Documentation/lockstat.txt

   1
   2 LOCK STATISTICS
   3
   4 - WHAT
   5
   6 As the name suggests, it provides statistics on locks.
   7
   8 - WHY
   9
  10 Because things like lock contention can severely impact performance.
  11
  12 - HOW
  13
  14 Lockdep already has hooks in the lock functions and maps lock instances to
  15 lock classes. We build on that (see Documentation/lockdep-design.txt).
  16 The graph below shows the relation between the lock functions and the various
  17 hooks 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
  39 lock, unlock    - the regular lock functions
  40 __*             - the hooks
  41 <>              - states
  42
  43 With 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
  56 From these number various other statistics can be derived, such as:
  57
  58  hold time average = hold time total / acquisitions
  59
  60 These numbers are gathered per lock class, per read/write state (when
  61 applicable).
  62
  63 It also tracks 4 contention points per class. A contention point is a call site
  64 that had to wait on lock acquisition.
  65
  66  - CONFIGURATION
  67
  68 Lock statistics are enabled via CONFIG_LOCK_STAT.
  69
  70  - USAGE
  71
  72 Enable collection of statistics:
  73
  74 # echo 1 >/proc/sys/kernel/lock_stat
  75
  76 Disable collection of statistics:
  77
  78 # echo 0 >/proc/sys/kernel/lock_stat
  79
  80 Look 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
  87 01 lock_stat version 0.3
  88 02 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  89 03                               class name    con-bounces    contentions   waittime-min   waittime-max waittime-total    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total
  90 04 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  91 05
  92 06                          &mm->mmap_sem-W:           233            538 18446744073708       22924.27      607243.51           1342          45806           1.71        8595.89     1180582.34
  93 07                          &mm->mmap_sem-R:           205            587 18446744073708       28403.36      731975.00           1940         412426           0.58      187825.45     6307502.88
  94 08                          ---------------
  95 09                            &mm->mmap_sem            487          [<ffffffff8053491f>] do_page_fault+0x466/0x928
  96 10                            &mm->mmap_sem            179          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
  97 11                            &mm->mmap_sem            279          [<ffffffff80210a57>] sys_mmap+0x75/0xce
  98 12                            &mm->mmap_sem             76          [<ffffffff802a490b>] sys_munmap+0x32/0x59
  99 13                          ---------------
 100 14                            &mm->mmap_sem            270          [<ffffffff80210a57>] sys_mmap+0x75/0xce
 101 15                            &mm->mmap_sem            431          [<ffffffff8053491f>] do_page_fault+0x466/0x928
 102 16                            &mm->mmap_sem            138          [<ffffffff802a490b>] sys_munmap+0x32/0x59
 103 17                            &mm->mmap_sem            145          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
 104 18
 105 19 ...............................................................................................................................................................................................
 106 20
 107 21                              dcache_lock:           621            623           0.52         118.26        1053.02           6745          91930           0.29         316.29      118423.41
 108 22                              -----------
 109 23                              dcache_lock            179          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
 110 24                              dcache_lock            113          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
 111 25                              dcache_lock             99          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
 112 26                              dcache_lock            104          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
 113 27                              -----------
 114 28                              dcache_lock            192          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
 115 29                              dcache_lock             98          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
 116 30                              dcache_lock             72          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
 117 31                              dcache_lock            112          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
 118
 119 This excerpt shows the first two lock class statistics. Line 01 shows the
 120 output version - each time the format changes this will be updated. Line 02-04
 121 show the header with column descriptions. Lines 05-18 and 20-31 show the actual
 122 statistics. These statistics come in two parts; the actual stats separated by a
 123 short separator (line 08, 13) from the contention points.
 124
 125 The first lock (05-18) is a read/write lock, and shows two lines above the
 126 short separator. The contention points don't match the column descriptors,
 127 they have two: contentions and [<IP>] symbol. The second set of contention
 128 points are the points we're contending with.
 129
 130 The integer part of the time values is in us.
 131
 132 Dealing with nested locks, subclasses may appear:
 133
 134 32...............................................................................................................................................................................................
 135 33
 136 34                               &rq->lock:         13128          13128           0.43         190.53      103881.26          97454        3453404           0.00         401.11    13224683.11
 137 35                               ---------
 138 36                               &rq->lock            645          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
 139 37                               &rq->lock            297          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
 140 38                               &rq->lock            360          [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a
 141 39                               &rq->lock            428          [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb
 142 40                               ---------
 143 41                               &rq->lock             77          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
 144 42                               &rq->lock            174          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
 145 43                               &rq->lock           4715          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
 146 44                               &rq->lock            893          [<ffffffff81340524>] schedule+0x157/0x7b8
 147 45
 148 46...............................................................................................................................................................................................
 149 47
 150 48                             &rq->lock/1:         11526          11488           0.33         388.73      136294.31          21461          38404           0.00          37.93      109388.53
 151 49                             -----------
 152 50                             &rq->lock/1          11526          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
 153 51                             -----------
 154 52                             &rq->lock/1           5645          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
 155 53                             &rq->lock/1           1224          [<ffffffff81340524>] schedule+0x157/0x7b8
 156 54                             &rq->lock/1           4336          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
 157 55                             &rq->lock/1            181          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
 158
 159 Line 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,
 161 double_rq_lock actually acquires a nested lock of two spinlocks.
 162
 163 View 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
 177 Clear the statistics:
 178
 179 # echo 0 > /proc/lock_stat