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

[Barry Song]

On Wed, Oct 27, 2021 at 2:06 PM Waiman Long <longman@xxxxxxxxxx> wrote:
>
>
> On 10/26/21 7:58 PM, Barry Song wrote:
> >> 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 also able to reproduce on Ubuntu 21.04 LTS.
> >
> > all docker will be put in this cgroups and its child cgroups:
> > /sys/fs/cgroup/cpuset/docker
> >
> > disabling and enabling SMT by:
> > echo off > /sys/devices/system/cpu/smt/control
> > echo on > /sys/devices/system/cpu/smt/control
> >
> > or unpluging and pluging CPUs by:
> > echo 0 > /sys/devices/system/cpu/cpuX/online
> > echo 1 > /sys/devices/system/cpu/cpuX/online
> >
> > then all docker images will lose some CPUs.
> >
> > So should we document the broken behaviours somewhere?
>
> Is the special cpuset_v2_mode mount option able to fix the issue?
>
> This mode is documented in
>
> Documentation/admin-guide/cgroup-v1/cpuset.rst:
>
> The cpuset.effective_cpus and cpuset.effective_mems files are
> normally read-only copies of cpuset.cpus and cpuset.mems files
> respectively.  If the cpuset cgroup filesystem is mounted with the
> special "cpuset_v2_mode" option, the behavior of these files will become
> similar to the corresponding files in cpuset v2.  In other words, hotplug
> events will not change cpuset.cpus and cpuset.mems.  Those events will
> only affect cpuset.effective_cpus and cpuset.effective_mems which show
> the actual cpus and memory nodes that are currently used by this cpuset.
> See Documentation/admin-guide/cgroup-v2.rst for more information about
> cpuset v2 behavior.
>
> Maybe we can make it more visible.

Is it possible to make cpuset_v2_mode true in default? not quite sure if
it will harm something.

>
> Cheers,
> Longman

Thanks
Barry