205 lines
12 KiB
ReStructuredText
205 lines
12 KiB
ReStructuredText
===============
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Lock Statistics
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===============
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What
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====
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As the name suggests, it provides statistics on locks.
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Why
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===
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Because things like lock contention can severely impact performance.
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How
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===
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Lockdep already has hooks in the lock functions and maps lock instances to
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lock classes. We build on that (see Documentation/locking/lockdep-design.rst).
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The graph below shows the relation between the lock functions and the various
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hooks therein::
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__acquire
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lock _____
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| \
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| __contended
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| <wait>
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| _______/
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|/
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__acquired
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.
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<hold>
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.
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__release
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unlock
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lock, unlock - the regular lock functions
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__* - the hooks
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<> - states
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With these hooks we provide the following statistics:
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con-bounces
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- number of lock contention that involved x-cpu data
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contentions
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- number of lock acquisitions that had to wait
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wait time
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min
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- shortest (non-0) time we ever had to wait for a lock
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max
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- longest time we ever had to wait for a lock
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total
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- total time we spend waiting on this lock
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avg
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- average time spent waiting on this lock
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acq-bounces
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- number of lock acquisitions that involved x-cpu data
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acquisitions
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- number of times we took the lock
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hold time
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min
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- shortest (non-0) time we ever held the lock
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max
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- longest time we ever held the lock
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total
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- total time this lock was held
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avg
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- average time this lock was held
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These numbers are gathered per lock class, per read/write state (when
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applicable).
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It also tracks 4 contention points per class. A contention point is a call site
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that had to wait on lock acquisition.
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Configuration
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-------------
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Lock statistics are enabled via CONFIG_LOCK_STAT.
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Usage
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-----
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Enable collection of statistics::
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# echo 1 >/proc/sys/kernel/lock_stat
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Disable collection of statistics::
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# echo 0 >/proc/sys/kernel/lock_stat
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Look at the current lock statistics::
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( line numbers not part of actual output, done for clarity in the explanation
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below )
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# less /proc/lock_stat
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01 lock_stat version 0.4
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02-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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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
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04-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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05
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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
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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
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08 ---------------
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09 &mm->mmap_sem 1 [<ffffffff811502a7>] khugepaged_scan_mm_slot+0x57/0x280
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10 &mm->mmap_sem 96 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510
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11 &mm->mmap_sem 34 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0
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12 &mm->mmap_sem 17 [<ffffffff81127e71>] vm_munmap+0x41/0x80
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13 ---------------
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14 &mm->mmap_sem 1 [<ffffffff81046fda>] dup_mmap+0x2a/0x3f0
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15 &mm->mmap_sem 60 [<ffffffff81129e29>] SyS_mprotect+0xe9/0x250
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16 &mm->mmap_sem 41 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510
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17 &mm->mmap_sem 68 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0
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18
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19.............................................................................................................................................................................................................................
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20
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21 unix_table_lock: 110 112 0.21 49.24 163.91 1.46 21094 66312 0.12 624.42 31589.81 0.48
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22 ---------------
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23 unix_table_lock 45 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0
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24 unix_table_lock 47 [<ffffffff8150b111>] unix_release_sock+0x31/0x250
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25 unix_table_lock 15 [<ffffffff8150ca37>] unix_find_other+0x117/0x230
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26 unix_table_lock 5 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0
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27 ---------------
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28 unix_table_lock 39 [<ffffffff8150b111>] unix_release_sock+0x31/0x250
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29 unix_table_lock 49 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0
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30 unix_table_lock 20 [<ffffffff8150ca37>] unix_find_other+0x117/0x230
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31 unix_table_lock 4 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0
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This excerpt shows the first two lock class statistics. Line 01 shows the
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output version - each time the format changes this will be updated. Line 02-04
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show the header with column descriptions. Lines 05-18 and 20-31 show the actual
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statistics. These statistics come in two parts; the actual stats separated by a
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short separator (line 08, 13) from the contention points.
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Lines 09-12 show the first 4 recorded contention points (the code
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which tries to get the lock) and lines 14-17 show the first 4 recorded
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contended points (the lock holder). It is possible that the max
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con-bounces point is missing in the statistics.
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The first lock (05-18) is a read/write lock, and shows two lines above the
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short separator. The contention points don't match the column descriptors,
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they have two: contentions and [<IP>] symbol. The second set of contention
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points are the points we're contending with.
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The integer part of the time values is in us.
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Dealing with nested locks, subclasses may appear::
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32...........................................................................................................................................................................................................................
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33
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34 &rq->lock: 13128 13128 0.43 190.53 103881.26 7.91 97454 3453404 0.00 401.11 13224683.11 3.82
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35 ---------
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36 &rq->lock 645 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
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37 &rq->lock 297 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
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38 &rq->lock 360 [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a
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39 &rq->lock 428 [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb
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40 ---------
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41 &rq->lock 77 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
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42 &rq->lock 174 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
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43 &rq->lock 4715 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
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44 &rq->lock 893 [<ffffffff81340524>] schedule+0x157/0x7b8
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45
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46...........................................................................................................................................................................................................................
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47
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48 &rq->lock/1: 1526 11488 0.33 388.73 136294.31 11.86 21461 38404 0.00 37.93 109388.53 2.84
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49 -----------
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50 &rq->lock/1 11526 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
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51 -----------
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52 &rq->lock/1 5645 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
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53 &rq->lock/1 1224 [<ffffffff81340524>] schedule+0x157/0x7b8
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54 &rq->lock/1 4336 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
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55 &rq->lock/1 181 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
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Line 48 shows statistics for the second subclass (/1) of &rq->lock class
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(subclass starts from 0), since in this case, as line 50 suggests,
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double_rq_lock actually acquires a nested lock of two spinlocks.
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View the top contending locks::
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# grep : /proc/lock_stat | head
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clockevents_lock: 2926159 2947636 0.15 46882.81 1784540466.34 605.41 3381345 3879161 0.00 2260.97 53178395.68 13.71
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tick_broadcast_lock: 346460 346717 0.18 2257.43 39364622.71 113.54 3642919 4242696 0.00 2263.79 49173646.60 11.59
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&mapping->i_mmap_mutex: 203896 203899 3.36 645530.05 31767507988.39 155800.21 3361776 8893984 0.17 2254.15 14110121.02 1.59
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&rq->lock: 135014 136909 0.18 606.09 842160.68 6.15 1540728 10436146 0.00 728.72 17606683.41 1.69
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&(&zone->lru_lock)->rlock: 93000 94934 0.16 59.18 188253.78 1.98 1199912 3809894 0.15 391.40 3559518.81 0.93
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tasklist_lock-W: 40667 41130 0.23 1189.42 428980.51 10.43 270278 510106 0.16 653.51 3939674.91 7.72
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tasklist_lock-R: 21298 21305 0.20 1310.05 215511.12 10.12 186204 241258 0.14 1162.33 1179779.23 4.89
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rcu_node_1: 47656 49022 0.16 635.41 193616.41 3.95 844888 1865423 0.00 764.26 1656226.96 0.89
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&(&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
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rcu_node_0: 29203 30064 0.16 786.55 1555573.00 51.74 88963 244254 0.00 398.87 428872.51 1.76
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Clear the statistics::
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# echo 0 > /proc/lock_stat
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