[PATCH v4 09/14] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1

[Will Deacon]

On Tue, Dec 01, 2020 at 03:56:49PM +0000, Quentin Perret wrote:
> On Tuesday 01 Dec 2020 at 14:11:21 (+0000), Qais Yousef wrote:
> > For cpusets, if hotunplug results in an empty cpuset, then all tasks are moved
> > to the nearest ancestor if I read the code correctly. In our case, only 32bit
> > tasks have to move out to retain this behavior. Since now for the first time we
> > have tasks that can't run on all cpus.
> > 
> > Which by the way might be the right behavior for 64bit tasks execing 32bit
> > binary in a 64bit only cpuset. I suggested SIGKILL'ing them but maybe moving
> > them to the nearest ancestor too is more aligned with the behavior above.
> 
> Hmm, I guess that means putting all 32-bit-execd-from-64-bit tasks in
> the root group in Android. I'll try and check the implications, but that
> might be just fine... Sounds like a sensible behaviour to me anyways.

I'll look into this -- anything we can do to avoid forcefully resetting the
affinity mask to the arch_task_cpu_possible_mask() is worth considering.

Will