linux-kernel - Re: [PATCH RFC 00/11] lock monitor: Separate features related to lock

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Date:	Wed, 17 Mar 2010 16:30:53 +0900
From:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, h.mitake@...il.com,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Jason Baron <jbaron@...hat.com>
Subject: Re: [PATCH RFC 00/11] lock monitor: Separate features related to
 lock

On 03/17/10 10:32, Frederic Weisbecker wrote:
 > On Sun, Mar 14, 2010 at 07:13:55PM +0100, Peter Zijlstra wrote:
 >> On Sun, 2010-03-14 at 19:38 +0900, Hitoshi Mitake wrote:
 >>> Current lockdep is too complicated because,
 >>>   * dependency validation
 >>>   * statistics
 >>>   * event tracing
 >>> are all implemented by it.
 >>> This cause problem of overhead.
 >>> If user enables one of them, overhead of rests part is not avoidable.
 >>> (tracing is exception. If user enables validation or stat,
 >>> overhead of tracing doesn't occur.)
 >>>
 >>> So I suggest new subsystem "lock monitor".
 >>> This is a general purpose lock event hooking mechanism.
 >>>
 >>> lock monitor will be enable easy implementing and running
 >>> these features related to lock.
 >>>
 >>> And I'm hoping that lock monitor will reduce overhead of perf lock.
 >>> Because lock monitor separates dependency validation and event 
tracing clearly,
 >>> so calling of functions of lockdep (e.g. lock_acquire()) only for 
validation
 >>> will not occur lock events.
 >>>
 >>> I implemented it on the branch perf/inject of Frederic's 
random-tracing tree.
 >>> Because the branch is hottest place of lock and tracing :)
 >>
 >> OK, so I really don't like this much..
 >>
 >> Building a lockstat kernel (PROVE_LOCKING=n) should not have much more
 >> overhead than the proposed solution, if the simple lock acquistion
 >> tracking bothers you, you can do a patch to weaken that.
 >>
 >> I really really dislike how you add a monitor variable between
 >> everything for no reason what so ever.
 >>
 >> You use a new rwlock_t, which is an instant fail, those things are worse
 >> than useless.
 >>
 >> You add chained indirect calls into all lock ops, that's got to hurt.
 >
 >
 > Well, the idea was not bad at the first glance. It was separating
 > lockdep and lock events codes.
 >
 > But indeed, the indirect calls plus the locking are not good
 > for such a fast path.
 >
 > There is something else, it would be nice to keep the
 > lockdep_map ->  lockdep_class mapping so that we can
 > do lock profiling based on classes too. So we actually
 > need the lockdep code. What we don't need is the prove
 > locking or the lock stats. So I guess we can have a new
 > config to enable lock events and get rid of the prove
 > locking / lock stat code if we don't need it.
 >
 >

Thanks for your comments, Peter and Frederic.

My main motivation of writing this patch series was that
some kernel codes uses lockdep functions (e.g. lock_acquire()) directly,
so perf lock gets a lot of trace events without actual locks (e.g. 
might_lock_read()).
I think that these are confusable things for users.

But I noticed that these events can be reduced by
turning off CONFIG_PROVE_LOCKING. Yeah, my patch series was pointless... :)

Should perf lock warn not to use with CONFIG_PROVE_LOCKING?

Thanks,
	Hitoshi
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