linux-kernel - Re: [RFC] [Patch 4/4] lock contention tracking slimmed down

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Message-Id: <1181323622.5728.6.camel@lappy>
Date:	Fri, 08 Jun 2007 19:27:02 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Martin Peschke <mp3@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, jbaron@...hat.com,
	rostedt@...dmis.org, billh@...ppy.monkey.org, mingo@...e.hu,
	linux-s390@...r.kernel.org
Subject: Re: [RFC] [Patch 4/4] lock contention tracking slimmed down

On Fri, 2007-06-08 at 19:18 +0200, Martin Peschke wrote:
> Peter Zijlstra wrote:
> > On Fri, 2007-06-08 at 19:00 +0200, Martin Peschke wrote:
> >> Peter Zijlstra wrote:
> >>> I'm confused as to where the class->stat objects are initialised? Is
> >>> that done in lock_stat_init()? If so, then you have a bug.
> >> static struct lock_class lock_classes[MAX_LOCKDEP_KEYS];
> >>
> >> I assume this gets us class structures containing all zeros, doesn't it?
> >> Then class->stat is zeros as well, which is handled by lib/statistics.
> >> (In this case, data gathering hasn't been turned on yet, and statistic_inc()
> >> and similar functions don't access other areas of struct statistic.)
> > 
> > Who eventually calls percpu_alloc?
> 
> There is a small state machine calling percpu_alloc when users do
> 
>     echo state=on > /debug/statistics/lockdep/definition
> 
> So data gathering is off by default.
> 
> It might make sense to allow "state=on" as a default. Then allocation would
> be done in the context of statistic_attach().

Right, the problem here is that you iterate over all_lock_classes once
at init.

Contrary to what the name might suggest, it are not all possible
classes, just all active ones. So you'll only attach the classes which
have been used up until the init point. All other classes used later
will never be initialized.

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