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

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Message-Id: <1243927502.23657.5619.camel@twins>
Date:	Tue, 02 Jun 2009 09:25:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Daniel Walker <dwalker@...o99.com>,
	Linus Walleij <linus.ml.walleij@...il.com>,
	Andrew Victor <linux@...im.org.za>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	John Stultz <johnstul@...ux.vnet.ibm.com>,
	linux-arm-kernel@...ts.arm.linux.org.uk, linux-sh@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: sched_clock() clocksource handling.

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?
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