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 | | | | _______/ |/ | __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 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 [] do_page_fault+0x466/0x928 10 &mm->mmap_sem 179 [] sys_mprotect+0xcd/0x21d 11 &mm->mmap_sem 279 [] sys_mmap+0x75/0xce 12 &mm->mmap_sem 76 [] sys_munmap+0x32/0x59 13 --------------- 14 &mm->mmap_sem 270 [] sys_mmap+0x75/0xce 15 &mm->mmap_sem 431 [] do_page_fault+0x466/0x928 16 &mm->mmap_sem 138 [] sys_munmap+0x32/0x59 17 &mm->mmap_sem 145 [] 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 [] _atomic_dec_and_lock+0x34/0x54 24 dcache_lock 113 [] d_alloc+0x19a/0x1eb 25 dcache_lock 99 [] d_rehash+0x1b/0x44 26 dcache_lock 104 [] d_instantiate+0x36/0x8a 27 ----------- 28 dcache_lock 192 [] _atomic_dec_and_lock+0x34/0x54 29 dcache_lock 98 [] d_rehash+0x1b/0x44 30 dcache_lock 72 [] d_alloc+0x19a/0x1eb 31 dcache_lock 112 [] 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 [] 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 [] 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: 11526 11488 0.33 388.73 136294.31 21461 38404 0.00 37.93 109388.53 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 &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