linux-kernel - Re: [PATCH 1/2] cpusets,isolcpus: exclude isolcpus from load balancing in cpusets

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Message-ID: <20150227093231.GG21418@twins.programming.kicks-ass.net>
Date:	Fri, 27 Feb 2015 10:32:31 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, Clark Williams <williams@...hat.com>,
	Li Zefan <lizefan@...wei.com>, Ingo Molnar <mingo@...hat.com>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	cgroups@...r.kernel.org
Subject: Re: [PATCH 1/2] cpusets,isolcpus: exclude isolcpus from load
 balancing in cpusets

On Wed, Feb 25, 2015 at 11:38:07AM -0500, riel@...hat.com wrote:
> From: Rik van Riel <riel@...hat.com>
> 
> Ensure that cpus specified with the isolcpus= boot commandline
> option stay outside of the load balancing in the kernel scheduler.
> 
> Operations like load balancing can introduce unwanted latencies,
> which is exactly what the isolcpus= commandline is there to prevent.
> 
> Previously, simply creating a new cpuset, without even touching the
> cpuset.cpus field inside the new cpuset, would undo the effects of
> isolcpus=, by creating a scheduler domain spanning the whole system,
> and setting up load balancing inside that domain. The cpuset root
> cpuset.cpus file is read-only, so there was not even a way to undo
> that effect.
> 
> This does not impact the majority of cpusets users, since isolcpus=
> is a fairly specialized feature used for realtime purposes.
> 
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Clark Williams <williams@...hat.com>
> Cc: Li Zefan <lizefan@...wei.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Luiz Capitulino <lcapitulino@...hat.com>
> Cc: Mike Galbraith <umgwanakikbuti@...il.com>
> Cc: cgroups@...r.kernel.org
> Signed-off-by: Rik van Riel <riel@...hat.com>
> Tested-by: David Rientjes <rientjes@...gle.com>

Might I asked you to update Documentation/cgroups/cpusets.txt with this
knowledge? While it does mentions isolcpus it does not clarify the
interaction between it and cpusets.

Other than that,

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
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