Re: [PATCH 1/2] sched: Skip class callbacks with SCHED_FLAG_KEEP_PARAMS

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Andrea,

On 7/30/2026 11:20 AM, Andrea Righi wrote:
> SCHED_FLAG_KEEP_PARAMS suppresses scheduler parameter and class changes,
> but __sched_setscheduler() can still set DEQUEUE_CLASS when the class
> computed from the requested policy differs from the task's current
> class.
> 
> This causes the switching_from(), switched_from(), switching_to() and
> switched_to() callbacks to run even though p->sched_class remains
> unchanged.
> 
> Set DEQUEUE_CLASS only when the sched class is allowed to change.
> 
> Fixes: 637b0682821b ("sched: Fold sched_class::switch{ing,ed}_{to,from}() into the change pattern")
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/syscalls.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index b215b0ead9a60..903b47f5d0b74 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -675,7 +675,8 @@ 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;

It seems there also exists a SCHED_FLAG_KEEP_POLICY which should be used
to retain the policy. By a quick look at the syscall path,
SCHED_FLAG_KEEP_PARAMS should just do an equivalent of:

    get_params(p, &attr, 0)

and match the some properties of sched_attr with that of the task.

The task is still free to switch classes if the attr->sched_policy
differs given SCHED_FLAG_KEEP_PARAMS and SCHED_FLAG_KEEP_POLICY are two
distinct flags and there is a separate SCHED_FLAG_KEEP_ALL to combines
them both.

I'm having a sneaky suspicion that the bit in sched_change guard below
should actually check for SCHED_FLAG_KEEP_ALL like:

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index b215b0ead9a6..baf1fc59069e 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -680,7 +680,7 @@ int __sched_setscheduler(struct task_struct *p,
 
 	scoped_guard (sched_change, p, queue_flags) {
 
-		if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
+		if ((attr->sched_flags & SCHED_FLAG_KEEP_ALL) != SCHED_FLAG_KEEP_ALL) {
 			__setscheduler_params(p, attr);
 			p->sched_class = next_class;
 			p->prio = newprio;
---

>  
>  	scoped_guard (sched_change, p, queue_flags) {

-- 
Thanks and Regards,
Prateek