6 As the name suggests, it provides statistics on locks.
10 Because things like lock contention can severely impact performance.
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
39 lock, unlock - the regular lock functions
43 With these hooks we provide the following statistics:
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
56 From these number various other statistics can be derived, such as:
58 hold time average = hold time total / acquisitions
60 These numbers are gathered per lock class, per read/write state (when
63 It also tracks 4 contention points per class. A contention point is a call site
64 that had to wait on lock acquisition.
68 Lock statistics are enabled via CONFIG_LOCK_STAT.
72 Enable collection of statistics:
74 # echo 1 >/proc/sys/kernel/lock_stat
76 Disable collection of statistics:
78 # echo 0 >/proc/sys/kernel/lock_stat
80 Look at the current lock statistics:
82 ( line numbers not part of actual output, done for clarity in the explanation
85 # less /proc/lock_stat
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 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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
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
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
105 19 ...............................................................................................................................................................................................
107 21 dcache_lock: 621 623 0.52 118.26 1053.02 6745 91930 0.29 316.29 118423.41
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
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
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.
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.
130 The integer part of the time values is in us.
132 Dealing with nested locks, subclasses may appear:
134 32...............................................................................................................................................................................................
136 34 &rq->lock: 13128 13128 0.43 190.53 103881.26 97454 3453404 0.00 401.11 13224683.11
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
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
148 46...............................................................................................................................................................................................
150 48 &rq->lock/1: 11526 11488 0.33 388.73 136294.31 21461 38404 0.00 37.93 109388.53
152 50 &rq->lock/1 11526 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
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
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.
163 View the top contending locks:
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
177 Clear the statistics:
179 # echo 0 > /proc/lock_stat