Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/hid/hid
[sfrench/cifs-2.6.git] / Documentation / locking / lockstat.txt
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/locking/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 10                           &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 Lines 09-12 show the first 4 recorded contention points (the code
125 which tries to get the lock) and lines 14-17 show the first 4 recorded
126 contended points (the lock holder). It is possible that the max
127 con-bounces point is missing in the statistics.
128
129 The first lock (05-18) is a read/write lock, and shows two lines above the
130 short separator. The contention points don't match the column descriptors,
131 they have two: contentions and [<IP>] symbol. The second set of contention
132 points are the points we're contending with.
133
134 The integer part of the time values is in us.
135
136 Dealing with nested locks, subclasses may appear:
137
138 32...........................................................................................................................................................................................................................
139 33
140 34                               &rq->lock:       13128          13128           0.43         190.53      103881.26           7.91          97454        3453404           0.00         401.11    13224683.11           3.82
141 35                               ---------
142 36                               &rq->lock          645          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
143 37                               &rq->lock          297          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
144 38                               &rq->lock          360          [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a
145 39                               &rq->lock          428          [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb
146 40                               ---------
147 41                               &rq->lock           77          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
148 42                               &rq->lock          174          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
149 43                               &rq->lock         4715          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
150 44                               &rq->lock          893          [<ffffffff81340524>] schedule+0x157/0x7b8
151 45
152 46...........................................................................................................................................................................................................................
153 47
154 48                             &rq->lock/1:        1526          11488           0.33         388.73      136294.31          11.86          21461          38404           0.00          37.93      109388.53           2.84
155 49                             -----------
156 50                             &rq->lock/1        11526          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
157 51                             -----------
158 52                             &rq->lock/1         5645          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
159 53                             &rq->lock/1         1224          [<ffffffff81340524>] schedule+0x157/0x7b8
160 54                             &rq->lock/1         4336          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
161 55                             &rq->lock/1          181          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
162
163 Line 48 shows statistics for the second subclass (/1) of &rq->lock class
164 (subclass starts from 0), since in this case, as line 50 suggests,
165 double_rq_lock actually acquires a nested lock of two spinlocks.
166
167 View the top contending locks:
168
169 # grep : /proc/lock_stat | head
170                         clockevents_lock:       2926159        2947636           0.15       46882.81  1784540466.34         605.41        3381345        3879161           0.00        2260.97    53178395.68          13.71
171                      tick_broadcast_lock:        346460         346717           0.18        2257.43    39364622.71         113.54        3642919        4242696           0.00        2263.79    49173646.60          11.59
172                   &mapping->i_mmap_mutex:        203896         203899           3.36      645530.05 31767507988.39      155800.21        3361776        8893984           0.17        2254.15    14110121.02           1.59
173                                &rq->lock:        135014         136909           0.18         606.09      842160.68           6.15        1540728       10436146           0.00         728.72    17606683.41           1.69
174                &(&zone->lru_lock)->rlock:         93000          94934           0.16          59.18      188253.78           1.98        1199912        3809894           0.15         391.40     3559518.81           0.93
175                          tasklist_lock-W:         40667          41130           0.23        1189.42      428980.51          10.43         270278         510106           0.16         653.51     3939674.91           7.72
176                          tasklist_lock-R:         21298          21305           0.20        1310.05      215511.12          10.12         186204         241258           0.14        1162.33     1179779.23           4.89
177                               rcu_node_1:         47656          49022           0.16         635.41      193616.41           3.95         844888        1865423           0.00         764.26     1656226.96           0.89
178        &(&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
179                               rcu_node_0:         29203          30064           0.16         786.55     1555573.00          51.74          88963         244254           0.00         398.87      428872.51           1.76
180
181 Clear the statistics:
182
183 # echo 0 > /proc/lock_stat