===============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 (see Documentation/locking/lockdep-design.rst).The graph below shows the relation between the lock functions and the varioushooks 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 (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_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 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...........................................................................................................................................................................................................................  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