From: Chen Ridong <chenridong@huawei.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@huawei.com>
Acked-by: Waiman Long <longman@redhat.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 27adb04df675..3466ebbf1016 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,
@@ -3496,7 +3496,6 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
 	cpus_read_lock();
 	mutex_lock(&cpuset_mutex);
 
-	set_bit(CS_ONLINE, &cs->flags);
 	if (is_spread_page(parent))
 		set_bit(CS_SPREAD_PAGE, &cs->flags);
 	if (is_spread_slab(parent))
@@ -3571,7 +3570,6 @@ static void cpuset_css_offline(struct cgroup_subsys_state *css)
 		cpuset_update_flag(CS_SCHED_LOAD_BALANCE, cs, 0);
 
 	cpuset_dec();
-	clear_bit(CS_ONLINE, &cs->flags);
 
 	mutex_unlock(&cpuset_mutex);
 	cpus_read_unlock();
-- 
2.34.1

On Wed, Aug 13, 2025 at 08:29:01AM +0000, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.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@huawei.com>
> Acked-by: Waiman Long <longman@redhat.com>

Applied to cgroup/for-6.18.

Thanks.

-- 
tejun