linux-kernel - Re: BUG: using smp_processor_id() in preemptible [00000000] code: icedove-bin/5449

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Date:	Tue, 25 May 2010 11:47:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	piotr@...owicz.com, linux-kernel@...r.kernel.org,
	Jens Axboe <jens.axboe@...cle.com>,
	Divyesh Shah <dpshah@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: BUG: using smp_processor_id() in preemptible [00000000] code:
 icedove-bin/5449

On Tue, 2010-05-25 at 11:43 +0200, Ingo Molnar wrote:
> > Subject: sched_clock: Add local_clock()
> > From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Date: Tue May 25 10:48:51 CEST 2010
> > 
> > For people who otherwise get to write: cpu_clock(smp_processor_id()),
> > there is now: local_clock().
> 
> This doesnt fix the whole issue. cpu_clock() is local, while the measurements 
> done in the blk code are global ...
> 
> While the warning is fixed this way, the far more serious issue is still 
> there: time can go backwards if two points of time measurement are on 
> different CPUs and can mess up the statistics with negative values, etc...

cpu_clock() is synced on each tick, so the inter-cpu-drift should not
exceed 2 jiffies.

But yeah, if they want anything better, they'll have to start caring on
what cpu which timestamp got taken and use cpu_clock().
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