Re: [PATCH] sched: sched_clock() clocksource handling.

Peter Zijlstra
2009-06-02T07:25:28Z

On Tue, 2009-06-02 at 16:17 +0900, Paul Mundt wrote:

> @@ -362,6 +364,9 @@ 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;
> }
>
> @@ -440,7 +445,21 @@ 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) {
> + rcu_assign_pointer(sched_clocksource, &clocksource_jiffies);
> + spin_unlock_irqrestore(&clocksource_lock, flags);
> + synchronize_rcu();
> + return;
> + }
> +
> spin_unlock_irqrestore(&clocksource_lock, flags);
> }

What if there's multiple CLOCK_SOURCE_USER_FOR_SCHED_CLOCK [ damn, thats
a long name to type :-) ] ?

That is, should we have logic in select_clocksource that does:

if ((next->flags & ..) && next->prio > sched_clocksource->prio)

or whatever, so that it picks the best one?

Same for unregister, should we re-evaluate all clocksources before
falling back to basic?