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Documentation / lockstat.txt




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Based on kernel version 3.4. Page generated on 2012-05-21 22:07 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	 acq-bounces       - number of lock acquisitions that involved x-cpu data
51	 acquisitions      - number of times we took the lock
52	 hold time min     - shortest (non-0) time we ever held the lock
53	           max     - longest time we ever held the lock
54	           total   - total time this lock was held
55	
56	From these number various other statistics can be derived, such as:
57	
58	 hold time average = hold time total / acquisitions
59	
60	These numbers are gathered per lock class, per read/write state (when
61	applicable).
62	
63	It also tracks 4 contention points per class. A contention point is a call site
64	that had to wait on lock acquisition.
65	
66	 - CONFIGURATION
67	
68	Lock statistics are enabled via CONFIG_LOCK_STATS.
69	
70	 - USAGE
71	
72	Enable collection of statistics:
73	
74	# echo 1 >/proc/sys/kernel/lock_stat
75	
76	Disable collection of statistics:
77	
78	# echo 0 >/proc/sys/kernel/lock_stat
79	
80	Look at the current lock statistics:
81	
82	( line numbers not part of actual output, done for clarity in the explanation
83	  below )
84	
85	# less /proc/lock_stat
86	
87	01 lock_stat version 0.3
88	02 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
89	03                               class name    con-bounces    contentions   waittime-min   waittime-max waittime-total    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total
90	04 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
91	05
92	06                          &mm->mmap_sem-W:           233            538 18446744073708       22924.27      607243.51           1342          45806           1.71        8595.89     1180582.34
93	07                          &mm->mmap_sem-R:           205            587 18446744073708       28403.36      731975.00           1940         412426           0.58      187825.45     6307502.88
94	08                          ---------------
95	09                            &mm->mmap_sem            487          [<ffffffff8053491f>] do_page_fault+0x466/0x928
96	10                            &mm->mmap_sem            179          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
97	11                            &mm->mmap_sem            279          [<ffffffff80210a57>] sys_mmap+0x75/0xce
98	12                            &mm->mmap_sem             76          [<ffffffff802a490b>] sys_munmap+0x32/0x59
99	13                          ---------------
100	14                            &mm->mmap_sem            270          [<ffffffff80210a57>] sys_mmap+0x75/0xce
101	15                            &mm->mmap_sem            431          [<ffffffff8053491f>] do_page_fault+0x466/0x928
102	16                            &mm->mmap_sem            138          [<ffffffff802a490b>] sys_munmap+0x32/0x59
103	17                            &mm->mmap_sem            145          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
104	18
105	19 ...............................................................................................................................................................................................
106	20
107	21                              dcache_lock:           621            623           0.52         118.26        1053.02           6745          91930           0.29         316.29      118423.41
108	22                              -----------
109	23                              dcache_lock            179          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
110	24                              dcache_lock            113          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
111	25                              dcache_lock             99          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
112	26                              dcache_lock            104          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
113	27                              -----------
114	28                              dcache_lock            192          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
115	29                              dcache_lock             98          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
116	30                              dcache_lock             72          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
117	31                              dcache_lock            112          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
118	
119	This excerpt shows the first two lock class statistics. Line 01 shows the
120	output version - each time the format changes this will be updated. Line 02-04
121	show the header with column descriptions. Lines 05-18 and 20-31 show the actual
122	statistics. These statistics come in two parts; the actual stats separated by a
123	short separator (line 08, 13) from the contention points.
124	
125	The first lock (05-18) is a read/write lock, and shows two lines above the
126	short separator. The contention points don't match the column descriptors,
127	they have two: contentions and [<IP>] symbol. The second set of contention
128	points are the points we're contending with.
129	
130	The integer part of the time values is in us.
131	
132	Dealing with nested locks, subclasses may appear:
133	
134	32...............................................................................................................................................................................................
135	33
136	34                               &rq->lock:         13128          13128           0.43         190.53      103881.26          97454        3453404           0.00         401.11    13224683.11
137	35                               ---------
138	36                               &rq->lock            645          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
139	37                               &rq->lock            297          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
140	38                               &rq->lock            360          [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a
141	39                               &rq->lock            428          [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb
142	40                               ---------
143	41                               &rq->lock             77          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
144	42                               &rq->lock            174          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
145	43                               &rq->lock           4715          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
146	44                               &rq->lock            893          [<ffffffff81340524>] schedule+0x157/0x7b8
147	45
148	46...............................................................................................................................................................................................
149	47
150	48                             &rq->lock/1:         11526          11488           0.33         388.73      136294.31          21461          38404           0.00          37.93      109388.53
151	49                             -----------
152	50                             &rq->lock/1          11526          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
153	51                             -----------
154	52                             &rq->lock/1           5645          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
155	53                             &rq->lock/1           1224          [<ffffffff81340524>] schedule+0x157/0x7b8
156	54                             &rq->lock/1           4336          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
157	55                             &rq->lock/1            181          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
158	
159	Line 48 shows statistics for the second subclass (/1) of &rq->lock class
160	(subclass starts from 0), since in this case, as line 50 suggests,
161	double_rq_lock actually acquires a nested lock of two spinlocks.
162	
163	View the top contending locks:
164	
165	# grep : /proc/lock_stat | head
166	              &inode->i_data.tree_lock-W:            15          21657           0.18     1093295.30 11547131054.85             58          10415           0.16          87.51        6387.60
167	              &inode->i_data.tree_lock-R:             0              0           0.00           0.00           0.00          23302         231198           0.25           8.45       98023.38
168	                             dcache_lock:          1037           1161           0.38          45.32         774.51           6611         243371           0.15         306.48       77387.24
169	                         &inode->i_mutex:           161            286 18446744073709       62882.54     1244614.55           3653          20598 18446744073709       62318.60     1693822.74
170	                         &zone->lru_lock:            94             94           0.53           7.33          92.10           4366          32690           0.29          59.81       16350.06
171	              &inode->i_data.i_mmap_mutex:            79             79           0.40           3.77          53.03          11779          87755           0.28         116.93       29898.44
172	                        &q->__queue_lock:            48             50           0.52          31.62          86.31            774          13131           0.17         113.08       12277.52
173	                        &rq->rq_lock_key:            43             47           0.74          68.50         170.63           3706          33929           0.22         107.99       17460.62
174	                      &rq->rq_lock_key#2:            39             46           0.75           6.68          49.03           2979          32292           0.17         125.17       17137.63
175	                         tasklist_lock-W:            15             15           1.45          10.87          32.70           1201           7390           0.58          62.55       13648.47
176	
177	Clear the statistics:
178	
179	# echo 0 > /proc/lock_stat
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