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Subject: Re: [PATCH] sched: Support current clocksource handling in
 fallback sched_clock().
From: Peter Zijlstra <peterz@infradead.org>
To: Paul Mundt <lethal@linux-sh.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Daniel Walker <dwalker@fifo99.com>,
       Linus Walleij <linus.ml.walleij@gmail.com>, Ingo Molnar <mingo@elte.hu>,
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Date: Thu, 28 May 2009 14:59:30 +0200
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On Thu, 2009-05-28 at 21:42 +0900, Paul Mundt wrote:

>  unsigned long long __attribute__((weak)) sched_clock(void)
>  {
> -	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
> -					* (NSEC_PER_SEC / HZ);
> +	struct clocksource *clock = ACCESS_ONCE(sched_clocksource);
> +
> +	return cyc2ns(clock, clocksource_read(clock));
>  }
>  

> @@ -440,7 +444,17 @@ void clocksource_unregister(struct clocksource *cs)
>  	list_del(&cs->list);
>  	if (clocksource_override == cs)
>  		clocksource_override = NULL;
> +
>  	next_clocksource = select_clocksource();
> +
> +	/*
> +	 * If select_clocksource() fails to find another suitable
> +	 * clocksource for sched_clocksource and we are unregistering
> +	 * it, switch back to jiffies.
> +	 */
> +	if (sched_clocksource == cs)
> +		sched_clocksource = &clocksource_jiffies;
> +
>  	spin_unlock_irqrestore(&clocksource_lock, flags);
>  }


CPU0                           CPU1

clock = ACCESS_ONCE(sched_clocksource);

                               unload module
                                 clocksource_unregister()
                                   sched_clocksource = jiffies
                                 unmap data/text

cyc2ns(clock, clocksource_read(clock)) <--- fireworks