Documentation/filesystems/gfs2-glocks.rst

   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, except for those marked LM_FLAG_PRIORITY which are
  24 used only during recovery, and even then only for journal locks.
  25
  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:
  29
  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 ==========      ====== =====================================================
  38
  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:
  43
  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 ==========      ==========   ==============   ==========   ==============
  52
  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.
  56
  57 Table of glock operations and per type constants:
  58
  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 =============      =============================================================
  76
  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.
  85
  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.
  91
  92 Locking rules for glock operations:
  93
  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 =============    ======================    =============================
 106
 107 .. Note::
 108
 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.
 113
 114 Glock locking order within GFS2:
 115
 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!)
 125
 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.
 132
 133 Glock Statistics
 134 ----------------
 135
 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.
 141
 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.
 150
 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.
 160
 161 The three pairs of mean/variance measure the following
 162 things:
 163
 164  1. DLM lock time (non-blocking requests)
 165  2. DLM lock time (blocking requests)
 166  3. Inter-request time (again to the DLM)
 167
 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.
 175
 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.
 182
 183 So why gather these statistics? There are several reasons
 184 we'd like to get a better idea of these timings:
 185
 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")
 191
 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.
 197
 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.
 204
 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.
 210
 211 Per sb stats can be found here::
 212
 213     /sys/kernel/debug/gfs2/<fsname>/sbstats
 214
 215 Per glock stats can be found here::
 216
 217     /sys/kernel/debug/gfs2/<fsname>/glstats
 218
 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.
 222
 223 The abbreviations used in the output as are follows:
 224
 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 =========  ================================================================
 235
 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.
 240
 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:
 244
 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 ======   =======================================
 250
 251 (remaining fields as per above list)
 252
 253