lockstat: documentation (a560aa48) · Commits · e / devices / android_kernel_fairphone_FP3

Documentation/lockstat.txt

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		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. 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
		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

		From these number various other statistics can be derived, such as:

		hold time average = hold time total / acquisitions

		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.

		- USAGE

		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 0.2
		02 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
		03 class name con-bounces contentions waittime-min waittime-max waittime-total acq-bounces acquisitions holdtime-min holdtime-max holdtime-total
		04 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
		05
		06 &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60
		07 &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38
		08 --------------------------
		09 &inode->i_data.tree_lock 0 [<ffffffff8027c08f>] add_to_page_cache+0x5f/0x190
		10
		11 ...............................................................................................................................................................................................
		12
		13 dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24
		14 -----------
		15 dcache_lock 180 [<ffffffff802c0d7e>] sys_getcwd+0x11e/0x230
		16 dcache_lock 165 [<ffffffff802c002a>] d_alloc+0x15a/0x210
		17 dcache_lock 33 [<ffffffff8035818d>] _atomic_dec_and_lock+0x4d/0x70
		18 dcache_lock 1 [<ffffffff802beef8>] shrink_dcache_parent+0x18/0x130

		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 05-10 and 13-18 show the actual
		statistics. These statistics come in two parts; the actual stats separated by a
		short separator (line 08, 14) from the contention points.

		The first lock (05-10) 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.


		View the top contending locks:

		# grep : /proc/lock_stat \| head
		&inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60
		&inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38
		dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24
		&inode->i_mutex: 161 286 18446744073709 62882.54 1244614.55 3653 20598 18446744073709 62318.60 1693822.74
		&zone->lru_lock: 94 94 0.53 7.33 92.10 4366 32690 0.29 59.81 16350.06
		&inode->i_data.i_mmap_lock: 79 79 0.40 3.77 53.03 11779 87755 0.28 116.93 29898.44
		&q->__queue_lock: 48 50 0.52 31.62 86.31 774 13131 0.17 113.08 12277.52
		&rq->rq_lock_key: 43 47 0.74 68.50 170.63 3706 33929 0.22 107.99 17460.62
		&rq->rq_lock_key#2: 39 46 0.75 6.68 49.03 2979 32292 0.17 125.17 17137.63
		tasklist_lock-W: 15 15 1.45 10.87 32.70 1201 7390 0.58 62.55 13648.47

		Clear the statistics:

		# echo 0 > /proc/lock_stat

lib/Kconfig.debug

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Original line number	Diff line number	Diff line
		@@ -294,6 +294,8 @@ config LOCK_STAT
		help
		This feature enables tracking lock contention points

		For more details, see Documentation/lockstat.txt

		config DEBUG_LOCKDEP
		bool "Lock dependency engine debugging"
		depends on DEBUG_KERNEL && LOCKDEP