Re: [PATCH 04/15] sched: Skip class callbacks with SCHED_FLAG_KEEP_PARAMS

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Using SCHED_FLAG_KEEP_PARAMS with a new SCHED_DEADLINE policy on a non-deadline task causes a permanent deadline bandwidth leak in the root domain.
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--- Patch [4]: [PATCH 04/15] sched: Skip class callbacks with SCHED_FLAG_KEEP_PARAMS ---
commit 7456fab35f0046cf8daada3381577cde13d7dbe1
Author: Andrea Righi <arighi@nvidia.com>

sched: Skip class callbacks with SCHED_FLAG_KEEP_PARAMS

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 commit sets DEQUEUE_CLASS only when SCHED_FLAG_KEEP_PARAMS allows
the class to change.

> 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;
>  
>  	scoped_guard (sched_change, p, queue_flags) {

[Severity: High]
This is a pre-existing issue and was not introduced by the patch under review,
but does using SCHED_FLAG_KEEP_PARAMS with a new SCHED_DEADLINE policy on a
non-deadline task cause a permanent deadline bandwidth leak in the root
domain?

When a user with CAP_SYS_NICE calls sched_setattr() with
attr.sched_policy = SCHED_DEADLINE and SCHED_FLAG_KEEP_PARAMS on a
non-deadline task, sys_sched_setattr() fetches the current parameters:

sys_sched_setattr()
    if (attr.sched_flags & SCHED_FLAG_KEEP_PARAMS)
        get_params(p, &attr, 0);

This can replace attr.sched_runtime with the task's current fair slice while
leaving attr.sched_period user-provided, constructing a valid deadline ratio.

Earlier in __sched_setscheduler(), this requested bandwidth is validated and
permanently added to the global root domain accounting via __dl_add() inside
sched_dl_overflow():

    if ((dl_policy(policy) || dl_task(p)) && sched_dl_overflow(p, policy, attr)) {

However, because SCHED_FLAG_KEEP_PARAMS is set, the parameter and class
transition is skipped further down inside the scoped_guard block:

    if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
        __setscheduler_params(p, attr);
        p->sched_class = next_class;
        p->prio = newprio;
        __setscheduler_dl_pi(newprio, policy, p, scope);
    }

Since the task remains non-deadline and p->dl.dl_bw remains 0, when the task
eventually exits, it will subtract its dl_bw (0) instead of the allocated
bandwidth, leaving the globally added bandwidth permanently leaked.

Is there a way to prevent sched_dl_overflow() from allocating bandwidth if we
know SCHED_FLAG_KEEP_PARAMS will skip the parameter updates?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260728154425.1549660-1-arighi@nvidia.com?part=4