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

[Chen Ridong <chenridong@huaweicloud.com>]

On 2025/11/18 16:36, 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, thischeck did not account for remote partitions, which were
> introduced later.
> 
> After commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug processing
> synchronous"),cgroup v2's 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 the redundant partition effective_cpus check. Additionally, it adds
> a check and warningto 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 | 29 ++++++-----------------------
>  1 file changed, 6 insertions(+), 23 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index daf813386260..1ac58e3f26b4 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1084,11 +1084,10 @@ 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);
> @@ -1107,30 +1106,14 @@ void rebuild_sched_domains_locked(void)
>  	    !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
>  		return;
>  
> -	/*
> -	 * 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();
> -	}
> -
>  	/* Generate domain masks and attrs */
>  	ndoms = generate_sched_domains(&doms, &attr);
>  
> +	for (i = 0; i < ndoms; ++i) {
> +		if (WARN_ON_ONCE(!cpumask_subset(doms[i], cpu_active_mask)))
> +			return;
> +	}
> +
>  	/* Have scheduler rebuild the domains */
>  	partition_sched_domains(ndoms, doms, attr);
>  }

Friendly ping.

-- 
Best regards,
Ridong