On Fri, Jan 07, 2022 at 03:25:21PM +0100, Dietmar Eggemann wrote:
> 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.
Thanks! I tested the patch below and can confirm that I no longer see
any BUGs with this applied.
Tested-By: John Keeping <john@xxxxxxxxxxxx>
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -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);
if (cpu == -1 || cpu == rq->cpu)
return 0;
