1 Glock internal locking rules 2 ------------------------------ 3 4This documents the basic principles of the glock state machine 5internals. Each glock (struct gfs2_glock in fs/gfs2/incore.h) 6has two main (internal) locks: 7 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. 15 16The gl_holders list contains all the queued lock requests (not 17just the holders) associated with the glock. If there are any 18held locks, then they will be contiguous entries at the head 19of the list. Locks are granted in strictly the order that they 20are queued, except for those marked LM_FLAG_PRIORITY which are 21used only during recovery, and even then only for journal locks. 22 23There are three lock states that users of the glock layer can request, 24namely shared (SH), deferred (DF) and exclusive (EX). Those translate 25to the following DLM lock modes: 26 27Glock 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) 33 34Thus DF is basically a shared mode which is incompatible with the "normal" 35shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O 36operations. The glocks are basically a lock plus some routines which deal 37with cache management. The following rules apply for the cache: 38 39Glock 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 45 46These rules are implemented using the various glock operations which 47are defined for each type of glock. Not all types of glocks use 48all the modes. Only inode glocks use the DF mode for example. 49 50Table of glock operations and per type constants: 51 52Field | Purpose 53---------------------------------------------------------------------------- 54go_xmote_th | Called before remote state change (e.g. to sync dirty data) 55go_xmote_bh | Called after remote state change (e.g. to refill cache) 56go_inval | Called if remote state change requires invalidating the cache 57go_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) 59go_lock | Called for the first local holder of a lock 60go_unlock | Called on the final local unlock of a lock 61go_dump | Called to print content of object for debugfs file, or on 62 | error to dump glock to the log. 63go_type | The type of the glock, LM_TYPE_..... 64go_callback | Called if the DLM sends a callback to drop this lock 65go_flags | GLOF_ASPACE is set, if the glock has an address space 66 | associated with it 67 68The minimum hold time for each lock is the time after a remote lock 69grant for which we ignore remote demote requests. This is in order to 70prevent a situation where locks are being bounced around the cluster 71from node to node with none of the nodes making any progress. This 72tends to show up most with shared mmaped files which are being written 73to by multiple nodes. By delaying the demotion in response to a 74remote callback, that gives the userspace program time to make 75some progress before the pages are unmapped. 76 77There is a plan to try and remove the go_lock and go_unlock callbacks 78if possible, in order to try and speed up the fast path though the locking. 79Also, eventually we hope to make the glock "EX" mode locally shared 80such that any local locking will be done with the i_mutex as required 81rather than via the glock. 82 83Locking rules for glock operations: 84 85Operation | GLF_LOCK bit lock held | gl_lockref.lock spinlock held 86------------------------------------------------------------------------- 87go_xmote_th | Yes | No 88go_xmote_bh | Yes | No 89go_inval | Yes | No 90go_demote_ok | Sometimes | Yes 91go_lock | Yes | No 92go_unlock | Yes | No 93go_dump | Sometimes | Yes 94go_callback | Sometimes (N/A) | Yes 95 96N.B. Operations must not drop either the bit lock or the spinlock 97if its held on entry. go_dump and do_demote_ok must never block. 98Note that go_dump will only be called if the glock's state 99indicates that it is caching uptodate data. 100 101Glock locking order within GFS2: 102 103 1. i_rwsem (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. i_rw_mutex (if required) 111 7. Page lock (always last, very important!) 112 113There are two glocks per inode. One deals with access to the inode 114itself (locking order as above), and the other, known as the iopen 115glock is used in conjunction with the i_nlink field in the inode to 116determine the lifetime of the inode in question. Locking of inodes 117is on a per-inode basis. Locking of rgrps is on a per rgrp basis. 118In general we prefer to lock local locks prior to cluster locks. 119 120 Glock Statistics 121 ------------------ 122 123The stats are divided into two sets: those relating to the 124super block and those relating to an individual glock. The 125super block stats are done on a per cpu basis in order to 126try and reduce the overhead of gathering them. They are also 127further divided by glock type. All timings are in nanoseconds. 128 129In the case of both the super block and glock statistics, 130the same information is gathered in each case. The super 131block timing statistics are used to provide default values for 132the glock timing statistics, so that newly created glocks 133should have, as far as possible, a sensible starting point. 134The per-glock counters are initialised to zero when the 135glock is created. The per-glock statistics are lost when 136the glock is ejected from memory. 137 138The statistics are divided into three pairs of mean and 139variance, plus two counters. The mean/variance pairs are 140smoothed exponential estimates and the algorithm used is 141one which will be very familiar to those used to calculation 142of round trip times in network code. See "TCP/IP Illustrated, 143Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement", 144p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards. 145Unlike the TCP/IP Illustrated case, the mean and variance are 146not scaled, but are in units of integer nanoseconds. 147 148The three pairs of mean/variance measure the following 149things: 150 151 1. DLM lock time (non-blocking requests) 152 2. DLM lock time (blocking requests) 153 3. Inter-request time (again to the DLM) 154 155A non-blocking request is one which will complete right 156away, whatever the state of the DLM lock in question. That 157currently means any requests when (a) the current state of 158the lock is exclusive, i.e. a lock demotion (b) the requested 159state is either null or unlocked (again, a demotion) or (c) the 160"try lock" flag is set. A blocking request covers all the other 161lock requests. 162 163There are two counters. The first is there primarily to show 164how many lock requests have been made, and thus how much data 165has gone into the mean/variance calculations. The other counter 166is counting queuing of holders at the top layer of the glock 167code. Hopefully that number will be a lot larger than the number 168of dlm lock requests issued. 169 170So why gather these statistics? There are several reasons 171we'd like to get a better idea of these timings: 172 1731. To be able to better set the glock "min hold time" 1742. To spot performance issues more easily 1753. To improve the algorithm for selecting resource groups for 176allocation (to base it on lock wait time, rather than blindly 177using a "try lock") 178 179Due to the smoothing action of the updates, a step change in 180some input quantity being sampled will only fully be taken 181into account after 8 samples (or 4 for the variance) and this 182needs to be carefully considered when interpreting the 183results. 184 185Knowing both the time it takes a lock request to complete and 186the average time between lock requests for a glock means we 187can compute the total percentage of the time for which the 188node is able to use a glock vs. time that the rest of the 189cluster has its share. That will be very useful when setting 190the lock min hold time. 191 192Great care has been taken to ensure that we 193measure exactly the quantities that we want, as accurately 194as possible. There are always inaccuracies in any 195measuring system, but I hope this is as accurate as we 196can reasonably make it. 197 198Per sb stats can be found here: 199/sys/kernel/debug/gfs2/<fsname>/sbstats 200Per glock stats can be found here: 201/sys/kernel/debug/gfs2/<fsname>/glstats 202 203Assuming that debugfs is mounted on /sys/kernel/debug and also 204that <fsname> is replaced with the name of the gfs2 filesystem 205in question. 206 207The abbreviations used in the output as are follows: 208 209srtt - Smoothed round trip time for non-blocking dlm requests 210srttvar - Variance estimate for srtt 211srttb - Smoothed round trip time for (potentially) blocking dlm requests 212srttvarb - Variance estimate for srttb 213sirt - Smoothed inter-request time (for dlm requests) 214sirtvar - Variance estimate for sirt 215dlm - Number of dlm requests made (dcnt in glstats file) 216queue - Number of glock requests queued (qcnt in glstats file) 217 218The sbstats file contains a set of these stats for each glock type (so 8 lines 219for each type) and for each cpu (one column per cpu). The glstats file contains 220a set of these stats for each glock in a similar format to the glocks file, but 221using the format mean/variance for each of the timing stats. 222 223The gfs2_glock_lock_time tracepoint prints out the current values of the stats 224for the glock in question, along with some addition information on each dlm 225reply that is received: 226 227status - The status of the dlm request 228flags - The dlm request flags 229tdiff - The time taken by this specific request 230(remaining fields as per above list) 231 232 233