| =============== | |
| 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.rst). | |
| The graph below shows the relation between the lock functions and the various | |
| hooks therein:: | |
| __acquire | |
| | | |
| lock _____ | |
| | \ | |
| | __contended | |
| | | | |
| | <wait> | |
| | _______/ | |
| |/ | |
| | | |
| __acquired | |
| | | |
| . | |
| <hold> | |
| . | |
| | | |
| __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 [<ffffffff811502a7>] khugepaged_scan_mm_slot+0x57/0x280 | |
| 10 &mm->mmap_sem 96 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510 | |
| 11 &mm->mmap_sem 34 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0 | |
| 12 &mm->mmap_sem 17 [<ffffffff81127e71>] vm_munmap+0x41/0x80 | |
| 13 --------------- | |
| 14 &mm->mmap_sem 1 [<ffffffff81046fda>] dup_mmap+0x2a/0x3f0 | |
| 15 &mm->mmap_sem 60 [<ffffffff81129e29>] SyS_mprotect+0xe9/0x250 | |
| 16 &mm->mmap_sem 41 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510 | |
| 17 &mm->mmap_sem 68 [<ffffffff81113d77>] 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 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0 | |
| 24 unix_table_lock 47 [<ffffffff8150b111>] unix_release_sock+0x31/0x250 | |
| 25 unix_table_lock 15 [<ffffffff8150ca37>] unix_find_other+0x117/0x230 | |
| 26 unix_table_lock 5 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0 | |
| 27 --------------- | |
| 28 unix_table_lock 39 [<ffffffff8150b111>] unix_release_sock+0x31/0x250 | |
| 29 unix_table_lock 49 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0 | |
| 30 unix_table_lock 20 [<ffffffff8150ca37>] unix_find_other+0x117/0x230 | |
| 31 unix_table_lock 4 [<ffffffff8150a09f>] 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 [<IP>] 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 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75 | |
| 37 &rq->lock 297 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a | |
| 38 &rq->lock 360 [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a | |
| 39 &rq->lock 428 [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb | |
| 40 --------- | |
| 41 &rq->lock 77 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75 | |
| 42 &rq->lock 174 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a | |
| 43 &rq->lock 4715 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54 | |
| 44 &rq->lock 893 [<ffffffff81340524>] 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 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54 | |
| 51 ----------- | |
| 52 &rq->lock/1 5645 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54 | |
| 53 &rq->lock/1 1224 [<ffffffff81340524>] schedule+0x157/0x7b8 | |
| 54 &rq->lock/1 4336 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54 | |
| 55 &rq->lock/1 181 [<ffffffff8104ba65>] 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 |