Re: [PATCH v2] cgroup/cpuset-v1: Add missing support for cpuset_v2_mode

[Waiman Long]

On 4/16/25 1:55 PM, T.J. Mercier wrote:
> On Wed, Apr 16, 2025 at 2:19 AM Kamalesh Babulal
> <kamalesh.babulal@xxxxxxxxxx> wrote:
> > Hi,
> >
> > On 4/16/25 5:23 AM, T.J. Mercier wrote:
> > > Android has mounted the v1 cpuset controller using filesystem type
> > > "cpuset" (not "cgroup") since 2015 [1], and depends on the resulting
> > > behavior where the controller name is not added as a prefix for cgroupfs
> > > files. [2]
> > >
> > > Later, a problem was discovered where cpu hotplug onlining did not
> > > affect the cpuset/cpus files, which Android carried an out-of-tree patch
> > > to address for a while. An attempt was made to upstream this patch, but
> > > the recommendation was to use the "cpuset_v2_mode" mount option
> > > instead. [3]
> > >
> > > An effort was made to do so, but this fails with "cgroup: Unknown
> > > parameter 'cpuset_v2_mode'" because commit e1cba4b85daa ("cgroup: Add
> > > mount flag to enable cpuset to use v2 behavior in v1 cgroup") did not
> > > update the special cased cpuset_mount(), and only the cgroup (v1)
> > > filesystem type was updated.
> > >
> > > Add parameter parsing to the cpuset filesystem type so that
> > > cpuset_v2_mode works like the cgroup filesystem type:
> > >
> > > $ mkdir /dev/cpuset
> > > $ mount -t cpuset -ocpuset_v2_mode none /dev/cpuset
> > > $ mount|grep cpuset
> > > none on /dev/cpuset type cgroup (rw,relatime,cpuset,noprefix,cpuset_v2_mode,release_agent=/sbin/cpuset_release_agent)
> > >
> > > [1] https://cs.android.com/android/_/android/platform/system/core/+/b769c8d24fd7be96f8968aa4c80b669525b930d3
> > > [2] https://cs.android.com/android/platform/superproject/main/+/main:system/core/libprocessgroup/setup/cgroup_map_write.cpp;drc=2dac5d89a0f024a2d0cc46a80ba4ee13472f1681;l=192
> > > [3] https://lore.kernel.org/lkml/f795f8be-a184-408a-0b5a-553d26061385@xxxxxxxxxx/T/
> > >
> > > Fixes: e1cba4b85daa ("cgroup: Add mount flag to enable cpuset to use v2 behavior in v1 cgroup")
> > > Signed-off-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
> > The patch looks good to me, please feel free to add
> >
> > Reviewed-by: Kamalesh Babulal <kamalesh.babulal@xxxxxxxxxx>
> >
> > One nit below:
> >
> > > ---
> > >   kernel/cgroup/cgroup.c | 29 +++++++++++++++++++++++++++++
> > >   1 file changed, 29 insertions(+)
> > >
> > > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > > index 27f08aa17b56..cf30ff2e7d60 100644
> > > --- a/kernel/cgroup/cgroup.c
> > > +++ b/kernel/cgroup/cgroup.c
> > > @@ -2353,9 +2353,37 @@ static struct file_system_type cgroup2_fs_type = {
> > >   };
> > >
> > >   #ifdef CONFIG_CPUSETS_V1
> > > +enum cpuset_param {
> > > +     Opt_cpuset_v2_mode,
> > > +};
> > > +
> > > +const struct fs_parameter_spec cpuset_fs_parameters[] = {
> > > +     fsparam_flag  ("cpuset_v2_mode", Opt_cpuset_v2_mode),
> > > +     {}
> > > +};
> > A minor optimization you may want to convert the cpuset_fs_parameters into
> > a static const.
> Thanks, I copied from cgroup1_fs_parameters since that's where
> cpuset_v2_mode lives, which doesn't have the static currently
> (cgroup2_fs_parameters does). Let me update cpuset_fs_parameters in
> v3, and add a second patch for cgroup1_fs_parameters.

Besides not exposing the structure outside the current file or maybe a 
tiny bit of linker speedup, is there other performance benefit by adding 
"static"?

Regards,
Longman