linux-kernel - Re: One of these things (CONFIG

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Date:	Mon, 21 Jan 2013 22:49:00 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Matt Sealey <matt@...esi-usa.com>, Arnd Bergmann <arnd@...db.de>,
	Linux ARM Kernel ML <linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: One of these things (CONFIG_HZ) is not like the others..

On Mon, Jan 21, 2013 at 02:36:13PM -0800, John Stultz wrote:
> Well, Russell brought up a case that doesn't handle this. If a system  
> *can't* do HZ=100, but can do HZ=200.
>
> Though there are hacks, of course, that might get around this (skip  
> every other interrupt at 200HZ).

Note: in the early days of EBSA110 support, yes, we did that, so that
we could have HZ=100 everywhere.  _However_ it sufficiently peturbed
NTP that it basically was unable to slew the clock in any sane manner.
I never got to the bottom of why that was, and when USER_HZ was
decoupled from the kernel HZ, it allowed the problem to be fixed, and
the kernel code to become a _lot_ cleaner.
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