Documentation: Add statistics about nested locks

- 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

            |

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 second subclass (/1) of &rq->lock class

(subclass starts from 0), 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