linux-kernel - Re: [PATCH] perf lock: Fix state machine to recognize lock sequence

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Date:	Wed, 21 Apr 2010 18:12:06 +0900
From:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	mingo@...e.hu, linux-kernel@...r.kernel.org, h.mitake@...il.com,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Jason Baron <jbaron@...hat.com>,
	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Subject: Re: [PATCH] perf lock: Fix state machine to recognize lock sequence

On 04/21/10 10:26, Frederic Weisbecker wrote:
 > On Fri, Apr 16, 2010 at 05:44:06PM +0900, Hitoshi Mitake wrote:
 >> Hi Ingo,
 >>
 >> I'm developing the model to recognize the correct sequence of lock 
events.
 >> Previous state machine of perf lock was really broken.
 >> This patch improves it a little.
 >>
 >> This patch prepares the array of state machine represents lock 
sequence for each threads.
 >> These state machines represent one of these sequence:
 >>
 >>        1) acquire ->  acquired ->  release
 >>        2) acquire ->  contended ->  acquired ->  release
 >>        3) acquire (w/ try) ->  release
 >>        4) acquire (w/ read) ->  release
 >>
 >> The case of 4) is a little special.
 >> Double acquire of read lock is allowed, so state machine of sequence
 >> counts read lock number, and permit double acquire and release.
 >>
 >> But, things are not so simple. Something of my model is still wrong.
 >> I counted the number of lock instances with bad sequence,
 >> and ratio is like this (case of tracing whoami): bad:122, total:1956
 >
 >
 >
 > I just gave your patch a try and it's worse: almost every sequences
 > were reported bad (it wasn't working either before your patch :)
 >
 > This is not the fault of your patch though. Actually your patch seems to
 > be a nice improvement.

Thanks for your review, Frederic!

 >
 > In fact I just found two things:
 >
 > 1) We are working on tasks in pid basis. We should work on a task by 
using
 > its tid.
 > In fact we are processing the sequences of several threads in a 
process as
 > if we were dealing with a single task.
 >
 > If A and B are two threads belonging to a same process, and if we have:
 >
 > A: acquire lock 1, release lock 1
 > B: acquire lock 2, release lock 2
 >
 > ...then we are dealing with a random mess of sequences:
 >
 > AB: acquire lock 1, acquire lock 2, release lock 1, and any kind of 
random
 > things like this.


Ah, I missed tid. I'll fix this point.

 >
 > 2) I can't get lock_acquired traces. Not sure why yet...

Really? It's mystery... I'll seek the cause.

 >
 >
 >>
 >> There is another new bad thing.
 >> The size of array of state machine is equal to max depth lockdep 
defines.
 >> If perf lock record tries to record lock events of the programs with 
lots of
 >> system call like "perf bench sched messaging", the array will be 
exhausted :(
 >
 >
 >
 > Yeah, I suggest you use a list for that in fact. The max lockdep 
depth may
 > change in the future, or become variable, so we can't relay on that.

Yeah, I'll use list or hashtable.

 >
 > But that's still a cool improvement.
 >
 > I'm queuing this patch.

Thanks! But I have to fix some points based on your advice.
Should I send v2 patch or make fix on your tree?

Thanks,
	Hitoshi
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