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 (see Documentation/locking/lockdep-design.txt). 16The graph below shows the relation between the lock functions and the various 17hooks 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 39lock, unlock - the regular lock functions 40__* - the hooks 41<> - states 42 43With 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 avg - average time spent waiting on this lock 51 acq-bounces - number of lock acquisitions that involved x-cpu data 52 acquisitions - number of times we took the lock 53 hold time min - shortest (non-0) time we ever held the lock 54 max - longest time we ever held the lock 55 total - total time this lock was held 56 avg - average time this lock was held 57 58These numbers are gathered per lock class, per read/write state (when 59applicable). 60 61It also tracks 4 contention points per class. A contention point is a call site 62that had to wait on lock acquisition. 63 64 - CONFIGURATION 65 66Lock statistics are enabled via CONFIG_LOCK_STAT. 67 68 - USAGE 69 70Enable collection of statistics: 71 72# echo 1 >/proc/sys/kernel/lock_stat 73 74Disable collection of statistics: 75 76# echo 0 >/proc/sys/kernel/lock_stat 77 78Look at the current lock statistics: 79 80( line numbers not part of actual output, done for clarity in the explanation 81 below ) 82 83# less /proc/lock_stat 84 8501 lock_stat version 0.4 8602----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 8703 class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg 8804----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 8905 9006 &mm->mmap_sem-W: 46 84 0.26 939.10 16371.53 194.90 47291 2922365 0.16 2220301.69 17464026916.32 5975.99 9107 &mm->mmap_sem-R: 37 100 1.31 299502.61 325629.52 3256.30 212344 34316685 0.10 7744.91 95016910.20 2.77 9208 --------------- 9309 &mm->mmap_sem 1 [<ffffffff811502a7>] khugepaged_scan_mm_slot+0x57/0x280 9419 &mm->mmap_sem 96 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510 9511 &mm->mmap_sem 34 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0 9612 &mm->mmap_sem 17 [<ffffffff81127e71>] vm_munmap+0x41/0x80 9713 --------------- 9814 &mm->mmap_sem 1 [<ffffffff81046fda>] dup_mmap+0x2a/0x3f0 9915 &mm->mmap_sem 60 [<ffffffff81129e29>] SyS_mprotect+0xe9/0x250 10016 &mm->mmap_sem 41 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510 10117 &mm->mmap_sem 68 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0 10218 10319............................................................................................................................................................................................................................. 10420 10521 unix_table_lock: 110 112 0.21 49.24 163.91 1.46 21094 66312 0.12 624.42 31589.81 0.48 10622 --------------- 10723 unix_table_lock 45 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0 10824 unix_table_lock 47 [<ffffffff8150b111>] unix_release_sock+0x31/0x250 10925 unix_table_lock 15 [<ffffffff8150ca37>] unix_find_other+0x117/0x230 11026 unix_table_lock 5 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0 11127 --------------- 11228 unix_table_lock 39 [<ffffffff8150b111>] unix_release_sock+0x31/0x250 11329 unix_table_lock 49 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0 11430 unix_table_lock 20 [<ffffffff8150ca37>] unix_find_other+0x117/0x230 11531 unix_table_lock 4 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0 116 117 118This excerpt shows the first two lock class statistics. Line 01 shows the 119output version - each time the format changes this will be updated. Line 02-04 120show the header with column descriptions. Lines 05-18 and 20-31 show the actual 121statistics. These statistics come in two parts; the actual stats separated by a 122short separator (line 08, 13) from the contention points. 123 124Lines 09-12 show the first 4 recorded contention points (the code 125which tries to get the lock) and lines 14-17 show the first 4 recorded 126contended points (the lock holder). It is possible that the max 127con-bounces point is missing in the statistics. 128 129The first lock (05-18) is a read/write lock, and shows two lines above the 130short separator. The contention points don't match the column descriptors, 131they have two: contentions and [<IP>] symbol. The second set of contention 132points are the points we're contending with. 133 134The integer part of the time values is in us. 135 136Dealing with nested locks, subclasses may appear: 137 13832........................................................................................................................................................................................................................... 13933 14034 &rq->lock: 13128 13128 0.43 190.53 103881.26 7.91 97454 3453404 0.00 401.11 13224683.11 3.82 14135 --------- 14236 &rq->lock 645 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75 14337 &rq->lock 297 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a 14438 &rq->lock 360 [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a 14539 &rq->lock 428 [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb 14640 --------- 14741 &rq->lock 77 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75 14842 &rq->lock 174 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a 14943 &rq->lock 4715 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54 15044 &rq->lock 893 [<ffffffff81340524>] schedule+0x157/0x7b8 15145 15246........................................................................................................................................................................................................................... 15347 15448 &rq->lock/1: 1526 11488 0.33 388.73 136294.31 11.86 21461 38404 0.00 37.93 109388.53 2.84 15549 ----------- 15650 &rq->lock/1 11526 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54 15751 ----------- 15852 &rq->lock/1 5645 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54 15953 &rq->lock/1 1224 [<ffffffff81340524>] schedule+0x157/0x7b8 16054 &rq->lock/1 4336 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54 16155 &rq->lock/1 181 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a 162 163Line 48 shows statistics for the second subclass (/1) of &rq->lock class 164(subclass starts from 0), since in this case, as line 50 suggests, 165double_rq_lock actually acquires a nested lock of two spinlocks. 166 167View the top contending locks: 168 169# grep : /proc/lock_stat | head 170 clockevents_lock: 2926159 2947636 0.15 46882.81 1784540466.34 605.41 3381345 3879161 0.00 2260.97 53178395.68 13.71 171 tick_broadcast_lock: 346460 346717 0.18 2257.43 39364622.71 113.54 3642919 4242696 0.00 2263.79 49173646.60 11.59 172 &mapping->i_mmap_mutex: 203896 203899 3.36 645530.05 31767507988.39 155800.21 3361776 8893984 0.17 2254.15 14110121.02 1.59 173 &rq->lock: 135014 136909 0.18 606.09 842160.68 6.15 1540728 10436146 0.00 728.72 17606683.41 1.69 174 &(&zone->lru_lock)->rlock: 93000 94934 0.16 59.18 188253.78 1.98 1199912 3809894 0.15 391.40 3559518.81 0.93 175 tasklist_lock-W: 40667 41130 0.23 1189.42 428980.51 10.43 270278 510106 0.16 653.51 3939674.91 7.72 176 tasklist_lock-R: 21298 21305 0.20 1310.05 215511.12 10.12 186204 241258 0.14 1162.33 1179779.23 4.89 177 rcu_node_1: 47656 49022 0.16 635.41 193616.41 3.95 844888 1865423 0.00 764.26 1656226.96 0.89 178 &(&dentry->d_lockref.lock)->rlock: 39791 40179 0.15 1302.08 88851.96 2.21 2790851 12527025 0.10 1910.75 3379714.27 0.27 179 rcu_node_0: 29203 30064 0.16 786.55 1555573.00 51.74 88963 244254 0.00 398.87 428872.51 1.76 180 181Clear the statistics: 182 183# echo 0 > /proc/lock_stat 184