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

[Lai Jiangshan <laijs@cn.fujitsu.com>]

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_*().

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.