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

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Message-ID: <4CDBC60F.8070905@stericsson.com>
Date:	Thu, 11 Nov 2010 11:31:43 +0100
From:	Linus Walleij <linus.walleij@...ricsson.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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

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.

> Aside of that you are trading a bit less source code with extra code
> in the clock read() function, which is called pretty frequently.

Hm, yeah. I would have to go to metrics but it's O(n)
is it not? Shouldn't hurt a high-frequency SMP system
I believe.

Yours,
Linus Walleij
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