linux/Documentation/filesystems/gfs2-glocks.txt
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   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_spin) 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_spin 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_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!)
 111
 112There are two glocks per inode. One deals with access to the inode
 113itself (locking order as above), and the other, known as the iopen
 114glock is used in conjunction with the i_nlink field in the inode to
 115determine the lifetime of the inode in question. Locking of inodes
 116is on a per-inode basis. Locking of rgrps is on a per rgrp basis.
 117In general we prefer to lock local locks prior to cluster locks.
 118
 119                            Glock Statistics
 120                           ------------------
 121
 122The stats are divided into two sets: those relating to the
 123super block and those relating to an individual glock. The
 124super block stats are done on a per cpu basis in order to
 125try and reduce the overhead of gathering them. They are also
 126further divided by glock type. All timings are in nanoseconds.
 127
 128In the case of both the super block and glock statistics,
 129the same information is gathered in each case. The super
 130block timing statistics are used to provide default values for
 131the glock timing statistics, so that newly created glocks
 132should have, as far as possible, a sensible starting point.
 133The per-glock counters are initialised to zero when the
 134glock is created. The per-glock statistics are lost when
 135the glock is ejected from memory.
 136
 137The statistics are divided into three pairs of mean and
 138variance, plus two counters. The mean/variance pairs are
 139smoothed exponential estimates and the algorithm used is
 140one which will be very familiar to those used to calculation
 141of round trip times in network code. See "TCP/IP Illustrated,
 142Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement",
 143p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards.
 144Unlike the TCP/IP Illustrated case, the mean and variance are
 145not scaled, but are in units of integer nanoseconds.
 146
 147The three pairs of mean/variance measure the following
 148things:
 149
 150 1. DLM lock time (non-blocking requests)
 151 2. DLM lock time (blocking requests)
 152 3. Inter-request time (again to the DLM)
 153
 154A non-blocking request is one which will complete right
 155away, whatever the state of the DLM lock in question. That
 156currently means any requests when (a) the current state of
 157the lock is exclusive, i.e. a lock demotion (b) the requested
 158state is either null or unlocked (again, a demotion) or (c) the
 159"try lock" flag is set. A blocking request covers all the other
 160lock requests.
 161
 162There are two counters. The first is there primarily to show
 163how many lock requests have been made, and thus how much data
 164has gone into the mean/variance calculations. The other counter
 165is counting queuing of holders at the top layer of the glock
 166code. Hopefully that number will be a lot larger than the number
 167of dlm lock requests issued.
 168
 169So why gather these statistics? There are several reasons
 170we'd like to get a better idea of these timings:
 171
 1721. To be able to better set the glock "min hold time"
 1732. To spot performance issues more easily
 1743. To improve the algorithm for selecting resource groups for
 175allocation (to base it on lock wait time, rather than blindly
 176using a "try lock")
 177
 178Due to the smoothing action of the updates, a step change in
 179some input quantity being sampled will only fully be taken
 180into account after 8 samples (or 4 for the variance) and this
 181needs to be carefully considered when interpreting the
 182results.
 183
 184Knowing both the time it takes a lock request to complete and
 185the average time between lock requests for a glock means we
 186can compute the total percentage of the time for which the
 187node is able to use a glock vs. time that the rest of the
 188cluster has its share. That will be very useful when setting
 189the lock min hold time.
 190
 191Great care has been taken to ensure that we
 192measure exactly the quantities that we want, as accurately
 193as possible. There are always inaccuracies in any
 194measuring system, but I hope this is as accurate as we
 195can reasonably make it.
 196
 197Per sb stats can be found here:
 198/sys/kernel/debug/gfs2/<fsname>/sbstats
 199Per glock stats can be found here:
 200/sys/kernel/debug/gfs2/<fsname>/glstats
 201
 202Assuming that debugfs is mounted on /sys/kernel/debug and also
 203that <fsname> is replaced with the name of the gfs2 filesystem
 204in question.
 205
 206The abbreviations used in the output as are follows:
 207
 208srtt     - Smoothed round trip time for non-blocking dlm requests
 209srttvar  - Variance estimate for srtt
 210srttb    - Smoothed round trip time for (potentially) blocking dlm requests
 211srttvarb - Variance estimate for srttb
 212sirt     - Smoothed inter-request time (for dlm requests)
 213sirtvar  - Variance estimate for sirt
 214dlm      - Number of dlm requests made (dcnt in glstats file)
 215queue    - Number of glock requests queued (qcnt in glstats file)
 216
 217The sbstats file contains a set of these stats for each glock type (so 8 lines
 218for each type) and for each cpu (one column per cpu). The glstats file contains
 219a set of these stats for each glock in a similar format to the glocks file, but
 220using the format mean/variance for each of the timing stats.
 221
 222The gfs2_glock_lock_time tracepoint prints out the current values of the stats
 223for the glock in question, along with some addition information on each dlm
 224reply that is received:
 225
 226status - The status of the dlm request
 227flags  - The dlm request flags
 228tdiff  - The time taken by this specific request
 229(remaining fields as per above list)
 230
 231
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