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. 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. - USAGE 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 0.2 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 &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60 07 &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38 08 -------------------------- 09 &inode->i_data.tree_lock 0 [] add_to_page_cache+0x5f/0x190 10 11 ............................................................................................................................................................................................... 12 13 dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24 14 ----------- 15 dcache_lock 180 [] sys_getcwd+0x11e/0x230 16 dcache_lock 165 [] d_alloc+0x15a/0x210 17 dcache_lock 33 [] _atomic_dec_and_lock+0x4d/0x70 18 dcache_lock 1 [] shrink_dcache_parent+0x18/0x130 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 05-10 and 13-18 show the actual statistics. These statistics come in two parts; the actual stats separated by a short separator (line 08, 14) from the contention points. The first lock (05-10) 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. 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_lock: 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