Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Kuba Piecuch <jpiecuch@google.com>]

On Wed Apr 15, 2026 at 6:17 AM UTC, Cheng-Yang Chou wrote:
> From df62cc372cb9fab7aa9b666a6a127d8248923dd8 Mon Sep 17 00:00:00 2001
> From: Cheng-Yang Chou <yphbchou0911@gmail.com>
> Date: Mon, 13 Apr 2026 15:19:59 +0800
> Subject: [PATCH v3 sched_ext/for-7.1] tools/sched_ext: Handle migration-disabled tasks in
>  scx_central
>
> When a task calls migrate_disable(), p->cpus_ptr is not updated until
> migrate_disable_switch() runs during context switch, so dispatch_to_cpu()
> may dequeue such a task and dispatch it to a CPU it cannot run on.
>
> Extend the mismatch check in dispatch_to_cpu() to also test
> is_migration_disabled() alongside the cpumask check, so tasks in this
> window are bounced to the fallback DSQ.
>
> Suggested-by: Andrea Righi <arighi@nvidia.com>
> Suggested-by: Tejun Heo <tj@kernel.org>
> Suggested-by: Kuba Piecuch <jpiecuch@google.com>
> Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
> ---
> v3:
>  - Drop the select_cpu() shortcut, revert enqueue() to the kthread-only
>    forward-progress path, and expand the dispatch_to_cpu() mismatch
>    check to cover is_migration_disabled() (Tejun Heo)
>  - Drop kick_cpu() on mismatch (Andrea Righi)
>  - Reword commit subject and message with the fix
>
> v2:
>  - Add is_migration_disabled() check in enqueue() as p->nr_cpus_allowed
>    is not updated until migrate_disable_switch() runs during context
>    switch, which happens after ops.enqueue(). (Kuba Piecuch)
>
>  tools/sched_ext/scx_central.bpf.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
> index 4efcce099bd5..64dd60b3e922 100644
> --- a/tools/sched_ext/scx_central.bpf.c
> +++ b/tools/sched_ext/scx_central.bpf.c
> @@ -149,10 +149,14 @@ static bool dispatch_to_cpu(s32 cpu)
>  		}
>  
>  		/*
> -		 * If we can't run the task at the top, do the dumb thing and
> -		 * bounce it to the fallback dsq.
> +		 * If we can't run the task at the top for whatever reason,
> +		 * bounce it to the fallback dsq. Also check
> +		 * is_migration_disabled() explicitly as p->cpus_ptr may not
> +		 * reflect the migration-disabled state yet if
> +		 * migrate_disable_switch() hasn't run.
>  		 */
> -		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
> +		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr) ||
> +		    (is_migration_disabled(p) && scx_bpf_task_cpu(p) != cpu)) {
>  			__sync_fetch_and_add(&nr_mismatches, 1);
>  			scx_bpf_dsq_insert(p, FALLBACK_DSQ_ID, SCX_SLICE_INF, 0);
>  			bpf_task_release(p);

Looks good to me!

Minor email-related comment: I would prefer if patch revisions (v2, v3, ...)
were sent in separate emails with their own subject line, otherwise it may look
like I'm reviewing the v1, if you look solely at the subject line.

To be clear, the following tag applies to v3.

Reviewed-by: Kuba Piecuch <jpiecuch@google.com>