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/locking/lockdep-design.txt). The graph below shows the relation between the lock functions and the various hooks therein. __acquire | lock _____ | \ | __contended | | | | _______/ |/ | __acquired | . . | __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 avg - average time spent 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 avg - average time this lock was held 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_STAT. - 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 0.4 02----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 03 class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg 04----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 05 06 &mm->mmap_sem-W: 46 84 0.26 939.10 16371.53 194.90 47291 2922365 0.16 2220301.69 17464026916.32 5975.99 07 &mm->mmap_sem-R: 37 100 1.31 299502.61 325629.52 3256.30 212344 34316685 0.10 7744.91 95016910.20 2.77 08 --------------- 09 &mm->mmap_sem 1 [] khugepaged_scan_mm_slot+0x57/0x280 10 &mm->mmap_sem 96 [] __do_page_fault+0x1d4/0x510 11 &mm->mmap_sem 34 [] vm_mmap_pgoff+0x87/0xd0 12 &mm->mmap_sem 17 [] vm_munmap+0x41/0x80 13 --------------- 14 &mm->mmap_sem 1 [] dup_mmap+0x2a/0x3f0 15 &mm->mmap_sem 60 [] SyS_mprotect+0xe9/0x250 16 &mm->mmap_sem 41 [] __do_page_fault+0x1d4/0x510 17 &mm->mmap_sem 68 [] vm_mmap_pgoff+0x87/0xd0 18 19............................................................................................................................................................................................................................. 20 21 unix_table_lock: 110 112 0.21 49.24 163.91 1.46 21094 66312 0.12 624.42 31589.81 0.48 22 --------------- 23 unix_table_lock 45 [] unix_create1+0x16e/0x1b0 24 unix_table_lock 47 [] unix_release_sock+0x31/0x250 25 unix_table_lock 15 [] unix_find_other+0x117/0x230 26 unix_table_lock 5 [] unix_autobind+0x11f/0x1b0 27 --------------- 28 unix_table_lock 39 [] unix_release_sock+0x31/0x250 29 unix_table_lock 49 [] unix_create1+0x16e/0x1b0 30 unix_table_lock 20 [] unix_find_other+0x117/0x230 31 unix_table_lock 4 [] unix_autobind+0x11f/0x1b0 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. Lines 09-12 show the first 4 recorded contention points (the code which tries to get the lock) and lines 14-17 show the first 4 recorded contended points (the lock holder). It is possible that the max con-bounces point is missing in the statistics. 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 [] 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 7.91 97454 3453404 0.00 401.11 13224683.11 3.82 35 --------- 36 &rq->lock 645 [] task_rq_lock+0x43/0x75 37 &rq->lock 297 [] try_to_wake_up+0x127/0x25a 38 &rq->lock 360 [] select_task_rq_fair+0x1f0/0x74a 39 &rq->lock 428 [] scheduler_tick+0x46/0x1fb 40 --------- 41 &rq->lock 77 [] task_rq_lock+0x43/0x75 42 &rq->lock 174 [] try_to_wake_up+0x127/0x25a 43 &rq->lock 4715 [] double_rq_lock+0x42/0x54 44 &rq->lock 893 [] schedule+0x157/0x7b8 45 46........................................................................................................................................................................................................................... 47 48 &rq->lock/1: 1526 11488 0.33 388.73 136294.31 11.86 21461 38404 0.00 37.93 109388.53 2.84 49 ----------- 50 &rq->lock/1 11526 [] double_rq_lock+0x4f/0x54 51 ----------- 52 &rq->lock/1 5645 [] double_rq_lock+0x42/0x54 53 &rq->lock/1 1224 [] schedule+0x157/0x7b8 54 &rq->lock/1 4336 [] double_rq_lock+0x4f/0x54 55 &rq->lock/1 181 [] 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 clockevents_lock: 2926159 2947636 0.15 46882.81 1784540466.34 605.41 3381345 3879161 0.00 2260.97 53178395.68 13.71 tick_broadcast_lock: 346460 346717 0.18 2257.43 39364622.71 113.54 3642919 4242696 0.00 2263.79 49173646.60 11.59 &mapping->i_mmap_mutex: 203896 203899 3.36 645530.05 31767507988.39 155800.21 3361776 8893984 0.17 2254.15 14110121.02 1.59 &rq->lock: 135014 136909 0.18 606.09 842160.68 6.15 1540728 10436146 0.00 728.72 17606683.41 1.69 &(&zone->lru_lock)->rlock: 93000 94934 0.16 59.18 188253.78 1.98 1199912 3809894 0.15 391.40 3559518.81 0.93 tasklist_lock-W: 40667 41130 0.23 1189.42 428980.51 10.43 270278 510106 0.16 653.51 3939674.91 7.72 tasklist_lock-R: 21298 21305 0.20 1310.05 215511.12 10.12 186204 241258 0.14 1162.33 1179779.23 4.89 rcu_node_1: 47656 49022 0.16 635.41 193616.41 3.95 844888 1865423 0.00 764.26 1656226.96 0.89 &(&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 rcu_node_0: 29203 30064 0.16 786.55 1555573.00 51.74 88963 244254 0.00 398.87 428872.51 1.76 Clear the statistics: # echo 0 > /proc/lock_stat