Re: [PATCH -next 1/4] cpuset: remove redundant CS_ONLINE flag

[Waiman Long]

On 8/12/25 8:54 PM, Chen Ridong wrote:
> On 2025/8/12 22:44, Waiman Long wrote:
> > On 8/8/25 5:25 AM, Chen Ridong wrote:
> > > From: Chen Ridong <chenridong@xxxxxxxxxx>
> > >
> > > The CS_ONLINE flag was introduced prior to the CSS_ONLINE flag in the
> > > cpuset subsystem. Currently, the flag setting sequence is as follows:
> > >
> > > 1. cpuset_css_online() sets CS_ONLINE
> > > 2. css->flags gets CSS_ONLINE set
> > > ...
> > > 3. cgroup->kill_css sets CSS_DYING
> > > 4. cpuset_css_offline() clears CS_ONLINE
> > > 5. css->flags clears CSS_ONLINE
> > >
> > > The is_cpuset_online() check currently occurs between steps 1 and 3.
> > > However, it would be equally safe to perform this check between steps 2
> > > and 3, as CSS_ONLINE provides the same synchronization guarantee as
> > > CS_ONLINE.
> > >
> > > Since CS_ONLINE is redundant with CSS_ONLINE and provides no additional
> > > synchronization benefits, we can safely remove it to simplify the code.
> > >
> > > Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
> > > ---
> > >    include/linux/cgroup.h          | 5 +++++
> > >    kernel/cgroup/cpuset-internal.h | 3 +--
> > >    kernel/cgroup/cpuset.c          | 4 +---
> > >    3 files changed, 7 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> > > index b18fb5fcb38e..ae73dbb19165 100644
> > > --- a/include/linux/cgroup.h
> > > +++ b/include/linux/cgroup.h
> > > @@ -354,6 +354,11 @@ static inline bool css_is_dying(struct cgroup_subsys_state *css)
> > >        return css->flags & CSS_DYING;
> > >    }
> > >    +static inline bool css_is_online(struct cgroup_subsys_state *css)
> > > +{
> > > +    return css->flags & CSS_ONLINE;
> > > +}
> > > +
> > >    static inline bool css_is_self(struct cgroup_subsys_state *css)
> > >    {
> > >        if (css == &css->cgroup->self) {
> > > diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
> > > index 383963e28ac6..75b3aef39231 100644
> > > --- a/kernel/cgroup/cpuset-internal.h
> > > +++ b/kernel/cgroup/cpuset-internal.h
> > > @@ -38,7 +38,6 @@ enum prs_errcode {
> > >      /* bits in struct cpuset flags field */
> > >    typedef enum {
> > > -    CS_ONLINE,
> > >        CS_CPU_EXCLUSIVE,
> > >        CS_MEM_EXCLUSIVE,
> > >        CS_MEM_HARDWALL,
> > > @@ -202,7 +201,7 @@ static inline struct cpuset *parent_cs(struct cpuset *cs)
> > >    /* convenient tests for these bits */
> > >    static inline bool is_cpuset_online(struct cpuset *cs)
> > >    {
> > > -    return test_bit(CS_ONLINE, &cs->flags) && !css_is_dying(&cs->css);
> > > +    return css_is_online(&cs->css) && !css_is_dying(&cs->css);
> > >    }
> > >      static inline int is_cpu_exclusive(const struct cpuset *cs)
> > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > > index f74d04429a29..cf7cd2255265 100644
> > > --- a/kernel/cgroup/cpuset.c
> > > +++ b/kernel/cgroup/cpuset.c
> > > @@ -207,7 +207,7 @@ static inline void notify_partition_change(struct cpuset *cs, int old_prs)
> > >     * parallel, we may leave an offline CPU in cpu_allowed or some other masks.
> > >     */
> > >    static struct cpuset top_cpuset = {
> > > -    .flags = BIT(CS_ONLINE) | BIT(CS_CPU_EXCLUSIVE) |
> > > +    .flags = BIT(CS_CPU_EXCLUSIVE) |
> > >             BIT(CS_MEM_EXCLUSIVE) | BIT(CS_SCHED_LOAD_BALANCE),
> > >        .partition_root_state = PRS_ROOT,
> > >        .relax_domain_level = -1,
> > top_cpuset.css is not initialized like the one in the children. If you modify is_cpuset_online() to
> > test the css.flags, you will probably need to set the CSS_ONLINE flag in top_cpuset.css.flags. I do
> > doubt that we will apply the is_cpuset_online() test on top_cpuset. To be consistent, we should
> > support that.
> >
> > BTW, other statically allocated css'es in the cgroup root may have similar problem. If you make the
> > css_is_online() helper available to all other controllers, you will have to document that limitation.
> >
> > Cheers,
> > Longman
> Hi, Longman, thank you for your response.
>
> If I understand correctly, the CSS_ONLINE flag should be set in top_cpuset.css during the following
> process:
>
> css_create
>    css = ss->css_alloc(parent_css);  // cgroup root is static, unlike children
>    online_css(css);
>       ret = ss->css_online(css);     // css root may differ from children
>       css->flags |= CSS_ONLINE;      // css.flags is set with CSS_ONLINE, including the root css
>
> I think css online must be successful, and it's CSS_ONLINE flag must be set. Do I missing anything?

I am talking about just the top_cpuset which is statically allocated. It 
is not created by the css_create() call and so the CSS_ONLINE will not 
be set.

Cheers,
Longman