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

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Message-ID: <AANLkTimbh3CsxAh9egF0etp0-uMDGb5Z9qkoltPZ6Nay@mail.gmail.com>
Date:	Fri, 28 May 2010 11:11:24 -0700
From:	Chad Talbott <ctalbott@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
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, 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.

The function could be written as follows (thanks Salman Qazi):

u64 local_clock(unsigned int *where)
{
       u64 clock;
       unsigned long flags;

       local_irq_save(flags);
       *where = smp_processor_id();
       clock = sched_clock_cpu(*where);
       local_irq_restore(flags);

       return clock;
}

but I don't see the utility of the resulting routine.

Chad
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