linux/Documentation/lockstat.txt
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   2LOCK STATISTICS
   3
   4- WHAT
   5
   6As the name suggests, it provides statistics on locks.
   7
   8- WHY
   9
  10Because things like lock contention can severely impact performance.
  11
  12- HOW
  13
  14Lockdep already has hooks in the lock functions and maps lock instances to
  15lock classes. We build on that. The graph below shows the relation between
  16the lock functions and the various hooks therein.
  17
  18        __acquire
  19            |
  20           lock _____
  21            |        \
  22            |    __contended
  23            |         |
  24            |       <wait>
  25            | _______/
  26            |/
  27            |
  28       __acquired
  29            |
  30            .
  31          <hold>
  32            .
  33            |
  34       __release
  35            |
  36         unlock
  37
  38lock, unlock    - the regular lock functions
  39__*             - the hooks
  40<>              - states
  41
  42With these hooks we provide the following statistics:
  43
  44 con-bounces       - number of lock contention that involved x-cpu data
  45 contentions       - number of lock acquisitions that had to wait
  46 wait time min     - shortest (non-0) time we ever had to wait for a lock
  47           max     - longest time we ever had to wait for a lock
  48           total   - total time we spend waiting on this lock
  49 acq-bounces       - number of lock acquisitions that involved x-cpu data
  50 acquisitions      - number of times we took the lock
  51 hold time min     - shortest (non-0) time we ever held the lock
  52           max     - longest time we ever held the lock
  53           total   - total time this lock was held
  54
  55From these number various other statistics can be derived, such as:
  56
  57 hold time average = hold time total / acquisitions
  58
  59These numbers are gathered per lock class, per read/write state (when
  60applicable).
  61
  62It also tracks 4 contention points per class. A contention point is a call site
  63that had to wait on lock acquisition.
  64
  65 - CONFIGURATION
  66
  67Lock statistics are enabled via CONFIG_LOCK_STATS.
  68
  69 - USAGE
  70
  71Enable collection of statistics:
  72
  73# echo 1 >/proc/sys/kernel/lock_stat
  74
  75Disable collection of statistics:
  76
  77# echo 0 >/proc/sys/kernel/lock_stat
  78
  79Look at the current lock statistics:
  80
  81( line numbers not part of actual output, done for clarity in the explanation
  82  below )
  83
  84# less /proc/lock_stat
  85
  8601 lock_stat version 0.3
  8702 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  8803                               class name    con-bounces    contentions   waittime-min   waittime-max waittime-total    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total
  8904 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  9005
  9106                          &mm->mmap_sem-W:           233            538 18446744073708       22924.27      607243.51           1342          45806           1.71        8595.89     1180582.34
  9207                          &mm->mmap_sem-R:           205            587 18446744073708       28403.36      731975.00           1940         412426           0.58      187825.45     6307502.88
  9308                          ---------------
  9409                            &mm->mmap_sem            487          [<ffffffff8053491f>] do_page_fault+0x466/0x928
  9510                            &mm->mmap_sem            179          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
  9611                            &mm->mmap_sem            279          [<ffffffff80210a57>] sys_mmap+0x75/0xce
  9712                            &mm->mmap_sem             76          [<ffffffff802a490b>] sys_munmap+0x32/0x59
  9813                          ---------------
  9914                            &mm->mmap_sem            270          [<ffffffff80210a57>] sys_mmap+0x75/0xce
 10015                            &mm->mmap_sem            431          [<ffffffff8053491f>] do_page_fault+0x466/0x928
 10116                            &mm->mmap_sem            138          [<ffffffff802a490b>] sys_munmap+0x32/0x59
 10217                            &mm->mmap_sem            145          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
 10318
 10419 ...............................................................................................................................................................................................
 10520
 10621                              dcache_lock:           621            623           0.52         118.26        1053.02           6745          91930           0.29         316.29      118423.41
 10722                              -----------
 10823                              dcache_lock            179          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
 10924                              dcache_lock            113          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
 11025                              dcache_lock             99          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
 11126                              dcache_lock            104          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
 11227                              -----------
 11328                              dcache_lock            192          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
 11429                              dcache_lock             98          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
 11530                              dcache_lock             72          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
 11631                              dcache_lock            112          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
 117
 118This excerpt shows the first two lock class statistics. Line 01 shows the
 119output version - each time the format changes this will be updated. Line 02-04
 120show the header with column descriptions. Lines 05-18 and 20-31 show the actual
 121statistics. These statistics come in two parts; the actual stats separated by a
 122short separator (line 08, 13) from the contention points.
 123
 124The first lock (05-18) is a read/write lock, and shows two lines above the
 125short separator. The contention points don't match the column descriptors,
 126they have two: contentions and [<IP>] symbol. The second set of contention
 127points are the points we're contending with.
 128
 129The integer part of the time values is in us.
 130
 131View the top contending locks:
 132
 133# grep : /proc/lock_stat | head
 134              &inode->i_data.tree_lock-W:            15          21657           0.18     1093295.30 11547131054.85             58          10415           0.16          87.51        6387.60
 135              &inode->i_data.tree_lock-R:             0              0           0.00           0.00           0.00          23302         231198           0.25           8.45       98023.38
 136                             dcache_lock:          1037           1161           0.38          45.32         774.51           6611         243371           0.15         306.48       77387.24
 137                         &inode->i_mutex:           161            286 18446744073709       62882.54     1244614.55           3653          20598 18446744073709       62318.60     1693822.74
 138                         &zone->lru_lock:            94             94           0.53           7.33          92.10           4366          32690           0.29          59.81       16350.06
 139              &inode->i_data.i_mmap_lock:            79             79           0.40           3.77          53.03          11779          87755           0.28         116.93       29898.44
 140                        &q->__queue_lock:            48             50           0.52          31.62          86.31            774          13131           0.17         113.08       12277.52
 141                        &rq->rq_lock_key:            43             47           0.74          68.50         170.63           3706          33929           0.22         107.99       17460.62
 142                      &rq->rq_lock_key#2:            39             46           0.75           6.68          49.03           2979          32292           0.17         125.17       17137.63
 143                         tasklist_lock-W:            15             15           1.45          10.87          32.70           1201           7390           0.58          62.55       13648.47
 144
 145Clear the statistics:
 146
 147# echo 0 > /proc/lock_stat
 148
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