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Date: Tue, 21 Dec 2010 22:08:33 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Avi Kivity <avi@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
        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>
Subject: Re: [RFC PATCH 15/15] nohz_task: Procfs interface
Message-ID: <20101221210831.GS1750@nowhere>
References: <1292858662-5650-1-git-send-email-fweisbec@gmail.com>
 <1292858662-5650-16-git-send-email-fweisbec@gmail.com>
 <1292859744.5021.4.camel@laptop>
 <20101220155737.GA1742@nowhere>
 <1292861799.5021.27.camel@laptop>
 <20101221012418.GI1715@nowhere>
 <1292919280.5021.203.camel@laptop>
 <4D10B2E9.9040806@redhat.com>
 <20101221170512.GM1750@nowhere>
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On Tue, Dec 21, 2010 at 08:17:33PM +0200, Avi Kivity wrote:
> On 12/21/2010 07:05 PM, Frederic Weisbecker wrote:
> >>
> >>  For an example of a per-cpu flag that is checked on every exit with
> >>  zero additional overhead on the flag clear case, look at
> >>  TIF_USER_RETURN_NOTIFY.
> >
> >Right, but the problem is actually that if we want to automate the nohz
> >attribute on every tasks, then you need you have this flag set for
> >all of these threads.
> >
> >No problem with that, but if nobody wants the nohz attribute, we don't
> >need to force that slow path.
> 
> When the scheduler detects the task is all alone, it sets the flag;
> when it blocks, or if another task joins, it drops the flag (at most
> one task per cpu has the flag set).
> 
> Does that work?

Makes sense. And that integrates well with Peter's idea of creating a
new cpuset attribute for the nohz tasks.

But instead of making this detection from the scheduler, I think this
should be done from the tick: if there is only one task running, set
it the TF flag.

But anyway, that's an optimisation. We can start with setting that flag
on every task in that cpuset.
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