Re: [PATCH v2] sched: Reject policy changes with SCHED_FLAG_KEEP_PARAMS

[K Prateek Nayak]

Hello Andrea,

On 7/30/2026 7:28 PM, Andrea Righi wrote:
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index b215b0ead9a60..8fb8474d0a0ec 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -645,12 +645,19 @@ int __sched_setscheduler(struct task_struct *p,
>  		goto recheck;
>  	}
>  
> +	/* KEEP_PARAMS only makes sense if the scheduling policy is unchanged */
> +	if ((attr->sched_flags & SCHED_FLAG_KEEP_PARAMS) && policy != p->policy) {
> +		retval = -EINVAL;
> +		goto unlock;
> +	}
> +
>  	/*
>  	 * If setscheduling to SCHED_DEADLINE (or changing the parameters
>  	 * of a SCHED_DEADLINE task) we need to check if enough bandwidth
>  	 * is available.
>  	 */
> -	if ((dl_policy(policy) || dl_task(p)) && sched_dl_overflow(p, policy, attr)) {
> +	if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS) &&
> +	    (dl_policy(policy) || dl_task(p)) && sched_dl_overflow(p, policy, attr)) {
>  		retval = -EBUSY;
>  		goto unlock;
>  	}

On an unrelated side note, similar concern exists for p->reset_on_fork and
that it can be changed by a parallel sched_setscheduler() that finished
before and the one that is lagging can continue with a stale copy.

reset_on_fork is computed outside the rq_lock for KEEP_POLICY case and
p->reset_on_fork will be set to that if nothing else changes (same policy,
same attributes, no uclamp changes) in the early unlock case.

Peter, is that a concern?

> @@ -675,7 +682,7 @@ int __sched_setscheduler(struct task_struct *p,
>  	prev_class = p->sched_class;
>  	next_class = __setscheduler_class(policy, newprio);
>  
> -	if (prev_class != next_class)
> +	if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS) && prev_class != next_class)
>  		queue_flags |= DEQUEUE_CLASS;

Can this happen if we've already ensured policy is unchanged under
rq_lock for KEEP_PARAMS? The

    newprio = __normal_prio(policy, ...);

above would have fixed it under the rq_lock right based on policy
right?

>  
>  	scoped_guard (sched_change, p, queue_flags) {

-- 
Thanks and Regards,
Prateek