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

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Date:	Sun, 14 Mar 2010 19:13:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>
Cc:	fweisbec@...il.com, 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 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.

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