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1 .. SPDX-License-Identifier: GPL-2.0
3 ============================
4 Glock internal locking rules
5 ============================
7 This documents the basic principles of the glock state machine
8 internals. Each glock (struct gfs2_glock in fs/gfs2/incore.h)
9 has two main (internal) locks:
11 1. A spinlock (gl_lockref.lock) which protects the internal state such
12 as gl_state, gl_target and the list of holders (gl_holders)
13 2. A non-blocking bit lock, GLF_LOCK, which is used to prevent other
14 threads from making calls to the DLM, etc. at the same time. If a
15 thread takes this lock, it must then call run_queue (usually via the
16 workqueue) when it releases it in order to ensure any pending tasks
17 are completed.
19 The gl_holders list contains all the queued lock requests (not
20 just the holders) associated with the glock. If there are any
21 held locks, then they will be contiguous entries at the head
22 of the list. Locks are granted in strictly the order that they
23 are queued, except for those marked LM_FLAG_PRIORITY which are
24 used only during recovery, and even then only for journal locks.
26 There are three lock states that users of the glock layer can request,
27 namely shared (SH), deferred (DF) and exclusive (EX). Those translate
28 to the following DLM lock modes:
30 ========== ====== =====================================================
31 Glock mode DLM lock mode
32 ========== ====== =====================================================
33 UN IV/NL Unlocked (no DLM lock associated with glock) or NL
34 SH PR (Protected read)
35 DF CW (Concurrent write)
36 EX EX (Exclusive)
37 ========== ====== =====================================================
39 Thus DF is basically a shared mode which is incompatible with the "normal"
40 shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O
41 operations. The glocks are basically a lock plus some routines which deal
42 with cache management. The following rules apply for the cache:
44 ========== ========== ============== ========== ==============
45 Glock mode Cache data Cache Metadata Dirty Data Dirty Metadata
46 ========== ========== ============== ========== ==============
47 UN No No No No
48 SH Yes Yes No No
49 DF No Yes No No
50 EX Yes Yes Yes Yes
51 ========== ========== ============== ========== ==============
53 These rules are implemented using the various glock operations which
54 are defined for each type of glock. Not all types of glocks use
55 all the modes. Only inode glocks use the DF mode for example.
57 Table of glock operations and per type constants:
59 ============= =============================================================
60 Field Purpose
61 ============= =============================================================
62 go_xmote_th Called before remote state change (e.g. to sync dirty data)
63 go_xmote_bh Called after remote state change (e.g. to refill cache)
64 go_inval Called if remote state change requires invalidating the cache
65 go_demote_ok Returns boolean value of whether its ok to demote a glock
66 (e.g. checks timeout, and that there is no cached data)
67 go_lock Called for the first local holder of a lock
68 go_unlock Called on the final local unlock of a lock
69 go_dump Called to print content of object for debugfs file, or on
70 error to dump glock to the log.
71 go_type The type of the glock, ``LM_TYPE_*``
72 go_callback Called if the DLM sends a callback to drop this lock
73 go_flags GLOF_ASPACE is set, if the glock has an address space
74 associated with it
75 ============= =============================================================
77 The minimum hold time for each lock is the time after a remote lock
78 grant for which we ignore remote demote requests. This is in order to
79 prevent a situation where locks are being bounced around the cluster
80 from node to node with none of the nodes making any progress. This
81 tends to show up most with shared mmaped files which are being written
82 to by multiple nodes. By delaying the demotion in response to a
83 remote callback, that gives the userspace program time to make
84 some progress before the pages are unmapped.
86 There is a plan to try and remove the go_lock and go_unlock callbacks
87 if possible, in order to try and speed up the fast path though the locking.
88 Also, eventually we hope to make the glock "EX" mode locally shared
89 such that any local locking will be done with the i_mutex as required
90 rather than via the glock.
92 Locking rules for glock operations:
94 ============= ====================== =============================
95 Operation GLF_LOCK bit lock held gl_lockref.lock spinlock held
96 ============= ====================== =============================
97 go_xmote_th Yes No
98 go_xmote_bh Yes No
99 go_inval Yes No
100 go_demote_ok Sometimes Yes
101 go_lock Yes No
102 go_unlock Yes No
103 go_dump Sometimes Yes
104 go_callback Sometimes (N/A) Yes
105 ============= ====================== =============================
107 .. Note::
109 Operations must not drop either the bit lock or the spinlock
110 if its held on entry. go_dump and do_demote_ok must never block.
111 Note that go_dump will only be called if the glock's state
112 indicates that it is caching uptodate data.
114 Glock locking order within GFS2:
116 1. i_rwsem (if required)
117 2. Rename glock (for rename only)
118 3. Inode glock(s)
119 (Parents before children, inodes at "same level" with same parent in
120 lock number order)
121 4. Rgrp glock(s) (for (de)allocation operations)
122 5. Transaction glock (via gfs2_trans_begin) for non-read operations
123 6. i_rw_mutex (if required)
124 7. Page lock (always last, very important!)
126 There are two glocks per inode. One deals with access to the inode
127 itself (locking order as above), and the other, known as the iopen
128 glock is used in conjunction with the i_nlink field in the inode to
129 determine the lifetime of the inode in question. Locking of inodes
130 is on a per-inode basis. Locking of rgrps is on a per rgrp basis.
131 In general we prefer to lock local locks prior to cluster locks.
133 Glock Statistics
134 ----------------
136 The stats are divided into two sets: those relating to the
137 super block and those relating to an individual glock. The
138 super block stats are done on a per cpu basis in order to
139 try and reduce the overhead of gathering them. They are also
140 further divided by glock type. All timings are in nanoseconds.
142 In the case of both the super block and glock statistics,
143 the same information is gathered in each case. The super
144 block timing statistics are used to provide default values for
145 the glock timing statistics, so that newly created glocks
146 should have, as far as possible, a sensible starting point.
147 The per-glock counters are initialised to zero when the
148 glock is created. The per-glock statistics are lost when
149 the glock is ejected from memory.
151 The statistics are divided into three pairs of mean and
152 variance, plus two counters. The mean/variance pairs are
153 smoothed exponential estimates and the algorithm used is
154 one which will be very familiar to those used to calculation
155 of round trip times in network code. See "TCP/IP Illustrated,
156 Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement",
157 p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards.
158 Unlike the TCP/IP Illustrated case, the mean and variance are
159 not scaled, but are in units of integer nanoseconds.
161 The three pairs of mean/variance measure the following
162 things:
164 1. DLM lock time (non-blocking requests)
165 2. DLM lock time (blocking requests)
166 3. Inter-request time (again to the DLM)
168 A non-blocking request is one which will complete right
169 away, whatever the state of the DLM lock in question. That
170 currently means any requests when (a) the current state of
171 the lock is exclusive, i.e. a lock demotion (b) the requested
172 state is either null or unlocked (again, a demotion) or (c) the
173 "try lock" flag is set. A blocking request covers all the other
174 lock requests.
176 There are two counters. The first is there primarily to show
177 how many lock requests have been made, and thus how much data
178 has gone into the mean/variance calculations. The other counter
179 is counting queuing of holders at the top layer of the glock
180 code. Hopefully that number will be a lot larger than the number
181 of dlm lock requests issued.
183 So why gather these statistics? There are several reasons
184 we'd like to get a better idea of these timings:
186 1. To be able to better set the glock "min hold time"
187 2. To spot performance issues more easily
188 3. To improve the algorithm for selecting resource groups for
189 allocation (to base it on lock wait time, rather than blindly
190 using a "try lock")
192 Due to the smoothing action of the updates, a step change in
193 some input quantity being sampled will only fully be taken
194 into account after 8 samples (or 4 for the variance) and this
195 needs to be carefully considered when interpreting the
196 results.
198 Knowing both the time it takes a lock request to complete and
199 the average time between lock requests for a glock means we
200 can compute the total percentage of the time for which the
201 node is able to use a glock vs. time that the rest of the
202 cluster has its share. That will be very useful when setting
203 the lock min hold time.
205 Great care has been taken to ensure that we
206 measure exactly the quantities that we want, as accurately
207 as possible. There are always inaccuracies in any
208 measuring system, but I hope this is as accurate as we
209 can reasonably make it.
211 Per sb stats can be found here::
213 /sys/kernel/debug/gfs2/<fsname>/sbstats
215 Per glock stats can be found here::
217 /sys/kernel/debug/gfs2/<fsname>/glstats
219 Assuming that debugfs is mounted on /sys/kernel/debug and also
220 that <fsname> is replaced with the name of the gfs2 filesystem
221 in question.
223 The abbreviations used in the output as are follows:
225 ========= ================================================================
226 srtt Smoothed round trip time for non blocking dlm requests
227 srttvar Variance estimate for srtt
228 srttb Smoothed round trip time for (potentially) blocking dlm requests
229 srttvarb Variance estimate for srttb
230 sirt Smoothed inter request time (for dlm requests)
231 sirtvar Variance estimate for sirt
232 dlm Number of dlm requests made (dcnt in glstats file)
233 queue Number of glock requests queued (qcnt in glstats file)
234 ========= ================================================================
236 The sbstats file contains a set of these stats for each glock type (so 8 lines
237 for each type) and for each cpu (one column per cpu). The glstats file contains
238 a set of these stats for each glock in a similar format to the glocks file, but
239 using the format mean/variance for each of the timing stats.
241 The gfs2_glock_lock_time tracepoint prints out the current values of the stats
242 for the glock in question, along with some addition information on each dlm
243 reply that is received:
245 ====== =======================================
246 status The status of the dlm request
247 flags The dlm request flags
248 tdiff The time taken by this specific request
249 ====== =======================================
251 (remaining fields as per above list)