linux-kernel - Re: [PATCH 4/4] nohz: Set isolcpus when nohz

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Message-ID: <20150520203809.GA2940@afzalpc>
Date:	Thu, 21 May 2015 02:08:09 +0530
From:	Afzal Mohammed <afzal.mohd.ma@...il.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 4/4] nohz: Set isolcpus when nohz_full is set

Hi,

On Sun, May 17, 2015 at 07:30:50AM +0200, Mike Galbraith wrote:

> Yeah, tying nohz_full set to isolcpus set up an initial condition that
> you have to tear down with cpusets if you want those cpus returned to
> the general purpose pool.  I had considered the kernel setting initial
> state to be an optimization, but have reconsidered.

> Given that kernel initiated association to isolcpus, a user turning
> NO_HZ_FULL_ALL on had better not have much generic load to manage.  If

On a quad-core desktop system with NO_HZ_FULL_ALL, hackbench took 3x
time as compared to w/o this patch, except boot cpu every one else
jobless. Though NO_HZ_FULL_ALL (afaik) is not meant for generic load,
it was working fine, but not after this - it is now like a single core
system.

Regards
Afzal

> he/she does not have CPUSETS enabled, or should Rik's patch rendering
> isolcpus immutable be merged, he/she would quickly discover that the
> generic box has been transformed into an utterly inflexible specialist
> incapable of performing mundane tasks ever again.
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