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Documentation / lockstat.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.

1	
2	LOCK STATISTICS
3	
4	- WHAT
5	
6	As the name suggests, it provides statistics on locks.
7	
8	- WHY
9	
10	Because things like lock contention can severely impact performance.
11	
12	- HOW
13	
14	Lockdep already has hooks in the lock functions and maps lock instances to
15	lock classes. We build on that (see Documentation/lockdep-design.txt).
16	The graph below shows the relation between the lock functions and the various
17	hooks therein.
18	
19	        __acquire
20	            |
21	           lock _____
22	            |        \
23	            |    __contended
24	            |         |
25	            |       <wait>
26	            | _______/
27	            |/
28	            |
29	       __acquired
30	            |
31	            .
32	          <hold>
33	            .
34	            |
35	       __release
36	            |
37	         unlock
38	
39	lock, unlock	- the regular lock functions
40	__*		- the hooks
41	<> 		- states
42	
43	With these hooks we provide the following statistics:
44	
45	 con-bounces       - number of lock contention that involved x-cpu data
46	 contentions       - number of lock acquisitions that had to wait
47	 wait time min     - shortest (non-0) time we ever had to wait for a lock
48	           max     - longest time we ever had to wait for a lock
49		   total   - total time we spend waiting on this lock
50		   avg     - average time spent waiting on this lock
51	 acq-bounces       - number of lock acquisitions that involved x-cpu data
52	 acquisitions      - number of times we took the lock
53	 hold time min     - shortest (non-0) time we ever held the lock
54		   max     - longest time we ever held the lock
55		   total   - total time this lock was held
56		   avg     - average time this lock was held
57	
58	These numbers are gathered per lock class, per read/write state (when
59	applicable).
60	
61	It also tracks 4 contention points per class. A contention point is a call site
62	that had to wait on lock acquisition.
63	
64	 - CONFIGURATION
65	
66	Lock statistics are enabled via CONFIG_LOCK_STAT.
67	
68	 - USAGE
69	
70	Enable collection of statistics:
71	
72	# echo 1 >/proc/sys/kernel/lock_stat
73	
74	Disable collection of statistics:
75	
76	# echo 0 >/proc/sys/kernel/lock_stat
77	
78	Look at the current lock statistics:
79	
80	( line numbers not part of actual output, done for clarity in the explanation
81	  below )
82	
83	# less /proc/lock_stat
84	
85	01 lock_stat version 0.4
86	02-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
87	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
88	04-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
89	05
90	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
91	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
92	08                         ---------------
93	09                           &mm->mmap_sem              1          [<ffffffff811502a7>] khugepaged_scan_mm_slot+0x57/0x280
94	19                           &mm->mmap_sem             96          [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510
95	11                           &mm->mmap_sem             34          [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0
96	12                           &mm->mmap_sem             17          [<ffffffff81127e71>] vm_munmap+0x41/0x80
97	13                         ---------------
98	14                           &mm->mmap_sem              1          [<ffffffff81046fda>] dup_mmap+0x2a/0x3f0
99	15                           &mm->mmap_sem             60          [<ffffffff81129e29>] SyS_mprotect+0xe9/0x250
100	16                           &mm->mmap_sem             41          [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510
101	17                           &mm->mmap_sem             68          [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0
102	18
103	19.............................................................................................................................................................................................................................
104	20
105	21                         unix_table_lock:           110            112           0.21          49.24         163.91           1.46          21094          66312           0.12         624.42       31589.81           0.48
106	22                         ---------------
107	23                         unix_table_lock             45          [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0
108	24                         unix_table_lock             47          [<ffffffff8150b111>] unix_release_sock+0x31/0x250
109	25                         unix_table_lock             15          [<ffffffff8150ca37>] unix_find_other+0x117/0x230
110	26                         unix_table_lock              5          [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0
111	27                         ---------------
112	28                         unix_table_lock             39          [<ffffffff8150b111>] unix_release_sock+0x31/0x250
113	29                         unix_table_lock             49          [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0
114	30                         unix_table_lock             20          [<ffffffff8150ca37>] unix_find_other+0x117/0x230
115	31                         unix_table_lock              4          [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0
116	
117	
118	This excerpt shows the first two lock class statistics. Line 01 shows the
119	output version - each time the format changes this will be updated. Line 02-04
120	show the header with column descriptions. Lines 05-18 and 20-31 show the actual
121	statistics. These statistics come in two parts; the actual stats separated by a
122	short separator (line 08, 13) from the contention points.
123	
124	The first lock (05-18) is a read/write lock, and shows two lines above the
125	short separator. The contention points don't match the column descriptors,
126	they have two: contentions and [<IP>] symbol. The second set of contention
127	points are the points we're contending with.
128	
129	The integer part of the time values is in us.
130	
131	Dealing with nested locks, subclasses may appear:
132	
133	32...........................................................................................................................................................................................................................
134	33
135	34                               &rq->lock:       13128          13128           0.43         190.53      103881.26           7.91          97454        3453404           0.00         401.11    13224683.11           3.82
136	35                               ---------
137	36                               &rq->lock          645          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
138	37                               &rq->lock          297          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
139	38                               &rq->lock          360          [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a
140	39                               &rq->lock          428          [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb
141	40                               ---------
142	41                               &rq->lock           77          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
143	42                               &rq->lock          174          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
144	43                               &rq->lock         4715          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
145	44                               &rq->lock          893          [<ffffffff81340524>] schedule+0x157/0x7b8
146	45
147	46...........................................................................................................................................................................................................................
148	47
149	48                             &rq->lock/1:        1526          11488           0.33         388.73      136294.31          11.86          21461          38404           0.00          37.93      109388.53           2.84
150	49                             -----------
151	50                             &rq->lock/1        11526          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
152	51                             -----------
153	52                             &rq->lock/1         5645          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
154	53                             &rq->lock/1         1224          [<ffffffff81340524>] schedule+0x157/0x7b8
155	54                             &rq->lock/1         4336          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
156	55                             &rq->lock/1          181          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
157	
158	Line 48 shows statistics for the second subclass (/1) of &rq->lock class
159	(subclass starts from 0), since in this case, as line 50 suggests,
160	double_rq_lock actually acquires a nested lock of two spinlocks.
161	
162	View the top contending locks:
163	
164	# grep : /proc/lock_stat | head
165				clockevents_lock:       2926159        2947636           0.15       46882.81  1784540466.34         605.41        3381345        3879161           0.00        2260.97    53178395.68          13.71
166			     tick_broadcast_lock:        346460         346717           0.18        2257.43    39364622.71         113.54        3642919        4242696           0.00        2263.79    49173646.60          11.59
167			  &mapping->i_mmap_mutex:        203896         203899           3.36      645530.05 31767507988.39      155800.21        3361776        8893984           0.17        2254.15    14110121.02           1.59
168				       &rq->lock:        135014         136909           0.18         606.09      842160.68           6.15        1540728       10436146           0.00         728.72    17606683.41           1.69
169		       &(&zone->lru_lock)->rlock:         93000          94934           0.16          59.18      188253.78           1.98        1199912        3809894           0.15         391.40     3559518.81           0.93
170				 tasklist_lock-W:         40667          41130           0.23        1189.42      428980.51          10.43         270278         510106           0.16         653.51     3939674.91           7.72
171				 tasklist_lock-R:         21298          21305           0.20        1310.05      215511.12          10.12         186204         241258           0.14        1162.33     1179779.23           4.89
172				      rcu_node_1:         47656          49022           0.16         635.41      193616.41           3.95         844888        1865423           0.00         764.26     1656226.96           0.89
173	       &(&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
174				      rcu_node_0:         29203          30064           0.16         786.55     1555573.00          51.74          88963         244254           0.00         398.87      428872.51           1.76
175	
176	Clear the statistics:
177	
178	# echo 0 > /proc/lock_stat
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