| Message ID | 20220127154059.974729-1-valentin.schneider@arm.com |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] sched/rt: Plug rt_mutex_setprio() vs push_rt_task() race | expand |
On 27/01/2022 16:40, Valentin Schneider wrote: > John reported that push_rt_task() can end up invoking > find_lowest_rq(rq->curr) when curr is not an RT task (in this case a CFS > one), which causes mayhem down convert_prio(). > > This can happen when current gets demoted to e.g. CFS when releasing an > rt_mutex, and the local CPU gets hit with an rto_push_work irqwork before > getting the chance to reschedule. Exactly who triggers this work isn't > entirely clear to me - switched_from_rt() only invokes rt_queue_pull_task() > if there are no RT tasks on the local RQ, which means the local CPU can't > be in the rto_mask. > > My current suspected sequence is something along the lines of the below, > with the demoted task being current. > > mark_wakeup_next_waiter() > rt_mutex_adjust_prio() > rt_mutex_setprio() // deboost originally-CFS task > check_class_changed() > switched_from_rt() // Only rt_queue_pull_task() if !rq->rt.rt_nr_running > switched_to_fair() // Sets need_resched > __balance_callbacks() // if pull_rt_task(), tell_cpu_to_push() can't select local CPU per the above > raw_spin_rq_unlock(rq) > > // need_resched is set, so task_woken_rt() can't > // invoke push_rt_tasks(). Best I can come up with is > // local CPU has rt_nr_migratory >= 2 after the demotion, so stays > // in the rto_mask, and then: > > <some other CPU running rto_push_irq_work_func() queues rto_push_work on this CPU> > push_rt_task() > // breakage follows here as rq->curr is CFS > > Move an existing check to check rq->curr vs the next pushable task's > priority before getting anywhere near find_lowest_rq(). While at it, add an > explicit sched_class of rq->curr check prior to invoking > find_lowest_rq(rq->curr). Align the DL logic to also reschedule regardless > of next_task's migratability. > > Link: http://lore.kernel.org/r/Yb3vXx3DcqVOi+EA@donbot > Fixes: a7c81556ec4d ("sched: Fix migrate_disable() vs rt/dl balancing") > Reported-by: John Keeping <john@metanate.com> > Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> > Tested-by: John Keeping <john@metanate.com> > --- > v1 -> v2: Reworded comments, added DL part (Dietmar) > --- LGTM. Rescheduling in case rq->curr has lower prio (including CFS tasks) than next_task and bailing out in case rq->curr is DL or stop-task prevents the bug from happening. The only small issue is the fact that, unlike in push_rt_task(), the DL logic only compares DL tasks (if (dl_task(rq->curr) ...), so you miss rescheduling when rq->curr is a lower priority non-DL task. Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com> [...]
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index d2c072b0ef01..62f0cf842277 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -2240,12 +2240,6 @@ static int push_dl_task(struct rq *rq) return 0; retry: - if (is_migration_disabled(next_task)) - return 0; - - if (WARN_ON(next_task == rq->curr)) - return 0; - /* * If next_task preempts rq->curr, and rq->curr * can move away, it makes sense to just reschedule @@ -2258,6 +2252,12 @@ static int push_dl_task(struct rq *rq) return 0; } + if (is_migration_disabled(next_task)) + return 0; + + if (WARN_ON(next_task == rq->curr)) + return 0; + /* We might release rq lock */ get_task_struct(next_task); diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 7b4f4fbbb404..14f273c29518 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -2026,6 +2026,16 @@ static int push_rt_task(struct rq *rq, bool pull) return 0; retry: + /* + * It's possible that the next_task slipped in of + * higher priority than current. If that's the case + * just reschedule current. + */ + if (unlikely(next_task->prio < rq->curr->prio)) { + resched_curr(rq); + return 0; + } + if (is_migration_disabled(next_task)) { struct task_struct *push_task = NULL; int cpu; @@ -2033,6 +2043,18 @@ static int push_rt_task(struct rq *rq, bool pull) if (!pull || rq->push_busy) return 0; + /* + * Invoking find_lowest_rq() on anything but an RT task doesn't + * make sense. Per the above priority check, curr has to + * be of higher priority than next_task, so no need to + * reschedule when bailing out. + * + * Note that the stoppers are masqueraded as SCHED_FIFO + * (cf. sched_set_stop_task()), so we can't rely on rt_task(). + */ + if (rq->curr->sched_class != &rt_sched_class) + return 0; + cpu = find_lowest_rq(rq->curr); if (cpu == -1 || cpu == rq->cpu) return 0; @@ -2057,16 +2079,6 @@ static int push_rt_task(struct rq *rq, bool pull) if (WARN_ON(next_task == rq->curr)) return 0; - /* - * It's possible that the next_task slipped in of - * higher priority than current. If that's the case - * just reschedule current. - */ - if (unlikely(next_task->prio < rq->curr->prio)) { - resched_curr(rq); - return 0; - } - /* We might release rq lock */ get_task_struct(next_task);
John reported that push_rt_task() can end up invoking find_lowest_rq(rq->curr) when curr is not an RT task (in this case a CFS one), which causes mayhem down convert_prio(). This can happen when current gets demoted to e.g. CFS when releasing an rt_mutex, and the local CPU gets hit with an rto_push_work irqwork before getting the chance to reschedule. Exactly who triggers this work isn't entirely clear to me - switched_from_rt() only invokes rt_queue_pull_task() if there are no RT tasks on the local RQ, which means the local CPU can't be in the rto_mask. My current suspected sequence is something along the lines of the below, with the demoted task being current. mark_wakeup_next_waiter() rt_mutex_adjust_prio() rt_mutex_setprio() // deboost originally-CFS task check_class_changed() switched_from_rt() // Only rt_queue_pull_task() if !rq->rt.rt_nr_running switched_to_fair() // Sets need_resched __balance_callbacks() // if pull_rt_task(), tell_cpu_to_push() can't select local CPU per the above raw_spin_rq_unlock(rq) // need_resched is set, so task_woken_rt() can't // invoke push_rt_tasks(). Best I can come up with is // local CPU has rt_nr_migratory >= 2 after the demotion, so stays // in the rto_mask, and then: <some other CPU running rto_push_irq_work_func() queues rto_push_work on this CPU> push_rt_task() // breakage follows here as rq->curr is CFS Move an existing check to check rq->curr vs the next pushable task's priority before getting anywhere near find_lowest_rq(). While at it, add an explicit sched_class of rq->curr check prior to invoking find_lowest_rq(rq->curr). Align the DL logic to also reschedule regardless of next_task's migratability. Link: http://lore.kernel.org/r/Yb3vXx3DcqVOi+EA@donbot Fixes: a7c81556ec4d ("sched: Fix migrate_disable() vs rt/dl balancing") Reported-by: John Keeping <john@metanate.com> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> Tested-by: John Keeping <john@metanate.com> --- v1 -> v2: Reworded comments, added DL part (Dietmar) --- kernel/sched/deadline.c | 12 ++++++------ kernel/sched/rt.c | 32 ++++++++++++++++++++++---------- 2 files changed, 28 insertions(+), 16 deletions(-)