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    SubjectRe: [RT] BUG in sched/cpupri.c
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    On 07/01/2022 12:49, John Keeping wrote:
    > On Fri, Jan 07, 2022 at 11:46:45AM +0100, Dietmar Eggemann wrote:
    >> On 22/12/2021 20:48, Valentin Schneider wrote:
    >>> /*
    >>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
    >>> index ef8228d19382..8f3e3a1367b6 100644
    >>> --- a/kernel/sched/rt.c
    >>> +++ b/kernel/sched/rt.c
    >>> @@ -1890,6 +1890,16 @@ static int push_rt_task(struct rq *rq, bool pull)
    >>> if (!next_task)
    >>> return 0;
    >>>
    >>> + /*
    >>> + * It's possible that the next_task slipped in of higher priority than
    >>> + * current, or current has *just* changed priority. If that's the case
    >>> + * just reschedule current.
    >>> + */
    >>> + if (unlikely(next_task->prio < rq->curr->prio)) {
    >>> + resched_curr(rq);
    >>> + return 0;
    >>> + }
    >>
    >> IMHO, that's the bit which prevents the BUG.
    >>
    >> But this would also prevent the case in which rq->curr is an RT task
    >> with lower prio than next_task.
    >>
    >> Also `rq->curr = migration/X` goes still though which is somehow fine
    >> since find_lowest_rq() bails out for if (task->nr_cpus_allowed == 1).
    >>
    >> And DL tasks (like sugov:X go through and they can have
    >> task->nr_cpus_allowed > 1 (arm64 slow-switching boards with shared
    >> freuency domains with schedutil). cpupri_find_fitness()->convert_prio()
    >> can handle task_pri, p->prio = -1 (CPUPRI_INVALID) although its somehow
    >> by coincidence.
    >>
    >> So maybe something like this:
    >
    > Do you mean to replace just the one hunk from Valentin's patch with the
    > change below (keeping the rest), or are you saying that only the change
    > below is needed?

    The latter.

    I think Valentin wanted to see if something like this can also occur via
    sched_setscheduler() and maybe for this changes in switched_from_[rt/dl]
    will be necessary.

    >> @ -1898,6 +1898,11 @@ static int push_rt_task(struct rq *rq, bool pull)
    >> if (!pull || rq->push_busy)
    >> return 0;
    >>
    >> + if (rq->curr->sched_class != &rt_sched_class) {
    >> + resched_curr(rq);
    >> + return 0;
    >> + }
    >> +
    >> cpu = find_lowest_rq(rq->curr);
    >>
    >> [...

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