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Date:   Tue, 18 Sep 2018 15:38:05 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Jan =?iso-8859-1?Q?H=2E_Sch=F6nherr?= <jschoenh@amazon.de>
Cc:     Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Paul Turner <pjt@google.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Rik van Riel <riel@surriel.com>
Subject: Re: Task group cleanups and optimizations (was: Re: [RFC 00/60]
 Coscheduling for Linux)
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On Tue, Sep 18, 2018 at 03:22:13PM +0200, Jan H. Sch?nherr wrote:
> On 09/17/2018 11:48 AM, Peter Zijlstra wrote:
> > On Sat, Sep 15, 2018 at 10:48:20AM +0200, Jan H. Sch?nherr wrote:
> >> On 09/14/2018 06:25 PM, Jan H. Sch?nherr wrote:
> > 
> >>> b) ability to move CFS RQs between CPUs: someone changed the affinity of
> >>>    a cpuset? No problem, just attach the runqueue with all the tasks elsewhere.
> >>>    No need to touch each and every task.
> > 
> > Can't do that, tasks might have individual constraints that are tighter
> > than the cpuset.
> 
> AFAIK, changing the affinity of a cpuset overwrites the individual affinities of tasks
> within them. Thus, it shouldn't be an issue.

No, it only shrinks the set. Also nothing stops you calling
sched_setaffinity() once you're inside the cpuset. The only contraint is
that your mask is a subset of the cpuset mask.