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. The graph below shows the relation between 16the lock functions and the various hooks therein. 17 18 __acquire 19 | 20 lock _____ 21 | \ 22 | __contended 23 | | 24 | <wait> 25 | _______/ 26 |/ 27 | 28 __acquired 29 | 30 . 31 <hold> 32 . 33 | 34 __release 35 | 36 unlock 37 38lock, unlock - the regular lock functions 39__* - the hooks 40<> - states 41 42With these hooks we provide the following statistics: 43 44 con-bounces - number of lock contention that involved x-cpu data 45 contentions - number of lock acquisitions that had to wait 46 wait time min - shortest (non-0) time we ever had to wait for a lock 47 max - longest time we ever had to wait for a lock 48 total - total time we spend waiting on this lock 49 acq-bounces - number of lock acquisitions that involved x-cpu data 50 acquisitions - number of times we took the lock 51 hold time min - shortest (non-0) time we ever held the lock 52 max - longest time we ever held the lock 53 total - total time this lock was held 54 55From these number various other statistics can be derived, such as: 56 57 hold time average = hold time total / acquisitions 58 59These numbers are gathered per lock class, per read/write state (when 60applicable). 61 62It also tracks 4 contention points per class. A contention point is a call site 63that had to wait on lock acquisition. 64 65 - USAGE 66 67Look at the current lock statistics: 68 69( line numbers not part of actual output, done for clarity in the explanation 70 below ) 71 72# less /proc/lock_stat 73 7401 lock_stat version 0.3 7502 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 7603 class name con-bounces contentions waittime-min waittime-max waittime-total acq-bounces acquisitions holdtime-min holdtime-max holdtime-total 7704 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 7805 7906 &mm->mmap_sem-W: 233 538 18446744073708 22924.27 607243.51 1342 45806 1.71 8595.89 1180582.34 8007 &mm->mmap_sem-R: 205 587 18446744073708 28403.36 731975.00 1940 412426 0.58 187825.45 6307502.88 8108 --------------- 8209 &mm->mmap_sem 487 [<ffffffff8053491f>] do_page_fault+0x466/0x928 8310 &mm->mmap_sem 179 [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d 8411 &mm->mmap_sem 279 [<ffffffff80210a57>] sys_mmap+0x75/0xce 8512 &mm->mmap_sem 76 [<ffffffff802a490b>] sys_munmap+0x32/0x59 8613 --------------- 8714 &mm->mmap_sem 270 [<ffffffff80210a57>] sys_mmap+0x75/0xce 8815 &mm->mmap_sem 431 [<ffffffff8053491f>] do_page_fault+0x466/0x928 8916 &mm->mmap_sem 138 [<ffffffff802a490b>] sys_munmap+0x32/0x59 9017 &mm->mmap_sem 145 [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d 9118 9219 ............................................................................................................................................................................................... 9320 9421 dcache_lock: 621 623 0.52 118.26 1053.02 6745 91930 0.29 316.29 118423.41 9522 ----------- 9623 dcache_lock 179 [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54 9724 dcache_lock 113 [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb 9825 dcache_lock 99 [<ffffffff802ca0dc>] d_rehash+0x1b/0x44 9926 dcache_lock 104 [<ffffffff802cbca0>] d_instantiate+0x36/0x8a 10027 ----------- 10128 dcache_lock 192 [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54 10229 dcache_lock 98 [<ffffffff802ca0dc>] d_rehash+0x1b/0x44 10330 dcache_lock 72 [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb 10431 dcache_lock 112 [<ffffffff802cbca0>] d_instantiate+0x36/0x8a 105 106This excerpt shows the first two lock class statistics. Line 01 shows the 107output version - each time the format changes this will be updated. Line 02-04 108show the header with column descriptions. Lines 05-18 and 20-31 show the actual 109statistics. These statistics come in two parts; the actual stats separated by a 110short separator (line 08, 13) from the contention points. 111 112The first lock (05-18) is a read/write lock, and shows two lines above the 113short separator. The contention points don't match the column descriptors, 114they have two: contentions and [<IP>] symbol. The second set of contention 115points are the points we're contending with. 116 117The integer part of the time values is in us. 118 119View the top contending locks: 120 121# grep : /proc/lock_stat | head 122 &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60 123 &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38 124 dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24 125 &inode->i_mutex: 161 286 18446744073709 62882.54 1244614.55 3653 20598 18446744073709 62318.60 1693822.74 126 &zone->lru_lock: 94 94 0.53 7.33 92.10 4366 32690 0.29 59.81 16350.06 127 &inode->i_data.i_mmap_lock: 79 79 0.40 3.77 53.03 11779 87755 0.28 116.93 29898.44 128 &q->__queue_lock: 48 50 0.52 31.62 86.31 774 13131 0.17 113.08 12277.52 129 &rq->rq_lock_key: 43 47 0.74 68.50 170.63 3706 33929 0.22 107.99 17460.62 130 &rq->rq_lock_key#2: 39 46 0.75 6.68 49.03 2979 32292 0.17 125.17 17137.63 131 tasklist_lock-W: 15 15 1.45 10.87 32.70 1201 7390 0.58 62.55 13648.47 132 133Clear the statistics: 134 135# echo 0 > /proc/lock_stat 136