linux/Documentation/lockstat.txt
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   1
   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 (see Documentation/lockdep-design.txt).
  16The graph below shows the relation between the lock functions and the various
  17hooks therein.
  18
  19        __acquire
  20            |
  21           lock _____
  22            |        \
  23            |    __contended
  24            |         |
  25            |       <wait>
  26            | _______/
  27            |/
  28            |
  29       __acquired
  30            |
  31            .
  32          <hold>
  33            .
  34            |
  35       __release
  36            |
  37         unlock
  38
  39lock, unlock    - the regular lock functions
  40__*             - the hooks
  41<>              - states
  42
  43With these hooks we provide the following statistics:
  44
  45 con-bounces       - number of lock contention that involved x-cpu data
  46 contentions       - number of lock acquisitions that had to wait
  47 wait time min     - shortest (non-0) time we ever had to wait for a lock
  48           max     - longest time we ever had to wait for a lock
  49           total   - total time we spend waiting on this lock
  50 acq-bounces       - number of lock acquisitions that involved x-cpu data
  51 acquisitions      - number of times we took the lock
  52 hold time min     - shortest (non-0) time we ever held the lock
  53           max     - longest time we ever held the lock
  54           total   - total time this lock was held
  55
  56From these number various other statistics can be derived, such as:
  57
  58 hold time average = hold time total / acquisitions
  59
  60These numbers are gathered per lock class, per read/write state (when
  61applicable).
  62
  63It also tracks 4 contention points per class. A contention point is a call site
  64that had to wait on lock acquisition.
  65
  66 - CONFIGURATION
  67
  68Lock statistics are enabled via CONFIG_LOCK_STAT.
  69
  70 - USAGE
  71
  72Enable collection of statistics:
  73
  74# echo 1 >/proc/sys/kernel/lock_stat
  75
  76Disable collection of statistics:
  77
  78# echo 0 >/proc/sys/kernel/lock_stat
  79
  80Look at the current lock statistics:
  81
  82( line numbers not part of actual output, done for clarity in the explanation
  83  below )
  84
  85# less /proc/lock_stat
  86
  8701 lock_stat version 0.3
  8802 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  8903                               class name    con-bounces    contentions   waittime-min   waittime-max waittime-total    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total
  9004 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  9105
  9206                          &mm->mmap_sem-W:           233            538 18446744073708       22924.27      607243.51           1342          45806           1.71        8595.89     1180582.34
  9307                          &mm->mmap_sem-R:           205            587 18446744073708       28403.36      731975.00           1940         412426           0.58      187825.45     6307502.88
  9408                          ---------------
  9509                            &mm->mmap_sem            487          [<ffffffff8053491f>] do_page_fault+0x466/0x928
  9610                            &mm->mmap_sem            179          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
  9711                            &mm->mmap_sem            279          [<ffffffff80210a57>] sys_mmap+0x75/0xce
  9812                            &mm->mmap_sem             76          [<ffffffff802a490b>] sys_munmap+0x32/0x59
  9913                          ---------------
 10014                            &mm->mmap_sem            270          [<ffffffff80210a57>] sys_mmap+0x75/0xce
 10115                            &mm->mmap_sem            431          [<ffffffff8053491f>] do_page_fault+0x466/0x928
 10216                            &mm->mmap_sem            138          [<ffffffff802a490b>] sys_munmap+0x32/0x59
 10317                            &mm->mmap_sem            145          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
 10418
 10519 ...............................................................................................................................................................................................
 10620
 10721                              dcache_lock:           621            623           0.52         118.26        1053.02           6745          91930           0.29         316.29      118423.41
 10822                              -----------
 10923                              dcache_lock            179          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
 11024                              dcache_lock            113          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
 11125                              dcache_lock             99          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
 11226                              dcache_lock            104          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
 11327                              -----------
 11428                              dcache_lock            192          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
 11529                              dcache_lock             98          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
 11630                              dcache_lock             72          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
 11731                              dcache_lock            112          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
 118
 119This excerpt shows the first two lock class statistics. Line 01 shows the
 120output version - each time the format changes this will be updated. Line 02-04
 121show the header with column descriptions. Lines 05-18 and 20-31 show the actual
 122statistics. These statistics come in two parts; the actual stats separated by a
 123short separator (line 08, 13) from the contention points.
 124
 125The first lock (05-18) is a read/write lock, and shows two lines above the
 126short separator. The contention points don't match the column descriptors,
 127they have two: contentions and [<IP>] symbol. The second set of contention
 128points are the points we're contending with.
 129
 130The integer part of the time values is in us.
 131
 132Dealing with nested locks, subclasses may appear:
 133
 13432...............................................................................................................................................................................................
 13533
 13634                               &rq->lock:         13128          13128           0.43         190.53      103881.26          97454        3453404           0.00         401.11    13224683.11
 13735                               ---------
 13836                               &rq->lock            645          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
 13937                               &rq->lock            297          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
 14038                               &rq->lock            360          [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a
 14139                               &rq->lock            428          [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb
 14240                               ---------
 14341                               &rq->lock             77          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
 14442                               &rq->lock            174          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
 14543                               &rq->lock           4715          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
 14644                               &rq->lock            893          [<ffffffff81340524>] schedule+0x157/0x7b8
 14745
 14846...............................................................................................................................................................................................
 14947
 15048                             &rq->lock/1:         11526          11488           0.33         388.73      136294.31          21461          38404           0.00          37.93      109388.53
 15149                             -----------
 15250                             &rq->lock/1          11526          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
 15351                             -----------
 15452                             &rq->lock/1           5645          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
 15553                             &rq->lock/1           1224          [<ffffffff81340524>] schedule+0x157/0x7b8
 15654                             &rq->lock/1           4336          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
 15755                             &rq->lock/1            181          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
 158
 159Line 48 shows statistics for the second subclass (/1) of &rq->lock class
 160(subclass starts from 0), since in this case, as line 50 suggests,
 161double_rq_lock actually acquires a nested lock of two spinlocks.
 162
 163View the top contending locks:
 164
 165# grep : /proc/lock_stat | head
 166              &inode->i_data.tree_lock-W:            15          21657           0.18     1093295.30 11547131054.85             58          10415           0.16          87.51        6387.60
 167              &inode->i_data.tree_lock-R:             0              0           0.00           0.00           0.00          23302         231198           0.25           8.45       98023.38
 168                             dcache_lock:          1037           1161           0.38          45.32         774.51           6611         243371           0.15         306.48       77387.24
 169                         &inode->i_mutex:           161            286 18446744073709       62882.54     1244614.55           3653          20598 18446744073709       62318.60     1693822.74
 170                         &zone->lru_lock:            94             94           0.53           7.33          92.10           4366          32690           0.29          59.81       16350.06
 171              &inode->i_data.i_mmap_mutex:            79             79           0.40           3.77          53.03          11779          87755           0.28         116.93       29898.44
 172                        &q->__queue_lock:            48             50           0.52          31.62          86.31            774          13131           0.17         113.08       12277.52
 173                        &rq->rq_lock_key:            43             47           0.74          68.50         170.63           3706          33929           0.22         107.99       17460.62
 174                      &rq->rq_lock_key#2:            39             46           0.75           6.68          49.03           2979          32292           0.17         125.17       17137.63
 175                         tasklist_lock-W:            15             15           1.45          10.87          32.70           1201           7390           0.58          62.55       13648.47
 176
 177Clear the statistics:
 178
 179# echo 0 > /proc/lock_stat
 180
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