Message-ID: <95c78188-bf8d-4453-b74f-b8a7dc6ae14d@huaweicloud.com>
Date: Wed, 13 Aug 2025 09:20:23 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: 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 2025/8/13 9:00, Waiman Long wrote:
> 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

Hi Longman,

Apologies for the call stack earlier. Thank you for your patience in clarifying this matter.

The CSS root is brought online through the following initialization flow:

cgroup_init_subsys
  css = ss->css_alloc(NULL);       // css 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

My key point is that:
- The root CSS should be online by design.
- Root css CSS_ONLINE flag should be properly set during initialization.

Best regards,
Ridong

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