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