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

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Date:	Tue, 21 Dec 2010 08:50:47 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	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 Tue, 2010-12-21 at 01:13 +0100, Frederic Weisbecker wrote:
> On Mon, Dec 20, 2010 at 04:47:58PM +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 suspect TIMER_NOT_PINNED has been introduced to save some power by
> avoiding to wake up idle cpus.
> 
> This is used by mod_timer(), schedule_timeout(), mod_timer_pending()
> So that's widely used and removing that could have a deep impact on
> power.

Yeah so? Who said task_nohz had to have the bestest power savings
around? Its not a laptop feature by any means. Simply disable the thing:
echo 0 > /proc/sys/kernel/timer_migration, or better yet, teach it about
your cpuset constraints and avoid it migrating timers into your set and
keep timers pinned within the set.


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