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

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Message-ID: <5559F216.8050007@redhat.com>
Date:	Mon, 18 May 2015 10:07:18 -0400
From:	Rik van Riel <riel@...hat.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>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 4/4] nohz: Set isolcpus when nohz_full is set

On 05/17/2015 11:29 PM, Mike Galbraith wrote:
> On Sun, 2015-05-17 at 22:17 -0400, Rik van Riel wrote:
>> On 05/17/2015 01:30 AM, Mike Galbraith wrote:
>>
>>> 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
>>> he/she does not have CPUSETS enabled, or should Rik's patch rendering
>>> isolcpus immutable be merged, 
>>
>> My patch does not aim to make isolcpus immutable, it aims to make
>> isolcpus resistent to system management tools (like libvirt)
>> automatically undoing isolcpus the instant a cpuset with the default
>> cpus (inherited from the root group) is created.
> 
> Aim or not, if cpusets is the sole modifier, it'll render isolcpus
> immutable, no?  Cpusets could grow an override to the override I
> suppose, to regain control of the resource it thinks it manages.

The other way would be to make /sys/devices/system/cpu/isolcpus
(which Greg KH promised he would queue up for 4.2) writable.

I am all for making isolcpus changeable at run time. I just want
to prevent it being changed accidentally at run time, by system
tools that want to place their workloads in cpusets.

-- 
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