--- /dev/null
+
+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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