linux-kernel - Re: [PATCH 3/5] lockstat: core infrastructure

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Date:	Wed, 30 May 2007 19:16:20 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bill Huey <billh@...ppy.monkey.org>,
	Jason Baron <jbaron@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 3/5] lockstat: core infrastructure

On Wed, 2007-05-30 at 10:06 -0700, Daniel Walker wrote:
> On Wed, 2007-05-30 at 15:49 +0200, Ingo Molnar wrote:
> > * Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > [...] We can argue that sched_clock is "good enough".  If someone 
> > > wants better accounting of locks on some other arch, they can simply 
> > > change sched_clock to be more precise.
> > 
> > exactly. Imprecise sched_clock() if there's a better fast clock source 
> > available is a bug in the architecture code. If the only available 
> > clocksource is 1 msec resolution then there's no solution and nothing to 
> > talk about - lock statistics will be 1msec granular just as much as 
> > scheduling.
> 
> 
> I don't agree .. sched_clock() is obsoleted by timekeepings clocksource
> structure.. sched_clock() was a quick way to get lowlevel time stamps
> just for the scheduler. The timekeeping clocksource structure is a more
> complete solution.
> 
> >From the architecture perspective there are two low level clock hooks to
> implement one is sched_clock() , and at least one clocksource structure.
> Both do essentially the same thing. With timekeepings clocksource
> structure actually being easier to implement cause the math is built in.

I think you are mistaken here; the two are similar but not identical.

I see sched_clock() as fast first, accurate second. Whereas the
clocksource thing is accurate first, fast second.

There is room for both of them.

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