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   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. The graph below shows the relation between
  16 the 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
  38 lock, unlock    - the regular lock functions
  39 __*             - the hooks
  40 <>              - states
  41
  42 With 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
  55 From these number various other statistics can be derived, such as:
  56
  57  hold time average = hold time total / acquisitions
  58
  59 These numbers are gathered per lock class, per read/write state (when
  60 applicable).
  61
  62 It also tracks 4 contention points per class. A contention point is a call site
  63 that had to wait on lock acquisition.
  64
  65  - USAGE
  66
  67 Look at the current lock statistics:
  68
  69 ( line numbers not part of actual output, done for clarity in the explanation
  70   below )
  71
  72 # less /proc/lock_stat
  73
  74 01 lock_stat version 0.3
  75 02 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  76 03                               class name    con-bounces    contentions   waittime-min   waittime-max waittime-total    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total
  77 04 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  78 05
  79 06                          &mm->mmap_sem-W:           233            538 18446744073708       22924.27      607243.51           1342          45806           1.71        8595.89     1180582.34
  80 07                          &mm->mmap_sem-R:           205            587 18446744073708       28403.36      731975.00           1940         412426           0.58      187825.45     6307502.88
  81 08                          ---------------
  82 09                            &mm->mmap_sem            487          [<ffffffff8053491f>] do_page_fault+0x466/0x928
  83 10                            &mm->mmap_sem            179          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
  84 11                            &mm->mmap_sem            279          [<ffffffff80210a57>] sys_mmap+0x75/0xce
  85 12                            &mm->mmap_sem             76          [<ffffffff802a490b>] sys_munmap+0x32/0x59
  86 13                          ---------------
  87 14                            &mm->mmap_sem            270          [<ffffffff80210a57>] sys_mmap+0x75/0xce
  88 15                            &mm->mmap_sem            431          [<ffffffff8053491f>] do_page_fault+0x466/0x928
  89 16                            &mm->mmap_sem            138          [<ffffffff802a490b>] sys_munmap+0x32/0x59
  90 17                            &mm->mmap_sem            145          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
  91 18
  92 19 ...............................................................................................................................................................................................
  93 20
  94 21                              dcache_lock:           621            623           0.52         118.26        1053.02           6745          91930           0.29         316.29      118423.41
  95 22                              -----------
  96 23                              dcache_lock            179          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
  97 24                              dcache_lock            113          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
  98 25                              dcache_lock             99          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
  99 26                              dcache_lock            104          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
 100 27                              -----------
 101 28                              dcache_lock            192          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
 102 29                              dcache_lock             98          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
 103 30                              dcache_lock             72          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
 104 31                              dcache_lock            112          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
 105
 106 This excerpt shows the first two lock class statistics. Line 01 shows the
 107 output version - each time the format changes this will be updated. Line 02-04
 108 show the header with column descriptions. Lines 05-18 and 20-31 show the actual
 109 statistics. These statistics come in two parts; the actual stats separated by a
 110 short separator (line 08, 13) from the contention points.
 111
 112 The first lock (05-18) is a read/write lock, and shows two lines above the
 113 short separator. The contention points don't match the column descriptors,
 114 they have two: contentions and [<IP>] symbol. The second set of contention
 115 points are the points we're contending with.
 116
 117 The integer part of the time values is in us.
 118
 119 View the top contending locks:
 120
 121 # grep : /proc/lock_stat | head
 122               &inode->i_data.tree_lock-W:            15          21657           0.18     1093295.30 11547131054.85             58          10415           0.16          87.51        6387.60
 123               &inode->i_data.tree_lock-R:             0              0           0.00           0.00           0.00          23302         231198           0.25           8.45       98023.38
 124                              dcache_lock:          1037           1161           0.38          45.32         774.51           6611         243371           0.15         306.48       77387.24
 125                          &inode->i_mutex:           161            286 18446744073709       62882.54     1244614.55           3653          20598 18446744073709       62318.60     1693822.74
 126                          &zone->lru_lock:            94             94           0.53           7.33          92.10           4366          32690           0.29          59.81       16350.06
 127               &inode->i_data.i_mmap_lock:            79             79           0.40           3.77          53.03          11779          87755           0.28         116.93       29898.44
 128                         &q->__queue_lock:            48             50           0.52          31.62          86.31            774          13131           0.17         113.08       12277.52
 129                         &rq->rq_lock_key:            43             47           0.74          68.50         170.63           3706          33929           0.22         107.99       17460.62
 130                       &rq->rq_lock_key#2:            39             46           0.75           6.68          49.03           2979          32292           0.17         125.17       17137.63
 131                          tasklist_lock-W:            15             15           1.45          10.87          32.70           1201           7390           0.58          62.55       13648.47
 132
 133 Clear the statistics:
 134
 135 # echo 0 > /proc/lock_stat