Re: [PATCH 10/11] tracing/perf: Fix lock events recursions in thefast path

[Ingo Molnar]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Fri, 2010-02-05 at 10:45 +0100, Peter Zijlstra wrote:
> > On Fri, 2010-02-05 at 10:38 +0800, Lai Jiangshan wrote:
> > > Paul E. McKenney wrote:
> > > > On Wed, Feb 03, 2010 at 10:14:34AM +0100, Frederic Weisbecker wrote:
> > > >> There are rcu locked read side areas in the path where we submit
> > > >> a trace events. 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 pacth by using the non-lockdep versions of
> > > >> rcu_read_(un)lock.
> > > > 
> > > > Hmmm...  Perhaps I should rename __rcu_read_lock() to something more
> > > > meaningful if it is to be used outside of the RCU files.  In the
> > > > meantime:
> > > > 
> > > > Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > 
> > > 
> > > Perhaps we can use the existed rcu_read_lock_sched_notrace().
> > > 
> > > not relate to this patchset, but RCU & lockdep:
> > > 
> > > We need to remove lockdep from rcu_read_lock_*().
> > 
> > I'm not at all convinced we need to do any such thing, remember its
> > debugging stuff, performance, while nice, doesn't really count.
> 
> That said, I'm not at all happy about removing lockdep annotations to make 
> the tracer faster, that's really counter productive.

Are there no dynamic techniques that could be used here?

Lockdep obviously wants maximum instrumentation coverage - performance be 
damned.

Lock profiling/tracing/visualization wants the minimum subset of events it is 
interested in - everything else is unnecessary overhead.

	Ingo
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