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- 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_spin) 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_spin 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_mutex (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. 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)
-
-