linux-kernel - Re: [PATCH] sched: Support current clocksource handling in fallback sched

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Date:	Wed, 27 May 2009 09:25:25 -0700
From:	Daniel Walker <dwalker@...o99.com>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	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 Wed, 2009-05-27 at 09:15 +0900, Paul Mundt wrote:

>  unsigned long long __attribute__((weak)) sched_clock(void)
>  {
> +	/*
> +	 * Use the current clocksource when it becomes available later in
> +	 * the boot process, and ensure that it is usable for sched_clock().
> +	 */
> +	if (clock && (clock->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK))
> +		return cyc2ns(clock, clocksource_read(clock));
> +
> +	/* Otherwise just fall back on jiffies */
>  	return (unsigned long long)(jiffies - INITIAL_JIFFIES)

Do we really need all this complexity in a fast path? the jiffies
clocksource is static and always ready. Could we instead remove the
usage of "clock" and create a new pointer called "sched_clocksource" and
initialize it to the jiffies clock, and allow the clocksource management
code to update that new pointer based on the
CLOCK_SOURCE_USE_FOR_SCHED_CLOCK flag when new clocksources are
registered/removed/marked unstable etc..

that would eliminate all the code in sched_clock except one line,

unsigned long long __attribute__((weak)) sched_clock(void)
{
	return cyc2ns(sched_clocksource, clocksource_read(sched_clocksource));
}

Daniel

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