| LOCK STATISTICS | |
| - WHAT | |
| As the name suggests, it provides statistics on locks. | |
| - WHY | |
| Because things like lock contention can severely impact performance. | |
| - HOW | |
| Lockdep already has hooks in the lock functions and maps lock instances to | |
| lock classes. We build on that (see Documentation/lockdep-design.txt). | |
| The graph below shows the relation between the lock functions and the various | |
| hooks therein. | |
| __acquire | |
| | | |
| lock _____ | |
| | \ | |
| | __contended | |
| | | | |
| | <wait> | |
| | _______/ | |
| |/ | |
| | | |
| __acquired | |
| | | |
| . | |
| <hold> | |
| . | |
| | | |
| __release | |
| | | |
| unlock | |
| lock, unlock - the regular lock functions | |
| __* - the hooks | |
| <> - states | |
| With these hooks we provide the following statistics: | |
| con-bounces - number of lock contention that involved x-cpu data | |
| contentions - number of lock acquisitions that had to wait | |
| wait time min - shortest (non-0) time we ever had to wait for a lock | |
| max - longest time we ever had to wait for a lock | |
| total - total time we spend waiting on this lock | |
| acq-bounces - number of lock acquisitions that involved x-cpu data | |
| acquisitions - number of times we took the lock | |
| hold time min - shortest (non-0) time we ever held the lock | |
| max - longest time we ever held the lock | |
| total - total time this lock was held | |
| From these number various other statistics can be derived, such as: | |
| hold time average = hold time total / acquisitions | |
| These numbers are gathered per lock class, per read/write state (when | |
| applicable). | |
| It also tracks 4 contention points per class. A contention point is a call site | |
| that had to wait on lock acquisition. | |
| - CONFIGURATION | |
| Lock statistics are enabled via CONFIG_LOCK_STATS. | |
| - USAGE | |
| Enable collection of statistics: | |
| # echo 1 >/proc/sys/kernel/lock_stat | |
| Disable collection of statistics: | |
| # echo 0 >/proc/sys/kernel/lock_stat | |
| Look at the current lock statistics: | |
| ( line numbers not part of actual output, done for clarity in the explanation | |
| below ) | |
| # less /proc/lock_stat | |
| 01 lock_stat version 0.3 | |
| 02 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | |
| 03 class name con-bounces contentions waittime-min waittime-max waittime-total acq-bounces acquisitions holdtime-min holdtime-max holdtime-total | |
| 04 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | |
| 05 | |
| 06 &mm->mmap_sem-W: 233 538 18446744073708 22924.27 607243.51 1342 45806 1.71 8595.89 1180582.34 | |
| 07 &mm->mmap_sem-R: 205 587 18446744073708 28403.36 731975.00 1940 412426 0.58 187825.45 6307502.88 | |
| 08 --------------- | |
| 09 &mm->mmap_sem 487 [<ffffffff8053491f>] do_page_fault+0x466/0x928 | |
| 10 &mm->mmap_sem 179 [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d | |
| 11 &mm->mmap_sem 279 [<ffffffff80210a57>] sys_mmap+0x75/0xce | |
| 12 &mm->mmap_sem 76 [<ffffffff802a490b>] sys_munmap+0x32/0x59 | |
| 13 --------------- | |
| 14 &mm->mmap_sem 270 [<ffffffff80210a57>] sys_mmap+0x75/0xce | |
| 15 &mm->mmap_sem 431 [<ffffffff8053491f>] do_page_fault+0x466/0x928 | |
| 16 &mm->mmap_sem 138 [<ffffffff802a490b>] sys_munmap+0x32/0x59 | |
| 17 &mm->mmap_sem 145 [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d | |
| 18 | |
| 19 ............................................................................................................................................................................................... | |
| 20 | |
| 21 dcache_lock: 621 623 0.52 118.26 1053.02 6745 91930 0.29 316.29 118423.41 | |
| 22 ----------- | |
| 23 dcache_lock 179 [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54 | |
| 24 dcache_lock 113 [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb | |
| 25 dcache_lock 99 [<ffffffff802ca0dc>] d_rehash+0x1b/0x44 | |
| 26 dcache_lock 104 [<ffffffff802cbca0>] d_instantiate+0x36/0x8a | |
| 27 ----------- | |
| 28 dcache_lock 192 [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54 | |
| 29 dcache_lock 98 [<ffffffff802ca0dc>] d_rehash+0x1b/0x44 | |
| 30 dcache_lock 72 [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb | |
| 31 dcache_lock 112 [<ffffffff802cbca0>] d_instantiate+0x36/0x8a | |
| This excerpt shows the first two lock class statistics. Line 01 shows the | |
| output version - each time the format changes this will be updated. Line 02-04 | |
| show the header with column descriptions. Lines 05-18 and 20-31 show the actual | |
| statistics. These statistics come in two parts; the actual stats separated by a | |
| short separator (line 08, 13) from the contention points. | |
| The first lock (05-18) is a read/write lock, and shows two lines above the | |
| short separator. The contention points don't match the column descriptors, | |
| they have two: contentions and [<IP>] symbol. The second set of contention | |
| points are the points we're contending with. | |
| The integer part of the time values is in us. | |
| Dealing with nested locks, subclasses may appear: | |
| 32............................................................................................................................................................................................... | |
| 33 | |
| 34 &rq->lock: 13128 13128 0.43 190.53 103881.26 97454 3453404 0.00 401.11 13224683.11 | |
| 35 --------- | |
| 36 &rq->lock 645 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75 | |
| 37 &rq->lock 297 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a | |
| 38 &rq->lock 360 [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a | |
| 39 &rq->lock 428 [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb | |
| 40 --------- | |
| 41 &rq->lock 77 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75 | |
| 42 &rq->lock 174 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a | |
| 43 &rq->lock 4715 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54 | |
| 44 &rq->lock 893 [<ffffffff81340524>] schedule+0x157/0x7b8 | |
| 45 | |
| 46............................................................................................................................................................................................... | |
| 47 | |
| 48 &rq->lock/1: 11526 11488 0.33 388.73 136294.31 21461 38404 0.00 37.93 109388.53 | |
| 49 ----------- | |
| 50 &rq->lock/1 11526 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54 | |
| 51 ----------- | |
| 52 &rq->lock/1 5645 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54 | |
| 53 &rq->lock/1 1224 [<ffffffff81340524>] schedule+0x157/0x7b8 | |
| 54 &rq->lock/1 4336 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54 | |
| 55 &rq->lock/1 181 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a | |
| Line 48 shows statistics for the second subclass (/1) of &rq->lock class | |
| (subclass starts from 0), since in this case, as line 50 suggests, | |
| double_rq_lock actually acquires a nested lock of two spinlocks. | |
| View the top contending locks: | |
| # grep : /proc/lock_stat | head | |
| &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60 | |
| &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38 | |
| dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24 | |
| &inode->i_mutex: 161 286 18446744073709 62882.54 1244614.55 3653 20598 18446744073709 62318.60 1693822.74 | |
| &zone->lru_lock: 94 94 0.53 7.33 92.10 4366 32690 0.29 59.81 16350.06 | |
| &inode->i_data.i_mmap_mutex: 79 79 0.40 3.77 53.03 11779 87755 0.28 116.93 29898.44 | |
| &q->__queue_lock: 48 50 0.52 31.62 86.31 774 13131 0.17 113.08 12277.52 | |
| &rq->rq_lock_key: 43 47 0.74 68.50 170.63 3706 33929 0.22 107.99 17460.62 | |
| &rq->rq_lock_key#2: 39 46 0.75 6.68 49.03 2979 32292 0.17 125.17 17137.63 | |
| tasklist_lock-W: 15 15 1.45 10.87 32.70 1201 7390 0.58 62.55 13648.47 | |
| Clear the statistics: | |
| # echo 0 > /proc/lock_stat |