linux-kernel - Re: [PATCH -next 1/4] cpuset: remove redundant CS

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Message-ID: <a27c39d5-7470-475e-aefa-0841bd816675@redhat.com>
Date: Tue, 12 Aug 2025 21:00:30 -0400
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>, Waiman Long <llong@...hat.com>,
 tj@...nel.org, hannes@...xchg.org, mkoutny@...e.com, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 lujialin4@...wei.com, chenridong@...wei.com
Subject: Re: [PATCH -next 1/4] cpuset: remove redundant CS_ONLINE flag

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@...wei.com>
>>>
>>> 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@...wei.com>
>>> ---
>>>    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