linux-kernel - Re: [PATCH v2] cgroup/cpuset-v1: Add missing support for cpuset_v2

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Message-ID: <CABdmKX2Basoq0Sk6qemcP3Mne6-nJPNN0Mz9WYjvdKWNagoaZg@mail.gmail.com>
Date: Wed, 16 Apr 2025 11:27:06 -0700
From: "T.J. Mercier" <tjmercier@...gle.com>
To: Waiman Long <llong@...hat.com>
Cc: Kamalesh Babulal <kamalesh.babulal@...cle.com>, Tejun Heo <tj@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Koutný <mkoutny@...e.com>, 
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] cgroup/cpuset-v1: Add missing support for cpuset_v2_mode

On Wed, Apr 16, 2025 at 11:05 AM Waiman Long <llong@...hat.com> wrote:
>
> On 4/16/25 1:55 PM, T.J. Mercier wrote:
> > On Wed, Apr 16, 2025 at 2:19 AM Kamalesh Babulal
> > <kamalesh.babulal@...cle.com> 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@redhat.com/T/
> >>>
> >>> Fixes: e1cba4b85daa ("cgroup: Add mount flag to enable cpuset to use v2 behavior in v1 cgroup")
> >>> Signed-off-by: T.J. Mercier <tjmercier@...gle.com>
> >> The patch looks good to me, please feel free to add
> >>
> >> Reviewed-by: Kamalesh Babulal <kamalesh.babulal@...cle.com>
> >>
> >> 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
>

I thought it might decrease the text size a tiny bit, but it doesn't
because the symbol isn't exported and I guess the compiler knows to
just inline.