Re: [PATCH] sched: Support current clocksource handling in fallback sched_clock().

Peter Zijlstra
2009-05-28T13:00:48Z

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