linux-kernel - Re: [PATCH 0/2] cpusets,isolcpus: resolve conflict between cpusets and isolcpus

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Message-ID: <54EC86F0.8080107@redhat.com>
Date:	Tue, 24 Feb 2015 09:13:04 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] cpusets,isolcpus: resolve conflict between cpusets
 and isolcpus

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On 02/23/2015 09:18 PM, Mike Galbraith wrote:
> On Mon, 2015-02-23 at 16:45 -0500, riel@...hat.com wrote:
>> 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.
> 
> 3/3: nohz_full cpus become part of that unified isolated map?

There may be use cases where users want nohz_full, but still
want the scheduler to automatically load balance the CPU.

I am not sure whether we want nohz_full and isolcpus to always
overlap 100%.

On the other hand, any CPU that is isolated with isolcpus=
probably wants nohz_full...

- -- 
All rights reversed
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