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diff --git a/Documentation/filesystems/gfs2-glocks.txt b/Documentation/filesystems/gfs2-glocks.txt deleted file mode 100644 index 7059623635b2..000000000000 --- a/Documentation/filesystems/gfs2-glocks.txt +++ /dev/null @@ -1,232 +0,0 @@ - Glock internal locking rules - ------------------------------ - -This documents the basic principles of the glock state machine -internals. Each glock (struct gfs2_glock in fs/gfs2/incore.h) -has two main (internal) locks: - - 1. A spinlock (gl_lockref.lock) which protects the internal state such - as gl_state, gl_target and the list of holders (gl_holders) - 2. A non-blocking bit lock, GLF_LOCK, which is used to prevent other - threads from making calls to the DLM, etc. at the same time. If a - thread takes this lock, it must then call run_queue (usually via the - workqueue) when it releases it in order to ensure any pending tasks - are completed. - -The gl_holders list contains all the queued lock requests (not -just the holders) associated with the glock. If there are any -held locks, then they will be contiguous entries at the head -of the list. Locks are granted in strictly the order that they -are queued, except for those marked LM_FLAG_PRIORITY which are -used only during recovery, and even then only for journal locks. - -There are three lock states that users of the glock layer can request, -namely shared (SH), deferred (DF) and exclusive (EX). Those translate -to the following DLM lock modes: - -Glock mode | DLM lock mode ------------------------------- - UN | IV/NL Unlocked (no DLM lock associated with glock) or NL - SH | PR (Protected read) - DF | CW (Concurrent write) - EX | EX (Exclusive) - -Thus DF is basically a shared mode which is incompatible with the "normal" -shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O -operations. The glocks are basically a lock plus some routines which deal -with cache management. The following rules apply for the cache: - -Glock mode | Cache data | Cache Metadata | Dirty Data | Dirty Metadata --------------------------------------------------------------------------- - UN | No | No | No | No - SH | Yes | Yes | No | No - DF | No | Yes | No | No - EX | Yes | Yes | Yes | Yes - -These rules are implemented using the various glock operations which -are defined for each type of glock. Not all types of glocks use -all the modes. Only inode glocks use the DF mode for example. - -Table of glock operations and per type constants: - -Field | Purpose ----------------------------------------------------------------------------- -go_xmote_th | Called before remote state change (e.g. to sync dirty data) -go_xmote_bh | Called after remote state change (e.g. to refill cache) -go_inval | Called if remote state change requires invalidating the cache -go_demote_ok | Returns boolean value of whether its ok to demote a glock - | (e.g. checks timeout, and that there is no cached data) -go_lock | Called for the first local holder of a lock -go_unlock | Called on the final local unlock of a lock -go_dump | Called to print content of object for debugfs file, or on - | error to dump glock to the log. -go_type | The type of the glock, LM_TYPE_..... -go_callback | Called if the DLM sends a callback to drop this lock -go_flags | GLOF_ASPACE is set, if the glock has an address space - | associated with it - -The minimum hold time for each lock is the time after a remote lock -grant for which we ignore remote demote requests. This is in order to -prevent a situation where locks are being bounced around the cluster -from node to node with none of the nodes making any progress. This -tends to show up most with shared mmaped files which are being written -to by multiple nodes. By delaying the demotion in response to a -remote callback, that gives the userspace program time to make -some progress before the pages are unmapped. - -There is a plan to try and remove the go_lock and go_unlock callbacks -if possible, in order to try and speed up the fast path though the locking. -Also, eventually we hope to make the glock "EX" mode locally shared -such that any local locking will be done with the i_mutex as required -rather than via the glock. - -Locking rules for glock operations: - -Operation | GLF_LOCK bit lock held | gl_lockref.lock spinlock held -------------------------------------------------------------------------- -go_xmote_th | Yes | No -go_xmote_bh | Yes | No -go_inval | Yes | No -go_demote_ok | Sometimes | Yes -go_lock | Yes | No -go_unlock | Yes | No -go_dump | Sometimes | Yes -go_callback | Sometimes (N/A) | Yes - -N.B. Operations must not drop either the bit lock or the spinlock -if its held on entry. go_dump and do_demote_ok must never block. -Note that go_dump will only be called if the glock's state -indicates that it is caching uptodate data. - -Glock locking order within GFS2: - - 1. i_rwsem (if required) - 2. Rename glock (for rename only) - 3. Inode glock(s) - (Parents before children, inodes at "same level" with same parent in - lock number order) - 4. Rgrp glock(s) (for (de)allocation operations) - 5. Transaction glock (via gfs2_trans_begin) for non-read operations - 6. i_rw_mutex (if required) - 7. Page lock (always last, very important!) - -There are two glocks per inode. One deals with access to the inode -itself (locking order as above), and the other, known as the iopen -glock is used in conjunction with the i_nlink field in the inode to -determine the lifetime of the inode in question. Locking of inodes -is on a per-inode basis. Locking of rgrps is on a per rgrp basis. -In general we prefer to lock local locks prior to cluster locks. - - Glock Statistics - ------------------ - -The stats are divided into two sets: those relating to the -super block and those relating to an individual glock. The -super block stats are done on a per cpu basis in order to -try and reduce the overhead of gathering them. They are also -further divided by glock type. All timings are in nanoseconds. - -In the case of both the super block and glock statistics, -the same information is gathered in each case. The super -block timing statistics are used to provide default values for -the glock timing statistics, so that newly created glocks -should have, as far as possible, a sensible starting point. -The per-glock counters are initialised to zero when the -glock is created. The per-glock statistics are lost when -the glock is ejected from memory. - -The statistics are divided into three pairs of mean and -variance, plus two counters. The mean/variance pairs are -smoothed exponential estimates and the algorithm used is -one which will be very familiar to those used to calculation -of round trip times in network code. See "TCP/IP Illustrated, -Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement", -p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards. -Unlike the TCP/IP Illustrated case, the mean and variance are -not scaled, but are in units of integer nanoseconds. - -The three pairs of mean/variance measure the following -things: - - 1. DLM lock time (non-blocking requests) - 2. DLM lock time (blocking requests) - 3. Inter-request time (again to the DLM) - -A non-blocking request is one which will complete right -away, whatever the state of the DLM lock in question. That -currently means any requests when (a) the current state of -the lock is exclusive, i.e. a lock demotion (b) the requested -state is either null or unlocked (again, a demotion) or (c) the -"try lock" flag is set. A blocking request covers all the other -lock requests. - -There are two counters. The first is there primarily to show -how many lock requests have been made, and thus how much data -has gone into the mean/variance calculations. The other counter -is counting queuing of holders at the top layer of the glock -code. Hopefully that number will be a lot larger than the number -of dlm lock requests issued. - -So why gather these statistics? There are several reasons -we'd like to get a better idea of these timings: - -1. To be able to better set the glock "min hold time" -2. To spot performance issues more easily -3. To improve the algorithm for selecting resource groups for -allocation (to base it on lock wait time, rather than blindly -using a "try lock") - -Due to the smoothing action of the updates, a step change in -some input quantity being sampled will only fully be taken -into account after 8 samples (or 4 for the variance) and this -needs to be carefully considered when interpreting the -results. - -Knowing both the time it takes a lock request to complete and -the average time between lock requests for a glock means we -can compute the total percentage of the time for which the -node is able to use a glock vs. time that the rest of the -cluster has its share. That will be very useful when setting -the lock min hold time. - -Great care has been taken to ensure that we -measure exactly the quantities that we want, as accurately -as possible. There are always inaccuracies in any -measuring system, but I hope this is as accurate as we -can reasonably make it. - -Per sb stats can be found here: -/sys/kernel/debug/gfs2/<fsname>/sbstats -Per glock stats can be found here: -/sys/kernel/debug/gfs2/<fsname>/glstats - -Assuming that debugfs is mounted on /sys/kernel/debug and also -that <fsname> is replaced with the name of the gfs2 filesystem -in question. - -The abbreviations used in the output as are follows: - -srtt - Smoothed round trip time for non-blocking dlm requests -srttvar - Variance estimate for srtt -srttb - Smoothed round trip time for (potentially) blocking dlm requests -srttvarb - Variance estimate for srttb -sirt - Smoothed inter-request time (for dlm requests) -sirtvar - Variance estimate for sirt -dlm - Number of dlm requests made (dcnt in glstats file) -queue - Number of glock requests queued (qcnt in glstats file) - -The sbstats file contains a set of these stats for each glock type (so 8 lines -for each type) and for each cpu (one column per cpu). The glstats file contains -a set of these stats for each glock in a similar format to the glocks file, but -using the format mean/variance for each of the timing stats. - -The gfs2_glock_lock_time tracepoint prints out the current values of the stats -for the glock in question, along with some addition information on each dlm -reply that is received: - -status - The status of the dlm request -flags - The dlm request flags -tdiff - The time taken by this specific request -(remaining fields as per above list) - - |