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    SubjectRe: [PATCH 10/11] tracing/perf: Fix lock events recursions in the fast path
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    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@linux.vnet.ibm.com>
    > > >
    > >
    > > 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.





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