[PATCH] Documentation: statistics about nested locks

[Juri Lelli <juri.lelli@gmail.com>]

Hi all,
just a little patch to the Documentation. I had some trouble 
understanding the trailing "/1" on some lock class names of lock_stat 
output, so I added something on this inside lockstat documentation.

Signed-off-by: Juri Lelli <juri.lelli@gmail.com>

---
  Documentation/lockstat.txt |   36 ++++++++++++++++++++++++++++++++++--
  1 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/Documentation/lockstat.txt b/Documentation/lockstat.txt
index 65f4c79..75eeb65 100644
--- a/Documentation/lockstat.txt
+++ b/Documentation/lockstat.txt
@@ -12,8 +12,9 @@ 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.
+lock classes. We build on that (see Documentation/lockdep-design.txt).
+The graph below shows the relation between the lock functions and the 
various
+hooks therein.

          __acquire
              |
@@ -128,6 +129,37 @@ points are the points we're contending with.

  The integer part of the time values is in us.

+Dealing with nested locks, subclasses may appear:
+
+32...............................................................................................................................................................................................
+33
+34                               &rq->lock:         13128 
13128           0.43         190.53      103881.26          97454 
  3453404           0.00         401.11    13224683.11
+35                               ---------
+36                               &rq->lock            645 
[<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
+37                               &rq->lock            297 
[<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
+38                               &rq->lock            360 
[<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a
+39                               &rq->lock            428 
[<ffffffff81045f98>] scheduler_tick+0x46/0x1fb
+40                               ---------
+41                               &rq->lock             77 
[<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
+42                               &rq->lock            174 
[<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
+43                               &rq->lock           4715 
[<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
+44                               &rq->lock            893 
[<ffffffff81340524>] schedule+0x157/0x7b8
+45
+46...............................................................................................................................................................................................
+47
+48                             &rq->lock/1:         11526 
11488           0.33         388.73      136294.31          21461 
    38404           0.00          37.93      109388.53
+49                             -----------
+50                             &rq->lock/1          11526 
[<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
+51                             -----------
+52                             &rq->lock/1           5645 
[<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
+53                             &rq->lock/1           1224 
[<ffffffff81340524>] schedule+0x157/0x7b8
+54                             &rq->lock/1           4336 
[<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
+55                             &rq->lock/1            181 
[<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
+
+Line 48 shows statistics for the first subclass (/1) of &rq->lock 
class, since
+in this case, as line 50 suggests, double_rq_lock actually acquires a 
nested
+lock of two spinlocks.
+
  View the top contending locks:

  # grep : /proc/lock_stat | head
-- 
1.7.4.1