linux-kernel - Re: [RFC PATCH 02/15] nohz_task: Avoid nohz task cpu as non-idle timer target

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Message-ID: <20101221002005.GH1715@nowhere>
Date:	Tue, 21 Dec 2010 01:20:07 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 02/15] nohz_task: Avoid nohz task cpu as non-idle
 timer target

On Mon, Dec 20, 2010 at 05:12:04PM +0100, Peter Zijlstra wrote:
> On Mon, 2010-12-20 at 11:06 -0500, Steven Rostedt wrote:
> > On Mon, 2010-12-20 at 16:47 +0100, Peter Zijlstra wrote:
> > > On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote:
> > > > Unbound timers are preferably targeted for non idle cpu. If
> > > > possible though, prioritize idle cpus over nohz task cpus,
> > > > because the main point of nohz task is to avoid unnecessary
> > > > timer interrupts.
> > > 
> > > Oh is it?
> > > 
> > > I'd very much expect the cpu that arms the timer to get the interrupt. I
> > > mean, if the task doesn't want to get interrupted by timers,
> > > _DON'T_USE_TIMERS_ to begin with.
> > > 
> > > So no, don't much like this at all.
> > 
> > I think this comes from other tasks on other CPUs that are using timers.
> 
> Tasks on other CPUs should not cause timers on this CPU, _if_ that does
> happen, fix that.
> 
> > Although, I'm not sure what causes an "unbound" timer to happen. I
> > thought timers usually go off on the CPU that asked for it to go off.
> 
> They do, except if you enable some weird power management feature that
> migrates timers around so as to let CPUs sleep longer. But I doubt
> that's the reason for this here, and if it is, just disable that.

That seems to me the reason for that: avoid to wake up idle cpus.

I can certainly deactivate TIMER_NOT_PINNED and make it a no-op
if CONFIG_NO_HZ_TASK.
But I'm not sure why we would want to do this.
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