the tracer ---------------------------- @@ -1396,6 +1433,58 @@ enabling function tracing, we incur an added overhead. This overhead may extend the latency times. But nevertheless, this trace has provided some very helpful debugging information. +If we prefer function graph output instead of function, we can set +display-graph option:: + + with echo 1 > options/display-graph + + # tracer: irqsoff + # + # irqsoff latency trace v1.1.5 on 4.20.0-rc6+ + # -------------------------------------------------------------------- + # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4) + # ----------------- + # | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0) + # ----------------- + # => started at: free_debug_processing + # => ended at: return_to_handler + # + # + # _-----=> irqs-off + # / _----=> need-resched + # | / _---=> hardirq/softirq + # || / _--=> preempt-depth + # ||| / + # REL TIME CPU TASK/PID |||| DURATION FUNCTION CALLS + # | | | | |||| | | | | | | + 0 us | 0) bash-1507 | d... | 0.000 us | _raw_spin_lock_irqsave(); + 0 us | 0) bash-1507 | d..1 | 0.378 us | do_raw_spin_trylock(); + 1 us | 0) bash-1507 | d..2 | | set_track() { + 2 us | 0) bash-1507 | d..2 | | save_stack_trace() { + 2 us | 0) bash-1507 | d..2 | | __save_stack_trace() { + 3 us | 0) bash-1507 | d..2 | | __unwind_start() { + 3 us | 0) bash-1507 | d..2 | | get_stack_info() { + 3 us | 0) bash-1507 | d..2 | 0.351 us | in_task_stack(); + 4 us | 0) bash-1507 | d..2 | 1.107 us | } + [...] + 3750 us | 0) bash-1507 | d..1 | 0.516 us | do_raw_spin_unlock(); + 3750 us | 0) bash-1507 | d..1 | 0.000 us | _raw_spin_unlock_irqrestore(); + 3764 us | 0) bash-1507 | d..1 | 0.000 us | tracer_hardirqs_on(); + bash-1507 0d..1 3792us : <stack trace> + => free_debug_processing + => __slab_free + => kmem_cache_free + => vm_area_free + => remove_vma + => exit_mmap + => mmput + => flush_old_exec + => load_elf_binary + => search_binary_handler + => __do_execve_file.isra.32 + => __x64_sys_execve + => do_syscall_64 + => entry_SYSCALL_64_after_hwframe preemptoff ---------- @@ -2784,6 +2873,38 @@ Produces:: We can see that there's no more lock or preempt tracing. +Selecting function filters via index +------------------------------------ + +Because processing of strings is expensive (the address of the function +needs to be looked up before comparing to the string being passed in), +an index can be used as well to enable functions. This is useful in the +case of setting thousands of specific functions at a time. By passing +in a list of numbers, no string processing will occur. Instead, the function +at the specific location in the internal array (which corresponds to the +functions in the "available_filter_functions" file), is selected. + +:: + + # echo 1 > set_ftrace_filter + +Will select the first function listed in "available_filter_functions" + +:: + + # head -1 available_filter_functions + trace_initcall_finish_cb + + # cat set_ftrace_filter + trace_initcall_finish_cb + + # head -50 available_filter_functions | tail -1 + x86_pmu_commit_txn + + # echo 1 50 > set_ftrace_filter + # cat set_ftrace_filter + trace_initcall_finish_cb + x86_pmu_commit_txn Dynamic ftrace with the function graph tracer --------------------------------------------- |