linux-kernel - Re: [PATCH] RFC: nomadik: expand timesource to 63 bits

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Date:	Thu, 11 Nov 2010 12:05:03 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Walleij <linus.walleij@...ricsson.com>
cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Nicolas Pitre <nico@...xnic.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Colin Cross <ccross@...gle.com>,
	Rabin VINCENT <rabin.vincent@...ricsson.com>
Subject: Re: [PATCH] RFC: nomadik: expand timesource to 63 bits

On Thu, 11 Nov 2010, Linus Walleij wrote:

> Thomas Gleixner wrote:
> 
> > There is only one caveat. When nohz is on and you sleep longer than 16
> > seconds then the limitation we have in place does not work anymore, as
> > it would say that the long sleep time is less than the 63bit
> > wraparound time. With 32bit clocksource it limits the sleep correclty
> > to avoid the clocksource wrap issue.
> 
> Hm! So the .mask has that side effect (as I suspected), and with
> this the sleep will be limited to what time you can get into 63 bits
> (2200 years). And we sure want NOHZ...
> 
> But if I complement the solution with the keepwarm() timer from
> Orion doing just a dummy read() on the clocksource every say 15 secs
> it will work even if the system sleeps for so long.

Well, you don't need the keepwarm() timer, if you just call
sched_clock() in the idle wakeup path, so the magic in the NOHZ code
will ensure that you reread the thing timely.

Thanks,

	tglx
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