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tag: v2.6.25-rc5
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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
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