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

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Message-ID: <20101221135240.GD1750@nowhere>
Date:	Tue, 21 Dec 2010 14:52:44 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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, Dec 21, 2010 at 08:50:47AM +0100, Peter Zijlstra wrote:
> 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.

Right.

So I'll start by deactivating this through /proc/sys/kernel/timer_migration
and care about making it more automatically later.
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