```   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. 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 - USAGE
66
67Look 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
7401 lock_stat version 0.3
7502 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
7603                               class name    con-bounces    contentions   waittime-min   waittime-max waittime-total    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total
7704 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
7805
7906                          &mm->mmap_sem-W:           233            538 18446744073708       22924.27      607243.51           1342          45806           1.71        8595.89     1180582.34
8007                          &mm->mmap_sem-R:           205            587 18446744073708       28403.36      731975.00           1940         412426           0.58      187825.45     6307502.88
8108                          ---------------
8209                            &mm->mmap_sem            487          [<ffffffff8053491f>] do_page_fault+0x466/0x928
8310                            &mm->mmap_sem            179          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
8411                            &mm->mmap_sem            279          [<ffffffff80210a57>] sys_mmap+0x75/0xce
8512                            &mm->mmap_sem             76          [<ffffffff802a490b>] sys_munmap+0x32/0x59
8613                          ---------------
8714                            &mm->mmap_sem            270          [<ffffffff80210a57>] sys_mmap+0x75/0xce
8815                            &mm->mmap_sem            431          [<ffffffff8053491f>] do_page_fault+0x466/0x928
8916                            &mm->mmap_sem            138          [<ffffffff802a490b>] sys_munmap+0x32/0x59
9017                            &mm->mmap_sem            145          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
9118
9219 ...............................................................................................................................................................................................
9320
9421                              dcache_lock:           621            623           0.52         118.26        1053.02           6745          91930           0.29         316.29      118423.41
9522                              -----------
9623                              dcache_lock            179          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
9724                              dcache_lock            113          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
9825                              dcache_lock             99          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
9926                              dcache_lock            104          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
10027                              -----------
10128                              dcache_lock            192          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
10229                              dcache_lock             98          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
10330                              dcache_lock             72          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
10431                              dcache_lock            112          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
105
106This excerpt shows the first two lock class statistics. Line 01 shows the
107output version - each time the format changes this will be updated. Line 02-04
108show the header with column descriptions. Lines 05-18 and 20-31 show the actual
109statistics. These statistics come in two parts; the actual stats separated by a
110short separator (line 08, 13) from the contention points.
111
112The first lock (05-18) is a read/write lock, and shows two lines above the
113short separator. The contention points don't match the column descriptors,
114they have two: contentions and [<IP>] symbol. The second set of contention
115points are the points we're contending with.
116
117The integer part of the time values is in us.
118
119View 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
133Clear the statistics:
134
135# echo 0 > /proc/lock_stat
136```
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