linux-kernel - Re: [RFC PATCH 15/15] nohz

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Message-ID: <20101221210831.GS1750@nowhere>
Date:	Tue, 21 Dec 2010 22:08:33 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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 15/15] nohz_task: Procfs interface

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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