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

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Date:	Wed, 30 May 2007 15:03:26 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Bill Huey <billh@...ppy.monkey.org>,
	Jason Baron <jbaron@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 3/5] lockstat: core infrastructure

On Tue, 2007-05-29 at 13:28 -0700, Daniel Walker wrote:
> On Tue, 2007-05-29 at 14:52 +0200, Peter Zijlstra wrote:
> > +       now = sched_clock();
> > +       waittime = now - hlock->waittime_stamp;
> > + 
> 
> It looks like your using sched_clock() through out .. It's a little
> troubling considering the constraints on the function .. Most
> architecture implement a jiffies sched_clock() w/ 1 millisecond or worse
> resolution.. I'd imagine a millisecond hold time is pretty rare, even a
> millisecond wait time might be fairly rare too ..  There's also no
> guarantee that sched_clock timestamps off two different cpu's can be
> compared (or at least that's my understanding) ..

All valid points, however.. calling anything more expensive 2-3 times
per lock acquisition is going to be _very_ painful.

Also, IMHO the contention count vs the acquisition count is the most
interesting number, the times are just a nice bonus (if and when they
work).


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