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Subject: Re: [PATCH] sched: Support current clocksource handling in
	fallback sched_clock().
From: Daniel Walker <dwalker@fifo99.com>
To: Paul Mundt <lethal@linux-sh.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <peterz@infradead.org>,
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Date: Wed, 27 May 2009 09:25:25 -0700
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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