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

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Message-ID: <775ef75a-b796-4171-b208-df110a73c558@huaweicloud.com>
Date: Wed, 13 Aug 2025 08:54:26 +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 2025/8/12 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?

Best regards,
Ridong