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

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Message-ID: <20150520210026.GC6776@linux.vnet.ibm.com>
Date:	Wed, 20 May 2015 14:00:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Afzal Mohammed <afzal.mohd.ma@...il.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	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>,
	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

On Thu, May 21, 2015 at 02:08:09AM +0530, Afzal Mohammed wrote:
> 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.

I have to ask...  What is your use case?  What are you wanting NO_HZ_FULL
to do for you?

							Thanx, Paul

> 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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