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SubjectRe: [PATCH 1/3] lockdep: Move lock events under lockdep recursion protection
On Wed, Mar 03 2010, 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.

I went to try this on 2.6.34-rc1 to see how much it would improve things
here. With 2.6.34-rc1, a

$ time sudo perf lock record ls /dev/shm

essentially hangs the box, it ends up crashing hard (not just live
locked like before). With the patch in place, it does eventually finish:

real 0m21.301s
user 0m0.030s
sys 0m21.040s

The directory is empty.

--
Jens Axboe



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