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Subject: Re: Task group cleanups and optimizations (was: Re: [RFC 00/60]
 Coscheduling for Linux)
To:     Peter Zijlstra <peterz@infradead.org>
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>
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Date:   Tue, 18 Sep 2018 15:54:03 +0200
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On 09/18/2018 03:38 PM, Peter Zijlstra wrote:
> On Tue, Sep 18, 2018 at 03:22:13PM +0200, Jan H. Schönherr wrote:
>> 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.

I meant setting the affinity of the cpuset *after* setting an individual affinity.

Like this:

# mkdir /sys/fs/cgroup/cpuset/test
# cat /sys/fs/cgroup/cpuset/cpuset.mems > /sys/fs/cgroup/cpuset/test/cpuset.mems
# cat /sys/fs/cgroup/cpuset/cpuset.cpus > /sys/fs/cgroup/cpuset/test/cpuset.cpus
# echo $$ > /sys/fs/cgroup/cpuset/test/tasks
# taskset -c -p 0 $$
pid 4745's current affinity list: 0-3
pid 4745's new affinity list: 0
# echo "0-1" > /sys/fs/cgroup/cpuset/test/cpuset.cpus
# taskset -c -p $$
pid 4745's current affinity list: 0,1
#

The individual affinity of $$ is lost, despite it being a subset.