Re: [PATCH RFC] cpuset: Make cpusets get restored on hotplug

[Joel Fernandes <joel@joelfernandes.org>]

Hi Tejun,

On Thu, Mar 26, 2020 at 03:20:35PM -0400, Tejun Heo wrote:
> On Thu, Mar 26, 2020 at 03:16:23PM -0400, Joel Fernandes (Google) wrote:
> > This deliberately changes the behavior of the per-cpuset
> > cpus file to not be effected by hotplug. When a cpu is offlined,
> > it will be removed from the cpuset/cpus file. When a cpu is onlined,
> > if the cpuset originally requested that that cpu was part of the cpuset,
> > that cpu will be restored to the cpuset. The cpus files still
> > have to be hierachical, but the ranges no longer have to be out of
> > the currently online cpus, just the physically present cpus.
> 
> This is already the behavior on cgroup2 and I don't think we want to
> introduce this big a behavior change to cgroup1 cpuset at this point.

It is not really that big a change. Please go over the patch, we are not
changing anything with how ->cpus_allowed works and interacts with the rest
of the system and the scheduler. We have just introduced a new mask to keep
track of which CPUs were requested without them being affected by hotplug. On
CPU onlining, we restore the state of ->cpus_allowed as not be affected by
hotplug.

There's 3 companies that have this issue so that should tell you something.
We don't want to carry this patch forever. Many people consider the hotplug
behavior to be completely broken.

thanks,

 - Joel