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Date: Thu, 11 Nov 2010 13:31:52 -0500 (EST)
From: Nicolas Pitre <nico@fluxnic.net>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Walleij <linus.walleij@stericsson.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        Colin Cross <ccross@google.com>,
        Rabin VINCENT <rabin.vincent@stericsson.com>
Subject: Re: [PATCH] RFC: nomadik: expand timesource to 63 bits
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On Thu, 11 Nov 2010, Thomas Gleixner wrote:

> 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.

You still have to make sure you get out of idle before 16 secs has 
passed.


Nicolas
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