linux-kernel - Re: [RFC] exponential update_wall

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Message-ID: <Pine.LNX.4.64.0609280031550.6761@scrub.home>
Date:	Thu, 28 Sep 2006 01:04:09 +0200 (CEST)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	john stultz <johnstul@...ibm.com>
cc:	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [RFC] exponential update_wall_time

Hi,

On Wed, 27 Sep 2006, john stultz wrote:

> Accumulate time in update_wall_time exponentially. 
> This avoids long running loops seen with the dynticks patch
> as well as the problematic hang" seen on systems with broken 
> clocksources.

This is the wrong approach, second_overflow() should be called every HZ
increment steps and your patch breaks this.
There are other approaches oo accommodate dyntick. 
1. You could make HZ in ntp_update_frequency() dynamic and thus reduce the 
frequency with which update_wall_time() needs to be called (Note that 
other clock variables like cycle_interval have to be adjusted as well). 
2. If dynticks stops the timer interrupt for a long time, it could 
precalculate a few things, e.g. it could complete the second and then 
advance the time in full seconds.

bye, Roman
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