Re: [PATCH v3 2/2] cgroup/cpuset: reserve DL bandwidth only for root-domain moves

[Waiman Long <longman@redhat.com>]

On 5/9/26 6:20 AM, Guopeng Zhang wrote:
> cpuset_can_attach() currently adds the bandwidth of all migrating
> SCHED_DEADLINE tasks to sum_migrate_dl_bw. If the source and destination
> cpuset effective CPU masks do not overlap, the whole sum is then
> reserved in the destination root domain.
>
> set_cpus_allowed_dl(), however, subtracts bandwidth from the source
> root domain only when the affinity change really moves the task between
> root domains. A DL task can move between cpusets that are still in the
> same root domain, so including that task in sum_migrate_dl_bw can reserve
> destination bandwidth without a matching source-side subtraction.
>
> Share the root-domain move test with set_cpus_allowed_dl(). Keep
> nr_migrate_dl_tasks counting all migrating deadline tasks for cpuset DL
> task accounting, but add to sum_migrate_dl_bw only for tasks that need a
> root-domain bandwidth move. Keep using the destination cpuset effective
> CPU mask and leave the broader can_attach()/attach() transaction model
> unchanged.
>
> Fixes: 2ef269ef1ac0 ("cgroup/cpuset: Free DL BW in case can_attach() fails")
> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
> ---
>   include/linux/sched/deadline.h  |  9 +++++++++
>   kernel/cgroup/cpuset-internal.h |  1 +
>   kernel/cgroup/cpuset.c          | 33 ++++++++++++++++++---------------
>   kernel/sched/deadline.c         | 13 ++++++++++---
>   4 files changed, 38 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/sched/deadline.h b/include/linux/sched/deadline.h
> index 1198138cb839..273538200a44 100644
> --- a/include/linux/sched/deadline.h
> +++ b/include/linux/sched/deadline.h
> @@ -33,6 +33,15 @@ struct root_domain;
>   extern void dl_add_task_root_domain(struct task_struct *p);
>   extern void dl_clear_root_domain(struct root_domain *rd);
>   extern void dl_clear_root_domain_cpu(int cpu);
> +/*
> + * Return whether moving DL task @p to @new_mask requires moving DL
> + * bandwidth accounting between root domains. This helper is specific to
> + * DL bandwidth move accounting semantics and is shared by
> + * cpuset_can_attach() and set_cpus_allowed_dl() so both paths use the
> + * same source root-domain test.
> + */
> +extern bool dl_task_needs_bw_move(struct task_struct *p,
> +				  const struct cpumask *new_mask);
>   
>   extern u64 dl_cookie;
>   extern bool dl_bw_visited(int cpu, u64 cookie);
> diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
> index bb4e692bea30..f7aaf01f7cd5 100644
> --- a/kernel/cgroup/cpuset-internal.h
> +++ b/kernel/cgroup/cpuset-internal.h
> @@ -167,6 +167,7 @@ struct cpuset {
>   	 */
>   	int nr_deadline_tasks;
>   	int nr_migrate_dl_tasks;
> +	/* DL bandwidth that needs destination reservation for this attach. */
>   	u64 sum_migrate_dl_bw;
>   	/*
>   	 * CPU used for temporary DL bandwidth allocation during attach;
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index b9c839538900..23abfbbb4686 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2993,7 +2993,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>   	struct cpuset *cs, *oldcs;
>   	struct task_struct *task;
>   	bool setsched_check;
> -	int ret;
> +	int cpu, ret;
>   
>   	/* used later by cpuset_attach() */
>   	cpuset_attach_old_cs = task_cs(cgroup_taskset_first(tset, &css));
> @@ -3038,28 +3038,31 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>   		}
>   
>   		if (dl_task(task)) {
> +			/*
> +			 * Count all migrating DL tasks for cpuset task accounting.
> +			 * Only tasks that need a root-domain bandwidth move
> +			 * contribute to sum_migrate_dl_bw.
> +			 */
>   			cs->nr_migrate_dl_tasks++;
> -			cs->sum_migrate_dl_bw += task->dl.dl_bw;
> +			if (dl_task_needs_bw_move(task, cs->effective_cpus))
> +				cs->sum_migrate_dl_bw += task->dl.dl_bw;
>   		}
>   	}
>   
> -	if (!cs->nr_migrate_dl_tasks)
> +	if (!cs->sum_migrate_dl_bw)
>   		goto out_success;
>   
> -	if (!cpumask_intersects(oldcs->effective_cpus, cs->effective_cpus)) {
> -		int cpu = cpumask_any_and(cpu_active_mask, cs->effective_cpus);
> -
> -		if (unlikely(cpu >= nr_cpu_ids)) {
> -			ret = -EINVAL;
> -			goto out_unlock;
> -		}
> +	cpu = cpumask_any_and(cpu_active_mask, cs->effective_cpus);
> +	if (unlikely(cpu >= nr_cpu_ids)) {
> +		ret = -EINVAL;
> +		goto out_unlock;
> +	}
>   
> -		ret = dl_bw_alloc(cpu, cs->sum_migrate_dl_bw);
> -		if (ret)
> -			goto out_unlock;
> +	ret = dl_bw_alloc(cpu, cs->sum_migrate_dl_bw);
> +	if (ret)
> +		goto out_unlock;
>   
> -		cs->dl_bw_cpu = cpu;
> -	}
> +	cs->dl_bw_cpu = cpu;
>   
>   out_success:
>   	/*
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index edca7849b165..7db4c87df83b 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -3107,20 +3107,18 @@ static void task_woken_dl(struct rq *rq, struct task_struct *p)
>   static void set_cpus_allowed_dl(struct task_struct *p,
>   				struct affinity_context *ctx)
>   {
> -	struct root_domain *src_rd;
>   	struct rq *rq;
>   
>   	WARN_ON_ONCE(!dl_task(p));
>   
>   	rq = task_rq(p);
> -	src_rd = rq->rd;
>   	/*
>   	 * Migrating a SCHED_DEADLINE task between exclusive
>   	 * cpusets (different root_domains) entails a bandwidth
>   	 * update. We already made space for us in the destination
>   	 * domain (see cpuset_can_attach()).
>   	 */
> -	if (!cpumask_intersects(src_rd->span, ctx->new_mask)) {
> +	if (dl_task_needs_bw_move(p, ctx->new_mask)) {
>   		struct dl_bw *src_dl_b;
>   
>   		src_dl_b = dl_bw_of(cpu_of(rq));
> @@ -3137,6 +3135,15 @@ static void set_cpus_allowed_dl(struct task_struct *p,
>   	set_cpus_allowed_common(p, ctx);
>   }
>   
> +bool dl_task_needs_bw_move(struct task_struct *p,
> +			   const struct cpumask *new_mask)
> +{
> +	if (!dl_task(p))
> +		return false;
> +
> +	return !cpumask_intersects(task_rq(p)->rd->span, new_mask);
> +}
> +
>   /* Assumes rq->lock is held */
>   static void rq_online_dl(struct rq *rq)
>   {

LGTM

Reviewed-by: Waiman Long <longman@redhat.com>