Re: [PATCH 05/12] sched: Move sched_class::prio_changed() into the change pattern

[K Prateek Nayak]

Hello Peter,

On 1/13/2026 5:17 PM, Peter Zijlstra wrote:
> Hum... so this one is a little more tricky.
> 
> So the normal rules are that DEQUEUE_SAVE + ENQUEUE_RESTORE should be as
> invariant as possible.
> 
> But what I think happens here is that at the point of dequeue we are
> effectively ready to throttle/replenish, but we don't.
> 
> Then at enqueue, we do. The replenish changes the deadline and we're up
> a creek.

I've the following data from the scenario in which I observe
the same splat as Pierre splat wit the two fixes on top of tip:

    yes-4108    [194] d..2.    53.396872: get_prio_dl: get_prio_dl: clock(53060728757)
    yes-4108    [194] d..2.    53.396873: update_curr_dl_se: update_curr_dl_se: past throttle label
    yes-4108    [194] d..2.    53.396873: update_curr_dl_se: dl_throttled(0) dl_overrun(0) timer_queued(0) server?(0)
    yes-4108    [194] d..2.    53.396873: update_curr_dl_se: dl_se->runtime(190623) rq->dl.overloaded(0)
    yes-4108    [194] d..2.    53.396874: get_prio_dl: get_prio_dl: deadline(53060017809)

    yes-4108    [194] d..2.    53.396878: enqueue_dl_entity: ENQUEUE_RESTORE update_dl_entity
    yes-4108    [194] d..2.    53.396878: enqueue_dl_entity: setup_new_dl_entity
    yes-4108    [194] d..2.    53.396878: enqueue_dl_entity: Replenish: Old: 53060017809 dl_deadline(1000000)
    yes-4108    [194] d..2.    53.396879: enqueue_dl_entity: Replenish: New: 53061728757
    yes-4108    [194] d..2.    53.396882: prio_changed_dl.part.0: Woops! prio_changed_dl: CPU(194) clock(53060728757) overloaded(0): Task: yes(4108), Curr: yes(4108) deadline: 53060017809 -> 53061728757

get_prio_dl() sees "deadline < rq->clock" but dl_se->runtime is still
positive so update_curr_dl_se() doesn't fiddle with the deadline.

ENQUEUE_RESTORE sees "deadline" before "rq->clock" and calls
setup_new_dl_entity() which calls replenish.

sched_change_end() will call prio_changed() with the old deadline from
get_prio_dl() but enqueue advanced the deadline so we land in a
pickle.

> 
> Let me think about this for a bit...

Should prio_changed_dl() care about "dl_se->dl_deadline" having changed
within the sched_change guard since that is the attribute that can be
changed using sched_setattr() right?

-- 
Thanks and Regards,
Prateek