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 tolock classes. We build on that (seeRuntime locking correctness validator).The graph below shows the relation between the lock functions and the varioushooks therein:
__acquire | lock _____ | \ | __contended | | | <wait> | _______/ |/ | __acquired | . <hold> . | __release | unlocklock, 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 (whenapplicable).
It also tracks 4 contention points per class. A contention point is a call sitethat 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_stat01 lock_stat version 0.402-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------03 class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg04-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------0506 &mm->mmap_sem-W: 46 84 0.26 939.10 16371.53 194.90 47291 2922365 0.16 2220301.69 17464026916.32 5975.9907 &mm->mmap_sem-R: 37 100 1.31 299502.61 325629.52 3256.30 212344 34316685 0.10 7744.91 95016910.20 2.7708 ---------------09 &mm->mmap_sem 1 [<ffffffff811502a7>] khugepaged_scan_mm_slot+0x57/0x28010 &mm->mmap_sem 96 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x51011 &mm->mmap_sem 34 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd012 &mm->mmap_sem 17 [<ffffffff81127e71>] vm_munmap+0x41/0x8013 ---------------14 &mm->mmap_sem 1 [<ffffffff81046fda>] dup_mmap+0x2a/0x3f015 &mm->mmap_sem 60 [<ffffffff81129e29>] SyS_mprotect+0xe9/0x25016 &mm->mmap_sem 41 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x51017 &mm->mmap_sem 68 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd01819.............................................................................................................................................................................................................................2021 unix_table_lock: 110 112 0.21 49.24 163.91 1.46 21094 66312 0.12 624.42 31589.81 0.4822 ---------------23 unix_table_lock 45 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b024 unix_table_lock 47 [<ffffffff8150b111>] unix_release_sock+0x31/0x25025 unix_table_lock 15 [<ffffffff8150ca37>] unix_find_other+0x117/0x23026 unix_table_lock 5 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b027 ---------------28 unix_table_lock 39 [<ffffffff8150b111>] unix_release_sock+0x31/0x25029 unix_table_lock 49 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b030 unix_table_lock 20 [<ffffffff8150ca37>] unix_find_other+0x117/0x23031 unix_table_lock 4 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0
This excerpt shows the first two lock class statistics. Line 01 shows theoutput version - each time the format changes this will be updated. Line 02-04show the header with column descriptions. Lines 05-18 and 20-31 show the actualstatistics. These statistics come in two parts; the actual stats separated by ashort separator (line 08, 13) from the contention points.
Lines 09-12 show the first 4 recorded contention points (the codewhich tries to get the lock) and lines 14-17 show the first 4 recordedcontended points (the lock holder). It is possible that the maxcon-bounces point is missing in the statistics.
The first lock (05-18) is a read/write lock, and shows two lines above theshort separator. The contention points don’t match the column descriptors,they have two: contentions and [<IP>] symbol. The second set of contentionpoints 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...........................................................................................................................................................................................................................3334 &rq->lock: 13128 13128 0.43 190.53 103881.26 7.91 97454 3453404 0.00 401.11 13224683.11 3.8235 ---------36 &rq->lock 645 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x7537 &rq->lock 297 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a38 &rq->lock 360 [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a39 &rq->lock 428 [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb40 ---------41 &rq->lock 77 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x7542 &rq->lock 174 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a43 &rq->lock 4715 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x5444 &rq->lock 893 [<ffffffff81340524>] schedule+0x157/0x7b84546...........................................................................................................................................................................................................................4748 &rq->lock/1: 1526 11488 0.33 388.73 136294.31 11.86 21461 38404 0.00 37.93 109388.53 2.8449 -----------50 &rq->lock/1 11526 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x5451 -----------52 &rq->lock/1 5645 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x5453 &rq->lock/1 1224 [<ffffffff81340524>] schedule+0x157/0x7b854 &rq->lock/1 4336 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x5455 &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