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Subject: Re: BUG: using smp_processor_id() in preemptible [00000000] code: 
 icedove-bin/5449
From: Peter Zijlstra <peterz@infradead.org>
To: Divyesh Shah <dpshah@google.com>
Cc: piotr@hosowicz.com, linux-kernel@vger.kernel.org,
       Jens Axboe <jens.axboe@oracle.com>
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Date: Tue, 25 May 2010 23:35:41 +0200
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On Tue, 2010-05-25 at 11:07 -0700, Divyesh Shah wrote:
> Peter, this only affects some stats collection and no scheduling
> behavior should be affected by this. Besides this call at rq_init() to
> sched_clock() there are other calls in the request dispatch and the
> request completion path. Not sure if those are problematic too.
> 
> @Ingo, about your concern of time going backwards. This could happen
> with sched_clock() as well when successive measurements were made on
> different cpus which are slightly out of sync. This case is handled
> well in all places where this was used. 

You really shouldn't be using sched_clock(), that thing can exhibit
unbounded drift between cpus.