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

[Peter Zijlstra]

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.


> 1) rcu_read_lock() is deadlock-immunity,
>    we get very little benefit from lockdep.

Except it detects things like failing to unlock, or going into userspace
while holding an rcu_read_lock()

Also, Paul has been spending lots of effort on getting rcu_dereference()
annotated.

> rcu_read_lock()
>    lock_acquire(read=2,check=1)
> 
>  * Values for check:
>  *
>  *   0: disabled
>  *   1: simple checks (freeing, held-at-exit-time, etc.)
>  *   2: full validation
>  */
> 
> We can check it by other methods.
> 
> 2) popular distributions and some companies enable lockdep for their kernel.
>    rcu_read_lock_*() are the most frequent lock in kernel.
>    lock_acquire() is not fast enough, it is a big function for RCU.

Its debug stuff, get over it, we're not going to limit its coverage
because people do silly things.

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