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

[Waiman Long <llong@redhat.com>]

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!