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Date: Fri, 05 Feb 2010 10:38:25 +0800 From: Lai Jiangshan <laijs@...fujitsu.com> To: paulmck@...ux.vnet.ibm.com CC: Frederic Weisbecker <fweisbec@...il.com>, Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, Arnaldo Carvalho de Melo <acme@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, Paul Mackerras <paulus@...ba.org>, Hitoshi Mitake <mitake@....info.waseda.ac.jp>, Li Zefan <lizf@...fujitsu.com>, Masami Hiramatsu <mhiramat@...hat.com>, Jens Axboe <jens.axboe@...cle.com> Subject: Re: [PATCH 10/11] tracing/perf: Fix lock events recursions in the fast path 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@...ux.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_*(). 1) rcu_read_lock() is deadlock-immunity, we get very little benefit from lockdep. 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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