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

[Sonny Rao]

On Thu, Mar 26, 2020 at 12:57 PM Waiman Long <longman@xxxxxxxxxx> wrote:
>
> On 3/26/20 3:44 PM, Joel Fernandes wrote:
> > 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.
> >
> I think Tejun is concerned about a change in the default behavior of
> cpuset v1.
>
> There is a special v2 mode for cpuset that is enabled by the mount
> option "cpuset_v2_mode". This causes the cpuset v1 to adopt some of the
> v2 behavior. I introduced this v2 mode a while back to address, I think,
> a similar concern. Could you try that to see if it is able to address
> your problem? If not, you can make some code adjustment within the
> framework of the v2 mode. As long as it is an opt-in, I think we are
> open to further change.

I am surprised if anyone actually wants this behavior, we (Chrome OS)
found out about it accidentally, and then found that Android had been
carrying a patch to fix it.  And if it were a desirable behavior then
why isn't it an option in v2?

>
> Cheers,
> Longman
>