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

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Date:	Tue, 24 Feb 2015 15:22:16 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] cpusets,isolcpus: resolve conflict between cpusets
 and isolcpus

On Tue, 2015-02-24 at 09:13 -0500, Rik van Riel wrote:
> 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...

I can't imagine caring deeply about the tiny interference of the tick,
yet not caring about the massive interference of load balancing.

	-Mike


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