[PATCH -next v2] cpuset: Remove unnecessary checks in rebuild_sched_domains_locked

[PATCH -next v2] cpuset: Remove unnecessary checks in rebuild_sched_domains_locked

[Chen Ridong]

From: Chen Ridong <chenridong@huawei.com>

Commit 406100f3da08 ("cpuset: fix race between hotplug work and later CPU
offline") added a check for empty effective_cpus in partitions for cgroup
v2. However, this check did not account for remote partitions, which were
introduced later.

After commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug processing
synchronous"), cpuset hotplug handling is now synchronous. This eliminates
the race condition with subsequent CPU offline operations that the original
check aimed to fix.

Instead of extending the check to support remote partitions, this patch
removes all the redundant effective_cpus check. Additionally, it adds a
check and warning to verify that all generated sched domains consist of
active CPUs, preventing partition_sched_domains from being invoked with
offline CPUs.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
---
 kernel/cgroup/cpuset.c | 50 +++++++++++++-----------------------------
 1 file changed, 15 insertions(+), 35 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 6e6eb09b8db6..fea577b4016a 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1103,53 +1103,33 @@ void dl_rebuild_rd_accounting(void)
  */
 void rebuild_sched_domains_locked(void)
 {
-	struct cgroup_subsys_state *pos_css;
 	struct sched_domain_attr *attr;
 	cpumask_var_t *doms;
-	struct cpuset *cs;
 	int ndoms;
+	int i;
 
 	lockdep_assert_cpus_held();
 	lockdep_assert_held(&cpuset_mutex);
 	force_sd_rebuild = false;
 
-	/*
-	 * If we have raced with CPU hotplug, return early to avoid
-	 * passing doms with offlined cpu to partition_sched_domains().
-	 * Anyways, cpuset_handle_hotplug() will rebuild sched domains.
-	 *
-	 * With no CPUs in any subpartitions, top_cpuset's effective CPUs
-	 * should be the same as the active CPUs, so checking only top_cpuset
-	 * is enough to detect racing CPU offlines.
-	 */
-	if (cpumask_empty(subpartitions_cpus) &&
-	    !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
-		return;
+	/* Generate domain masks and attrs */
+	ndoms = generate_sched_domains(&doms, &attr);
 
 	/*
-	 * With subpartition CPUs, however, the effective CPUs of a partition
-	 * root should be only a subset of the active CPUs.  Since a CPU in any
-	 * partition root could be offlined, all must be checked.
-	 */
-	if (!cpumask_empty(subpartitions_cpus)) {
-		rcu_read_lock();
-		cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
-			if (!is_partition_valid(cs)) {
-				pos_css = css_rightmost_descendant(pos_css);
-				continue;
-			}
-			if (!cpumask_subset(cs->effective_cpus,
-					    cpu_active_mask)) {
-				rcu_read_unlock();
-				return;
-			}
-		}
-		rcu_read_unlock();
+	* cpuset_hotplug_workfn is invoked synchronously now, thus this
+	* function should not race with CPU hotplug. And the effective CPUs
+	* must not include any offline CPUs. Passing an offline CPU in the
+	* doms to partition_sched_domains() will trigger a kernel panic.
+	*
+	* We perform a final check here: if the doms contains any
+	* offline CPUs, a warning is emitted and we return directly to
+	* prevent the panic.
+	*/
+	for (i = 0; i < ndoms; ++i) {
+		if (WARN_ON_ONCE(!cpumask_subset(doms[i], cpu_active_mask)))
+			return;
 	}
 
-	/* Generate domain masks and attrs */
-	ndoms = generate_sched_domains(&doms, &attr);
-
 	/* Have scheduler rebuild the domains */
 	partition_sched_domains(ndoms, doms, attr);
 }
-- 
2.34.1

Re: [PATCH -next v2] cpuset: Remove unnecessary checks in rebuild_sched_domains_locked

[Waiman Long]

On 11/26/25 4:11 AM, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.com>
>
> Commit 406100f3da08 ("cpuset: fix race between hotplug work and later CPU
> offline") added a check for empty effective_cpus in partitions for cgroup
> v2. However, this check did not account for remote partitions, which were
> introduced later.
>
> After commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug processing
> synchronous"), cpuset hotplug handling is now synchronous. This eliminates
> the race condition with subsequent CPU offline operations that the original
> check aimed to fix.
>
> Instead of extending the check to support remote partitions, this patch
> removes all the redundant effective_cpus check. Additionally, it adds a
> check and warning to verify that all generated sched domains consist of
> active CPUs, preventing partition_sched_domains from being invoked with
> offline CPUs.
>
> Signed-off-by: Chen Ridong <chenridong@huawei.com>
> ---
>   kernel/cgroup/cpuset.c | 50 +++++++++++++-----------------------------
>   1 file changed, 15 insertions(+), 35 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 6e6eb09b8db6..fea577b4016a 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1103,53 +1103,33 @@ void dl_rebuild_rd_accounting(void)
>    */
>   void rebuild_sched_domains_locked(void)
>   {
> -	struct cgroup_subsys_state *pos_css;
>   	struct sched_domain_attr *attr;
>   	cpumask_var_t *doms;
> -	struct cpuset *cs;
>   	int ndoms;
> +	int i;
>   
>   	lockdep_assert_cpus_held();
>   	lockdep_assert_held(&cpuset_mutex);
>   	force_sd_rebuild = false;
>   
> -	/*
> -	 * If we have raced with CPU hotplug, return early to avoid
> -	 * passing doms with offlined cpu to partition_sched_domains().
> -	 * Anyways, cpuset_handle_hotplug() will rebuild sched domains.
> -	 *
> -	 * With no CPUs in any subpartitions, top_cpuset's effective CPUs
> -	 * should be the same as the active CPUs, so checking only top_cpuset
> -	 * is enough to detect racing CPU offlines.
> -	 */
> -	if (cpumask_empty(subpartitions_cpus) &&
> -	    !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
> -		return;
> +	/* Generate domain masks and attrs */
> +	ndoms = generate_sched_domains(&doms, &attr);
>   
>   	/*
> -	 * With subpartition CPUs, however, the effective CPUs of a partition
> -	 * root should be only a subset of the active CPUs.  Since a CPU in any
> -	 * partition root could be offlined, all must be checked.
> -	 */
> -	if (!cpumask_empty(subpartitions_cpus)) {
> -		rcu_read_lock();
> -		cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
> -			if (!is_partition_valid(cs)) {
> -				pos_css = css_rightmost_descendant(pos_css);
> -				continue;
> -			}
> -			if (!cpumask_subset(cs->effective_cpus,
> -					    cpu_active_mask)) {
> -				rcu_read_unlock();
> -				return;
> -			}
> -		}
> -		rcu_read_unlock();
> +	* cpuset_hotplug_workfn is invoked synchronously now, thus this
> +	* function should not race with CPU hotplug. And the effective CPUs
> +	* must not include any offline CPUs. Passing an offline CPU in the
> +	* doms to partition_sched_domains() will trigger a kernel panic.
> +	*
> +	* We perform a final check here: if the doms contains any
> +	* offline CPUs, a warning is emitted and we return directly to
> +	* prevent the panic.
> +	*/
> +	for (i = 0; i < ndoms; ++i) {
> +		if (WARN_ON_ONCE(!cpumask_subset(doms[i], cpu_active_mask)))
> +			return;
>   	}
>   
> -	/* Generate domain masks and attrs */
> -	ndoms = generate_sched_domains(&doms, &attr);
> -
>   	/* Have scheduler rebuild the domains */
>   	partition_sched_domains(ndoms, doms, attr);
>   }
Reviewed-by: Waiman Long <longman@redhat.com>

Thanks!

Re: [PATCH -next v2] cpuset: Remove unnecessary checks in rebuild_sched_domains_locked

[Chen Ridong]

On 2025/11/27 3:47, Waiman Long wrote:
> On 11/26/25 4:11 AM, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@huawei.com>
>>
>> Commit 406100f3da08 ("cpuset: fix race between hotplug work and later CPU
>> offline") added a check for empty effective_cpus in partitions for cgroup
>> v2. However, this check did not account for remote partitions, which were
>> introduced later.
>>
>> After commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug processing
>> synchronous"), cpuset hotplug handling is now synchronous. This eliminates
>> the race condition with subsequent CPU offline operations that the original
>> check aimed to fix.
>>
>> Instead of extending the check to support remote partitions, this patch
>> removes all the redundant effective_cpus check. Additionally, it adds a
>> check and warning to verify that all generated sched domains consist of
>> active CPUs, preventing partition_sched_domains from being invoked with
>> offline CPUs.
>>
>> Signed-off-by: Chen Ridong <chenridong@huawei.com>
>> ---
>>   kernel/cgroup/cpuset.c | 50 +++++++++++++-----------------------------
>>   1 file changed, 15 insertions(+), 35 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 6e6eb09b8db6..fea577b4016a 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -1103,53 +1103,33 @@ void dl_rebuild_rd_accounting(void)
>>    */
>>   void rebuild_sched_domains_locked(void)
>>   {
>> -    struct cgroup_subsys_state *pos_css;
>>       struct sched_domain_attr *attr;
>>       cpumask_var_t *doms;
>> -    struct cpuset *cs;
>>       int ndoms;
>> +    int i;
>>         lockdep_assert_cpus_held();
>>       lockdep_assert_held(&cpuset_mutex);
>>       force_sd_rebuild = false;
>>   -    /*
>> -     * If we have raced with CPU hotplug, return early to avoid
>> -     * passing doms with offlined cpu to partition_sched_domains().
>> -     * Anyways, cpuset_handle_hotplug() will rebuild sched domains.
>> -     *
>> -     * With no CPUs in any subpartitions, top_cpuset's effective CPUs
>> -     * should be the same as the active CPUs, so checking only top_cpuset
>> -     * is enough to detect racing CPU offlines.
>> -     */
>> -    if (cpumask_empty(subpartitions_cpus) &&
>> -        !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
>> -        return;
>> +    /* Generate domain masks and attrs */
>> +    ndoms = generate_sched_domains(&doms, &attr);
>>         /*
>> -     * With subpartition CPUs, however, the effective CPUs of a partition
>> -     * root should be only a subset of the active CPUs.  Since a CPU in any
>> -     * partition root could be offlined, all must be checked.
>> -     */
>> -    if (!cpumask_empty(subpartitions_cpus)) {
>> -        rcu_read_lock();
>> -        cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
>> -            if (!is_partition_valid(cs)) {
>> -                pos_css = css_rightmost_descendant(pos_css);
>> -                continue;
>> -            }
>> -            if (!cpumask_subset(cs->effective_cpus,
>> -                        cpu_active_mask)) {
>> -                rcu_read_unlock();
>> -                return;
>> -            }
>> -        }
>> -        rcu_read_unlock();
>> +    * cpuset_hotplug_workfn is invoked synchronously now, thus this
>> +    * function should not race with CPU hotplug. And the effective CPUs
>> +    * must not include any offline CPUs. Passing an offline CPU in the
>> +    * doms to partition_sched_domains() will trigger a kernel panic.
>> +    *
>> +    * We perform a final check here: if the doms contains any
>> +    * offline CPUs, a warning is emitted and we return directly to
>> +    * prevent the panic.
>> +    */
>> +    for (i = 0; i < ndoms; ++i) {
>> +        if (WARN_ON_ONCE(!cpumask_subset(doms[i], cpu_active_mask)))
>> +            return;
>>       }
>>   -    /* Generate domain masks and attrs */
>> -    ndoms = generate_sched_domains(&doms, &attr);
>> -
>>       /* Have scheduler rebuild the domains */
>>       partition_sched_domains(ndoms, doms, attr);
>>   }
> Reviewed-by: Waiman Long <longman@redhat.com>
> 
> Thanks!
> 

Thanks.

-- 
Best regards,
Ridong

Re: [PATCH -next v2] cpuset: Remove unnecessary checks in rebuild_sched_domains_locked

[Chen Ridong]

On 2025/11/27 3:47, Waiman Long wrote:
> On 11/26/25 4:11 AM, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@huawei.com>
>>
>> Commit 406100f3da08 ("cpuset: fix race between hotplug work and later CPU
>> offline") added a check for empty effective_cpus in partitions for cgroup
>> v2. However, this check did not account for remote partitions, which were
>> introduced later.
>>
>> After commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug processing
>> synchronous"), cpuset hotplug handling is now synchronous. This eliminates
>> the race condition with subsequent CPU offline operations that the original
>> check aimed to fix.
>>
>> Instead of extending the check to support remote partitions, this patch
>> removes all the redundant effective_cpus check. Additionally, it adds a
>> check and warning to verify that all generated sched domains consist of
>> active CPUs, preventing partition_sched_domains from being invoked with
>> offline CPUs.
>>
>> Signed-off-by: Chen Ridong <chenridong@huawei.com>
>> ---
>>   kernel/cgroup/cpuset.c | 50 +++++++++++++-----------------------------
>>   1 file changed, 15 insertions(+), 35 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 6e6eb09b8db6..fea577b4016a 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -1103,53 +1103,33 @@ void dl_rebuild_rd_accounting(void)
>>    */
>>   void rebuild_sched_domains_locked(void)
>>   {
>> -    struct cgroup_subsys_state *pos_css;
>>       struct sched_domain_attr *attr;
>>       cpumask_var_t *doms;
>> -    struct cpuset *cs;
>>       int ndoms;
>> +    int i;
>>         lockdep_assert_cpus_held();
>>       lockdep_assert_held(&cpuset_mutex);
>>       force_sd_rebuild = false;
>>   -    /*
>> -     * If we have raced with CPU hotplug, return early to avoid
>> -     * passing doms with offlined cpu to partition_sched_domains().
>> -     * Anyways, cpuset_handle_hotplug() will rebuild sched domains.
>> -     *
>> -     * With no CPUs in any subpartitions, top_cpuset's effective CPUs
>> -     * should be the same as the active CPUs, so checking only top_cpuset
>> -     * is enough to detect racing CPU offlines.
>> -     */
>> -    if (cpumask_empty(subpartitions_cpus) &&
>> -        !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
>> -        return;
>> +    /* Generate domain masks and attrs */
>> +    ndoms = generate_sched_domains(&doms, &attr);
>>         /*
>> -     * With subpartition CPUs, however, the effective CPUs of a partition
>> -     * root should be only a subset of the active CPUs.  Since a CPU in any
>> -     * partition root could be offlined, all must be checked.
>> -     */
>> -    if (!cpumask_empty(subpartitions_cpus)) {
>> -        rcu_read_lock();
>> -        cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
>> -            if (!is_partition_valid(cs)) {
>> -                pos_css = css_rightmost_descendant(pos_css);
>> -                continue;
>> -            }
>> -            if (!cpumask_subset(cs->effective_cpus,
>> -                        cpu_active_mask)) {
>> -                rcu_read_unlock();
>> -                return;
>> -            }
>> -        }
>> -        rcu_read_unlock();
>> +    * cpuset_hotplug_workfn is invoked synchronously now, thus this
>> +    * function should not race with CPU hotplug. And the effective CPUs
>> +    * must not include any offline CPUs. Passing an offline CPU in the
>> +    * doms to partition_sched_domains() will trigger a kernel panic.
>> +    *
>> +    * We perform a final check here: if the doms contains any
>> +    * offline CPUs, a warning is emitted and we return directly to
>> +    * prevent the panic.
>> +    */
>> +    for (i = 0; i < ndoms; ++i) {
>> +        if (WARN_ON_ONCE(!cpumask_subset(doms[i], cpu_active_mask)))
>> +            return;
>>       }
>>   -    /* Generate domain masks and attrs */
>> -    ndoms = generate_sched_domains(&doms, &attr);
>> -
>>       /* Have scheduler rebuild the domains */
>>       partition_sched_domains(ndoms, doms, attr);
>>   }
> Reviewed-by: Waiman Long <longman@redhat.com>
> 
> Thanks!
> 

Hi Tj,

Just checking, can this patch be applied? Wanted to make sure you saw it.

-- 
Best regards,
Ridong

Re: [PATCH -next v2] cpuset: Remove unnecessary checks in rebuild_sched_domains_locked

[Tejun Heo]

On Tue, Dec 02, 2025 at 08:57:45AM +0800, Chen Ridong wrote:
> Just checking, can this patch be applied? Wanted to make sure you saw it.

Yeah, I missed it. Will apply after merge window.

Thanks.

-- 
tejun

Re: [PATCH -next v2] cpuset: Remove unnecessary checks in rebuild_sched_domains_locked

[Chen Ridong]

On 2025/12/2 13:29, Tejun Heo wrote:
> On Tue, Dec 02, 2025 at 08:57:45AM +0800, Chen Ridong wrote:
>> Just checking, can this patch be applied? Wanted to make sure you saw it.
> 
> Yeah, I missed it. Will apply after merge window.
> 
> Thanks.
> 

Thank you.

-- 
Best regards,
Ridong

Re: [PATCH v2] cpuset: Remove unnecessary checks in rebuild_sched_domains_locked

[Tejun Heo]

Applied to cgroup/for-6.20.

Thanks.
--
tejun