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Message-Id: <1243503281.23657.80.camel@twins>
Date: Thu, 28 May 2009 11:34:41 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Paul Mundt <lethal@...ux-sh.org>
Cc: Daniel Walker <dwalker@...o99.com>,
Thomas Gleixner <tglx@...utronix.de>,
Linus Walleij <linus.ml.walleij@...il.com>,
Ingo Molnar <mingo@...e.hu>,
Andrew Victor <linux@...im.org.za>,
Haavard Skinnemoen <hskinnemoen@...el.com>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org,
linux-arm-kernel@...ts.arm.linux.org.uk,
John Stultz <johnstul@...ux.vnet.ibm.com>
Subject: Re: [PATCH] sched: Support current clocksource handling in
fallback sched_clock().
On Thu, 2009-05-28 at 18:19 +0900, Paul Mundt wrote:
> Ok, there were some ordering problems with the early platform code, but
> I've played with this a bit more and got it to the point where this now
> also works. I can live with this over the v3 version if people prefer
> this approach instead.
>
> --
> @@ -38,8 +39,7 @@
> */
> unsigned long long __attribute__((weak)) sched_clock(void)
> {
> - return (unsigned long long)(jiffies - INITIAL_JIFFIES)
> - * (NSEC_PER_SEC / HZ);
> + return cyc2ns(sched_clocksource, clocksource_read(sched_clocksource));
> }
>
> static __read_mostly int sched_clock_running;
> @@ -362,6 +363,9 @@ static struct clocksource *select_clocksource(void)
> if (next == curr_clocksource)
> return NULL;
>
> + if (next->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK)
> + sched_clocksource = next;
> +
> return next;
> }
>
That's a single assignment, vs two reads on use. Should we be worried
about the SMP race where we observe two different sched_clocksource
pointers on read?
I would suggest we write it as:
u64 __weak sched_clock(void)
{
struct clocksource *clock = ACCESS_ONCE(sched_clocksource);
return cyc2ns(clock, clocksource_read(clock));
}
--
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