1 .. SPDX-License-Identifier: GPL-2.0 2 3 ============================ 4 Glock internal locking rules 5 ============================ 6 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: 10 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. 18 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. 24 25 There are three lock states that users of the glock layer can request, 26 namely shared (SH), deferred (DF) and exclusive (EX). Those translate 27 to the following DLM lock modes: 28 29 ========== ====== ===================================================== 30 Glock mode DLM lock mode 31 ========== ====== ===================================================== 32 UN IV/NL Unlocked (no DLM lock associated with glock) or NL 33 SH PR (Protected read) 34 DF CW (Concurrent write) 35 EX EX (Exclusive) 36 ========== ====== ===================================================== 37 38 Thus DF is basically a shared mode which is incompatible with the "normal" 39 shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O 40 operations. The glocks are basically a lock plus some routines which deal 41 with cache management. The following rules apply for the cache: 42 43 ========== ============== ========== ========== ============== 44 Glock mode Cache Metadata Cache data Dirty Data Dirty Metadata 45 ========== ============== ========== ========== ============== 46 UN No No No No 47 DF Yes No No No 48 SH Yes Yes No No 49 EX Yes Yes Yes Yes 50 ========== ============== ========== ========== ============== 51 52 These rules are implemented using the various glock operations which 53 are defined for each type of glock. Not all types of glocks use 54 all the modes. Only inode glocks use the DF mode for example. 55 56 Table of glock operations and per type constants: 57 58 ============== ============================================================= 59 Field Purpose 60 ============== ============================================================= 61 go_sync Called before remote state change (e.g. to sync dirty data) 62 go_xmote_bh Called after remote state change (e.g. to refill cache) 63 go_inval Called if remote state change requires invalidating the cache 64 go_instantiate Called when a glock has been acquired 65 go_held Called every time a glock holder is acquired 66 go_dump Called to print content of object for debugfs file, or on 67 error to dump glock to the log. 68 go_callback Called if the DLM sends a callback to drop this lock 69 go_unlocked Called when a glock is unlocked (dlm_unlock()) 70 go_type The type of the glock, ``LM_TYPE_*`` 71 go_flags GLOF_ASPACE is set, if the glock has an address space 72 associated with it 73 ============== ============================================================= 74 75 The minimum hold time for each lock is the time after a remote lock 76 grant for which we ignore remote demote requests. This is in order to 77 prevent a situation where locks are being bounced around the cluster 78 from node to node with none of the nodes making any progress. This 79 tends to show up most with shared mmapped files which are being written 80 to by multiple nodes. By delaying the demotion in response to a 81 remote callback, that gives the userspace program time to make 82 some progress before the pages are unmapped. 83 84 Eventually, we hope to make the glock "EX" mode locally shared such that any 85 local locking will be done with the i_mutex as required rather than via the 86 glock. 87 88 Locking rules for glock operations: 89 90 ============== ====================== ============================= 91 Operation GLF_LOCK bit lock held gl_lockref.lock spinlock held 92 ============== ====================== ============================= 93 go_sync Yes No 94 go_xmote_bh Yes No 95 go_inval Yes No 96 go_instantiate No No 97 go_held No No 98 go_dump Sometimes Yes 99 go_callback Sometimes (N/A) Yes 100 go_unlocked Yes No 101 ============== ====================== ============================= 102 103 .. Note:: 104 105 Operations must not drop either the bit lock or the spinlock 106 if its held on entry. go_dump and do_demote_ok must never block. 107 Note that go_dump will only be called if the glock's state 108 indicates that it is caching uptodate data. 109 110 Glock locking order within GFS2: 111 112 1. i_rwsem (if required) 113 2. Rename glock (for rename only) 114 3. Inode glock(s) 115 (Parents before children, inodes at "same level" with same parent in 116 lock number order) 117 4. Rgrp glock(s) (for (de)allocation operations) 118 5. Transaction glock (via gfs2_trans_begin) for non-read operations 119 6. i_rw_mutex (if required) 120 7. Page lock (always last, very important!) 121 122 There are two glocks per inode. One deals with access to the inode 123 itself (locking order as above), and the other, known as the iopen 124 glock is used in conjunction with the i_nlink field in the inode to 125 determine the lifetime of the inode in question. Locking of inodes 126 is on a per-inode basis. Locking of rgrps is on a per rgrp basis. 127 In general we prefer to lock local locks prior to cluster locks. 128 129 Glock Statistics 130 ---------------- 131 132 The stats are divided into two sets: those relating to the 133 super block and those relating to an individual glock. The 134 super block stats are done on a per cpu basis in order to 135 try and reduce the overhead of gathering them. They are also 136 further divided by glock type. All timings are in nanoseconds. 137 138 In the case of both the super block and glock statistics, 139 the same information is gathered in each case. The super 140 block timing statistics are used to provide default values for 141 the glock timing statistics, so that newly created glocks 142 should have, as far as possible, a sensible starting point. 143 The per-glock counters are initialised to zero when the 144 glock is created. The per-glock statistics are lost when 145 the glock is ejected from memory. 146 147 The statistics are divided into three pairs of mean and 148 variance, plus two counters. The mean/variance pairs are 149 smoothed exponential estimates and the algorithm used is 150 one which will be very familiar to those used to calculation 151 of round trip times in network code. See "TCP/IP Illustrated, 152 Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement", 153 p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards. 154 Unlike the TCP/IP Illustrated case, the mean and variance are 155 not scaled, but are in units of integer nanoseconds. 156 157 The three pairs of mean/variance measure the following 158 things: 159 160 1. DLM lock time (non-blocking requests) 161 2. DLM lock time (blocking requests) 162 3. Inter-request time (again to the DLM) 163 164 A non-blocking request is one which will complete right 165 away, whatever the state of the DLM lock in question. That 166 currently means any requests when (a) the current state of 167 the lock is exclusive, i.e. a lock demotion (b) the requested 168 state is either null or unlocked (again, a demotion) or (c) the 169 "try lock" flag is set. A blocking request covers all the other 170 lock requests. 171 172 There are two counters. The first is there primarily to show 173 how many lock requests have been made, and thus how much data 174 has gone into the mean/variance calculations. The other counter 175 is counting queuing of holders at the top layer of the glock 176 code. Hopefully that number will be a lot larger than the number 177 of dlm lock requests issued. 178 179 So why gather these statistics? There are several reasons 180 we'd like to get a better idea of these timings: 181 182 1. To be able to better set the glock "min hold time" 183 2. To spot performance issues more easily 184 3. To improve the algorithm for selecting resource groups for 185 allocation (to base it on lock wait time, rather than blindly 186 using a "try lock") 187 188 Due to the smoothing action of the updates, a step change in 189 some input quantity being sampled will only fully be taken 190 into account after 8 samples (or 4 for the variance) and this 191 needs to be carefully considered when interpreting the 192 results. 193 194 Knowing both the time it takes a lock request to complete and 195 the average time between lock requests for a glock means we 196 can compute the total percentage of the time for which the 197 node is able to use a glock vs. time that the rest of the 198 cluster has its share. That will be very useful when setting 199 the lock min hold time. 200 201 Great care has been taken to ensure that we 202 measure exactly the quantities that we want, as accurately 203 as possible. There are always inaccuracies in any 204 measuring system, but I hope this is as accurate as we 205 can reasonably make it. 206 207 Per sb stats can be found here:: 208 209 /sys/kernel/debug/gfs2/<fsname>/sbstats 210 211 Per glock stats can be found here:: 212 213 /sys/kernel/debug/gfs2/<fsname>/glstats 214 215 Assuming that debugfs is mounted on /sys/kernel/debug and also 216 that <fsname> is replaced with the name of the gfs2 filesystem 217 in question. 218 219 The abbreviations used in the output as are follows: 220 221 ========= ================================================================ 222 srtt Smoothed round trip time for non blocking dlm requests 223 srttvar Variance estimate for srtt 224 srttb Smoothed round trip time for (potentially) blocking dlm requests 225 srttvarb Variance estimate for srttb 226 sirt Smoothed inter request time (for dlm requests) 227 sirtvar Variance estimate for sirt 228 dlm Number of dlm requests made (dcnt in glstats file) 229 queue Number of glock requests queued (qcnt in glstats file) 230 ========= ================================================================ 231 232 The sbstats file contains a set of these stats for each glock type (so 8 lines 233 for each type) and for each cpu (one column per cpu). The glstats file contains 234 a set of these stats for each glock in a similar format to the glocks file, but 235 using the format mean/variance for each of the timing stats. 236 237 The gfs2_glock_lock_time tracepoint prints out the current values of the stats 238 for the glock in question, along with some addition information on each dlm 239 reply that is received: 240 241 ====== ======================================= 242 status The status of the dlm request 243 flags The dlm request flags 244 tdiff The time taken by this specific request 245 ====== ======================================= 246 247 (remaining fields as per above list) 248 249
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.