Subject: Re: [RFC PATCH 02/15] nohz_task: Avoid nohz task cpu as non-idle
 timer target
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anton Blanchard <anton@au1.ibm.com>,
        Tim Pepper <lnxninja@linux.vnet.ibm.com>
In-Reply-To: <20101221001354.GG1715@nowhere>
References: <1292858662-5650-1-git-send-email-fweisbec@gmail.com>
	 <1292858662-5650-3-git-send-email-fweisbec@gmail.com>
	 <1292860078.5021.6.camel@laptop>  <20101221001354.GG1715@nowhere>
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Date: Tue, 21 Dec 2010 08:50:47 +0100
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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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