Re: [PATCH 1/2] sched_ext: Auto-register/unregister dl_server reservations

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 21, 2026 at 07:33:56PM +0200, Andrea Righi wrote:

> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 9c458552d14ff..15ba49fcba9af 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -6061,6 +6061,7 @@ static void scx_root_disable(struct scx_sched *sch)
>  {
>  	struct scx_task_iter sti;
>  	struct task_struct *p;
> +	bool was_switched_all;
>  	int cpu;
>  
>  	/* guarantee forward progress and wait for descendants to be disabled */
> @@ -6087,6 +6088,13 @@ static void scx_root_disable(struct scx_sched *sch)
>  	 */
>  	mutex_lock(&scx_enable_mutex);
>  
> +	/*
> +	 * Snapshot the full vs partial mode before clearing the static
> +	 * branch, so the dl_server re-balance below knows whether the
> +	 * fair_server reservation needs to be reinstated.
> +	 */
> +	was_switched_all = scx_switched_all();
> +
>  	static_branch_disable(&__scx_switched_all);
>  	WRITE_ONCE(scx_switching_all, false);
>  
> @@ -6136,10 +6144,24 @@ static void scx_root_disable(struct scx_sched *sch)
>  	/*
>  	 * Invalidate all the rq clocks to prevent getting outdated
>  	 * rq clocks from a previous scx scheduler.
> +	 *
> +	 * Also re-balance the dl_server bandwidth reservations: detach
> +	 * ext_server (no more sched_ext tasks) and reinstate fair_server
> +	 * if it was previously detached because we were running in full
> +	 * mode. Detach before attach to avoid a transient overflow of the
> +	 * root domain's bandwidth capacity.
>  	 */
>  	for_each_possible_cpu(cpu) {
>  		struct rq *rq = cpu_rq(cpu);
> +
>  		scx_rq_clock_invalidate(rq);
> +
> +		scoped_guard(rq_lock_irqsave, rq) {
> +			dl_server_detach_bw(&rq->ext_server);
> +			if (was_switched_all &&
> +			    WARN_ON_ONCE(dl_server_attach_bw(&rq->fair_server)))
> +				pr_warn("failed to re-attach fair_server on CPU %d\n", cpu);
> +		}
>  	}
>  
>  	/* no task is on scx, turn off all the switches and flush in-progress calls */
> @@ -7314,6 +7336,27 @@ static void scx_root_enable_workfn(struct kthread_work *work)
>  	if (!(ops->flags & SCX_OPS_SWITCH_PARTIAL))
>  		static_branch_enable(&__scx_switched_all);
>  
> +	/*
> +	 * Re-balance the dl_server bandwidth reservations.
> +	 *
> +	 * In full mode (!SCX_OPS_SWITCH_PARTIAL) no task will ever run in
> +	 * the fair class, so detach the fair_server reservation and give
> +	 * that bandwidth back to the RT class. Always attach the
> +	 * ext_server reservation since sched_ext tasks are now possible.
> +	 *
> +	 * Detach before attach to avoid a transient overflow of the root
> +	 * domain's bandwidth capacity.
> +	 */
> +	for_each_possible_cpu(cpu) {
> +		struct rq *rq = cpu_rq(cpu);
> +
> +		guard(rq_lock_irqsave)(rq);
> +		if (scx_switched_all())
> +			dl_server_detach_bw(&rq->fair_server);
> +		if (WARN_ON_ONCE(dl_server_attach_bw(&rq->ext_server)))
> +			pr_warn("failed to attach ext_server on CPU %d\n", cpu);
> +	}
> +
>  	pr_info("sched_ext: BPF scheduler \"%s\" enabled%s\n",
>  		sch->ops.name, scx_switched_all() ? "" : " (partial)");
>  	kobject_uevent(&sch->kobj, KOBJ_ADD);

For switching *to* scx, I think it makes sense to attach ext_server
early and fail the switch if the attach fails. And only after the
switch, conditionally detach fair_server.

Since switching back to fair is a recovery path, this isn't really an
option -- the only actual option is keeping the fair_server reservation,
but that isn't ideal either.