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| -rw-r--r-- | Documentation/filesystems/gfs2-glocks.txt | 232 |
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diff --git a/Documentation/filesystems/gfs2-glocks.txt b/Documentation/filesystems/gfs2-glocks.txt new file mode 100644 index 000000000..705962363 --- /dev/null +++ b/Documentation/filesystems/gfs2-glocks.txt @@ -0,0 +1,232 @@ + 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) + + |