linux-kernel - Re: [PATCH] sched_clock: Provide local

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Message-ID: <1275070953.1645.391.camel@laptop>
Date:	Fri, 28 May 2010 20:22:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chad Talbott <ctalbott@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Divyesh Shah <dpshah@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched_clock: Provide local_clock() and improve 
 documentation

On Fri, 2010-05-28 at 11:11 -0700, Chad Talbott wrote:
> On Fri, May 28, 2010 at 6:13 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > + * local_clock()      -- is cpu_clock() on the current cpu.
> 
> Pretty sure this should read, "is cpu_clock() on *some* cpu," since
> there is no guarantee in a preemptible kernel that local_clock()
> returns on the same CPU that it was called from. The caller has to do
> the preempt protection itself.

Current cpu simply has no meaning if preemption isn't disabled. If it
is, it has and the result is useful.

Some of the local_clock() users are strictly per-cpu (and clearly have
preemption disabled), for some there simply is no better clock and
simply cope with the small incoherency in time.



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