Re: [PATCH 01/12] sched/core: Skip migration disabled tasks in proxy execution

[K Prateek Nayak]

Hello Andrea,

On 7/2/2026 10:39 PM, Andrea Righi wrote:
> Never attempt to migrate migration-disabled tasks or tasks that can only
> run on a single CPU when switching donor's execution context, preventing
> task pinning violations.
> 
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
> ---
>  kernel/sched/core.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3cc6fb1d20547..8a3eecc7caf5d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6936,6 +6936,20 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  			 */
>  			if (curr_in_chain)
>  				return proxy_resched_idle(rq);
> +			/*
> +			 * Tasks pinned to a single CPU (per-CPU kthreads via
> +			 * kthread_bind(), tasks under migrate_disable()) cannot
> +			 * be moved to @owner_cpu. proxy_migrate_task() uses

We only move it to donate the vruntime context. It is never actually
run there.

> +			 * __set_task_cpu() which would silently violate the
> +			 * pinning and leave the task to run on a CPU outside
> +			 * its cpus_ptr once it is unblocked. Deactivate it on

For the task to run as normal, p->is_blocked needs to be cleared. It is
only done in the wakeup path (and sometimes in find_proxy_task() if
the task is rq->curr) which ensures all the affinity / migrate disable
bits get fixed when the task gets to actually run.

Where / how is this being violated?

> +			 * this CPU; the owner running elsewhere will wake @p
> +			 * back up when the mutex becomes available.
> +			 */
> +			if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {
> +				__clear_task_blocked_on(p, NULL);
> +				goto deactivate;
> +			}

Proxy depends on the ability to migrate the donor context to owner's
CPU without actually running the task. The task is superficially on
the CPU only to give it's runtime share to the lock owner. Feels like
you are tripping a shortcoming in ext core if this is a problem.

>  			goto migrate_task;
>  		}
>  

-- 
Thanks and Regards,
Prateek