linux-kernel - Re: SMP timer issue

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Message-Id: <1231338051.11687.293.camel@twins>
Date:	Wed, 07 Jan 2009 15:20:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	melwyn lobo <linux.melwyn@...il.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: SMP timer issue

On Wed, 2009-01-07 at 10:38 +0530, melwyn lobo wrote:
> Hello All,
> Following reply from Russell on the ARM mailing list, I have posted my
> question on this list.
> 
> We have ported the arm kernel 2.6.24 to ARM Cortex dual core
> processor. My question is regarding timer optimization.
> At present we have three timers for the two cores as follows;
> 1. MTU which provides the system tick and in the absence of local
> timers provides IPI_TIMER messages to other processors.
> 2. Localtimer for CPU0
> 3. Localtimer for CPU1
> 
> So presently (when local timers are enabled) there are 3 interrupts
> for every tick. I want to optimize the logic performed by MTU into the
> local timer of CPU0 so that this one serves as the system tick. Thus
> there would be no need for MTU interrupts and it would serve only as a
> clock source.
> 
> My question in particular:
> 1. Is there some platform which has already done this ?
> 2. Is this possible without modifying existing kernel framework?
> 3. How to update jiffies during boot when localtimer has not yet been setup?
> 
> Eagerly awaiting for some advice/comments.

There is plenty code in the hrtimer stuff to make the cpu local timers
(which I presume are used to implement hrtimer clockevents) do the
jiffie tick.

I've no idea how to hook all that up, but added some folks who might
know to the CC.
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