lockstat: documentation
Provide some documentation for CONFIG_LOCK_STAT. Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Acked-by: Ingo Molnar <mingo@elte.hu> Cc: "Randy.Dunlap" <rdunlap@xenotime.net> Cc: Rob Landley <rob@landley.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Peter Zijlstra
Linus Torvalds
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Documentation/lockstat.txt
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Documentation/lockstat.txt
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LOCK STATISTICS
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- WHAT
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As the name suggests, it provides statistics on locks.
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- WHY
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Because things like lock contention can severely impact performance.
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- HOW
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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. The graph below shows the relation between
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the lock functions and the various 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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| |
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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 - number of lock contention that involved x-cpu data
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contentions - number of lock acquisitions that had to wait
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wait time min - shortest (non-0) time we ever had to wait for a lock
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max - longest time we ever had to wait for a lock
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total - total time we spend waiting on this lock
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acq-bounces - number of lock acquisitions that involved x-cpu data
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acquisitions - number of times we took the lock
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hold time min - shortest (non-0) time we ever held the lock
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max - longest time we ever held the lock
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total - total time this lock was held
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From these number various other statistics can be derived, such as:
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hold time average = hold time total / acquisitions
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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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- USAGE
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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 0.2
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02 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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03 class name con-bounces contentions waittime-min waittime-max waittime-total acq-bounces acquisitions holdtime-min holdtime-max holdtime-total
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04 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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05
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06 &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60
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07 &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38
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08 --------------------------
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09 &inode->i_data.tree_lock 0 [<ffffffff8027c08f>] add_to_page_cache+0x5f/0x190
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10
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11 ...............................................................................................................................................................................................
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12
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13 dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24
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14 -----------
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15 dcache_lock 180 [<ffffffff802c0d7e>] sys_getcwd+0x11e/0x230
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16 dcache_lock 165 [<ffffffff802c002a>] d_alloc+0x15a/0x210
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17 dcache_lock 33 [<ffffffff8035818d>] _atomic_dec_and_lock+0x4d/0x70
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18 dcache_lock 1 [<ffffffff802beef8>] shrink_dcache_parent+0x18/0x130
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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 05-10 and 13-18 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, 14) from the contention points.
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The first lock (05-10) 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.
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View the top contending locks:
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# grep : /proc/lock_stat | head
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&inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60
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&inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38
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dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24
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&inode->i_mutex: 161 286 18446744073709 62882.54 1244614.55 3653 20598 18446744073709 62318.60 1693822.74
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&zone->lru_lock: 94 94 0.53 7.33 92.10 4366 32690 0.29 59.81 16350.06
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&inode->i_data.i_mmap_lock: 79 79 0.40 3.77 53.03 11779 87755 0.28 116.93 29898.44
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&q->__queue_lock: 48 50 0.52 31.62 86.31 774 13131 0.17 113.08 12277.52
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&rq->rq_lock_key: 43 47 0.74 68.50 170.63 3706 33929 0.22 107.99 17460.62
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&rq->rq_lock_key#2: 39 46 0.75 6.68 49.03 2979 32292 0.17 125.17 17137.63
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tasklist_lock-W: 15 15 1.45 10.87 32.70 1201 7390 0.58 62.55 13648.47
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Clear the statistics:
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# echo 0 > /proc/lock_stat
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@ -294,6 +294,8 @@ config LOCK_STAT
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help
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This feature enables tracking lock contention points
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For more details, see Documentation/lockstat.txt
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config DEBUG_LOCKDEP
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bool "Lock dependency engine debugging"
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depends on DEBUG_KERNEL && LOCKDEP
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