Re: [PATCH 1/3] lockdep: Move lock events under lockdep recursionprotection

[Hitoshi Mitake]

On 03/03/10 15:55, Frederic Weisbecker wrote:
> There are rcu locked read side areas in the path where we submit
> a trace event. And these rcu_read_(un)lock() trigger lock events,
> which create recursive events.
>
> One pair in do_perf_sw_event:
>
> __lock_acquire
>        |
>        |--96.11%-- lock_acquire
>        |          |
>        |          |--27.21%-- do_perf_sw_event
>        |          |          perf_tp_event
>        |          |          |
>        |          |          |--49.62%-- ftrace_profile_lock_release
>        |          |          |          lock_release
>        |          |          |          |
>        |          |          |          |--33.85%-- _raw_spin_unlock
>
> Another pair in perf_output_begin/end:
>
> __lock_acquire
>        |--23.40%-- perf_output_begin
>        |          |          __perf_event_overflow
>        |          |          perf_swevent_overflow
>        |          |          perf_swevent_add
>        |          |          perf_swevent_ctx_event
>        |          |          do_perf_sw_event
>        |          |          perf_tp_event
>        |          |          |
>        |          |          |--55.37%-- ftrace_profile_lock_acquire
>        |          |          |          lock_acquire
>        |          |          |          |
>        |          |          |          |--37.31%-- _raw_spin_lock
>
> The problem is not that much the trace recursion itself, as we have a
> recursion protection already (though it's always wasteful to recurse).
> But the trace events are outside the lockdep recursion protection, then
> each lockdep event triggers a lock trace, which will trigger two
> other lockdep events. Here the recursive lock trace event won't
> be taken because of the trace recursion, so the recursion stops there
> but lockdep will still analyse these new events:
>
> To sum up, for each lockdep events we have:
>
> 	lock_*()
> 	     |
>               trace lock_acquire
>                    |
>                    ----- rcu_read_lock()
>                    |          |
>                    |          lock_acquire()
>                    |          |
>                    |          trace_lock_acquire() (stopped)
>                    |          |
> 		  |          lockdep analyze
>                    |
>                    ----- rcu_read_unlock()
>                               |
>                               lock_release
>                               |
>                               trace_lock_release() (stopped)
>                               |
>                               lockdep analyze
>
> And you can repeat the above two times as we have two rcu read side
> sections when we submit an event.
>
> This is fixed in this patch by moving the lock trace event under
> the lockdep recursion protection.

Thanks a lot, Frederic!

I tested perf lock with your patch, result is like this,

Typical scores:

before:
% sudo ./perf lock record ./perf bench sched messaging
# Running sched/messaging benchmark...
# 20 sender and receiver processes per group
# 10 groups == 400 processes run

     Total time: 3.265 [sec]
[ perf record: Woken up 0 times to write data ]
[ perf record: Captured and wrote 143.952 MB perf.data (~6289344 samples) ]

after:
% sudo ./perf lock record ./perf bench sched messaging
# Running sched/messaging benchmark...
# 20 sender and receiver processes per group
# 10 groups == 400 processes run

     Total time: 1.943 [sec]                       <--- about x1.5 faster!
[ perf record: Woken up 0 times to write data ]
[ perf record: Captured and wrote 98.161 MB perf.data (~4288734 samples) 
]   <--- size of perf.data is also reduced

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