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

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Message-ID: <4D10EF3D.3070000@redhat.com>
Date:	Tue, 21 Dec 2010 20:17:33 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.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 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?

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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