tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1
2LOCK STATISTICS
3
4- WHAT
5
6As the name suggests, it provides statistics on locks.
7
8- WHY
9
10Because things like lock contention can severely impact performance.
11
12- HOW
13
14Lockdep already has hooks in the lock functions and maps lock instances to
15lock classes. We build on that. The graph below shows the relation between
16the lock functions and the various hooks therein.
17
18 __acquire
19 |
20 lock _____
21 | \
22 | __contended
23 | |
24 | <wait>
25 | _______/
26 |/
27 |
28 __acquired
29 |
30 .
31 <hold>
32 .
33 |
34 __release
35 |
36 unlock
37
38lock, unlock - the regular lock functions
39__* - the hooks
40<> - states
41
42With these hooks we provide the following statistics:
43
44 con-bounces - number of lock contention that involved x-cpu data
45 contentions - number of lock acquisitions that had to wait
46 wait time min - shortest (non-0) time we ever had to wait for a lock
47 max - longest time we ever had to wait for a lock
48 total - total time we spend waiting on this lock
49 acq-bounces - number of lock acquisitions that involved x-cpu data
50 acquisitions - number of times we took the lock
51 hold time min - shortest (non-0) time we ever held the lock
52 max - longest time we ever held the lock
53 total - total time this lock was held
54
55From these number various other statistics can be derived, such as:
56
57 hold time average = hold time total / acquisitions
58
59These numbers are gathered per lock class, per read/write state (when
60applicable).
61
62It also tracks 4 contention points per class. A contention point is a call site
63that had to wait on lock acquisition.
64
65 - USAGE
66
67Look at the current lock statistics:
68
69( line numbers not part of actual output, done for clarity in the explanation
70 below )
71
72# less /proc/lock_stat
73
7401 lock_stat version 0.2
7502 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
7603 class name con-bounces contentions waittime-min waittime-max waittime-total acq-bounces acquisitions holdtime-min holdtime-max holdtime-total
7704 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
7805
7906 &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60
8007 &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38
8108 --------------------------
8209 &inode->i_data.tree_lock 0 [<ffffffff8027c08f>] add_to_page_cache+0x5f/0x190
8310
8411 ...............................................................................................................................................................................................
8512
8613 dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24
8714 -----------
8815 dcache_lock 180 [<ffffffff802c0d7e>] sys_getcwd+0x11e/0x230
8916 dcache_lock 165 [<ffffffff802c002a>] d_alloc+0x15a/0x210
9017 dcache_lock 33 [<ffffffff8035818d>] _atomic_dec_and_lock+0x4d/0x70
9118 dcache_lock 1 [<ffffffff802beef8>] shrink_dcache_parent+0x18/0x130
92
93This excerpt shows the first two lock class statistics. Line 01 shows the
94output version - each time the format changes this will be updated. Line 02-04
95show the header with column descriptions. Lines 05-10 and 13-18 show the actual
96statistics. These statistics come in two parts; the actual stats separated by a
97short separator (line 08, 14) from the contention points.
98
99The first lock (05-10) is a read/write lock, and shows two lines above the
100short separator. The contention points don't match the column descriptors,
101they have two: contentions and [<IP>] symbol.
102
103
104View the top contending locks:
105
106# grep : /proc/lock_stat | head
107 &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60
108 &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38
109 dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24
110 &inode->i_mutex: 161 286 18446744073709 62882.54 1244614.55 3653 20598 18446744073709 62318.60 1693822.74
111 &zone->lru_lock: 94 94 0.53 7.33 92.10 4366 32690 0.29 59.81 16350.06
112 &inode->i_data.i_mmap_lock: 79 79 0.40 3.77 53.03 11779 87755 0.28 116.93 29898.44
113 &q->__queue_lock: 48 50 0.52 31.62 86.31 774 13131 0.17 113.08 12277.52
114 &rq->rq_lock_key: 43 47 0.74 68.50 170.63 3706 33929 0.22 107.99 17460.62
115 &rq->rq_lock_key#2: 39 46 0.75 6.68 49.03 2979 32292 0.17 125.17 17137.63
116 tasklist_lock-W: 15 15 1.45 10.87 32.70 1201 7390 0.58 62.55 13648.47
117
118Clear the statistics:
119
120# echo 0 > /proc/lock_stat