7.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: K Prateek Nayak [ Upstream commit 118a2cf4ced60777a2408001e2b4838fed1cea55 ] Paul reported recent RCU changes exposed a lost resched where CPU idles with a rcutorture thread waiting while being completely runnable. Dumping the rq state at the time of idling with runnable tasks showed the following: rq->next_class = &fair_sched_class rq->nr_running = 1 rq->cfs.h_nr_queued = 0 rq->rt.rt_nr_running = 1 Although the waiting task was an RT task, "rq->next_class" did not point to rt_sched_class and was stuck at the lower priority fair_sched_class. Inspecting sched_change_end() revealed the following race with __sched_setscheduler() for queued, but non-running tasks: CPU0 CPU1 ==== ==== __schedule() /* * rq->donor is a fair task that blocks. * rq->next_class = &fair_sched_class */ rq_lock(rq0) pick_next_task() /* nr_running = 0 */ pick_task_fair() sched_balance_newidle() raw_spin_rq_unlock(rq0) rq_lock(rq0) activate_task(rq0, p) /* p is a fair task */ rq0->nr_running++; rq0->cfs.h_nr_queued++; wakeup_preempt(rq, p) rq->next_class = &fair_sched_class resched_curr() rq_unlock(rq0) __sched_setscheduler(p) /* p is moved to SCHED_RR */ task_rq_lock(p) sched_change_begin() rq0->cfs.h_nr_queued--; rq0->rt.rt_nr_running++; sched_change_end() /* * p->on_rq = 1 (ctx->queued) * but p is not rq->donor * (!ctx->running) hence * resched_curr() is and * rq->next_class upgrade is * skipped. */ task_rq_unlock() raw_spin_rq_lock(rq0) /* * rq->nr_running = 1 * rq->cfs.h_nr_queued = 0 * rq->rt.rt_nr_running = 1 * rq->next_class = &fair_sched_class */ if (rq->cfs.h_nr_queued && ...) /* Above is false */ if (rq_modified_above(this_rq, &fair_sched_class)) /* Also false since rq->next_class is still fair */ /* * sched_balance_newidle() is considered to * have failed to pull any fair tasks and no * other wakeups were detected. */ return NULL; /* pick_task_fair() */ pick_task_idle() return rq->idle; next = rq->idle; /* Preemption indicators are lost */ clear_tsk_need_resched(prev); clear_preempt_need_resched(); rq->curr = rq->idle; !!! rq->idle switches in with rq->running == 1 but !need_resched() !!! In simple words, enqueue, followed by a class change of the newly queued task goes unnoticed in rq->next_class indicator and sched_balance_newidle() sees no fair task, nor any higher priority task enqueued. The above sequence will lead to CPU idling with a runnable task on the CPU. Call wakeup_preempt() for ctx->queued cases in sched_change_end() to re-evaluate rq->next_class after class change (ENQUEUE_CLASS) and set the NEED_RESCHED indicator accordingly. Newidle balance will correctly see an enqueue of a higher priority task if a queued task was upgraded when the rq_lock() was dropped and will retry the pick. Closes: https://lore.kernel.org/lkml/8fa7f795-518c-4507-b684-60b0c1cac2c5@paulmck-laptop/ Fixes: 704069649b5b ("sched/core: Rework sched_class::wakeup_preempt() and rq_modified_*()") Reported-by: Paul E. McKenney Tested-by: Paul E. McKenney Signed-off-by: K Prateek Nayak Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Vincent Guittot Reviewed-by: Phil Auld Link: https://patch.msgid.link/20261013152455.9039-1-kprateek.nayak@amd.com Signed-off-by: Sasha Levin --- kernel/sched/core.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index c104562638e33..383ced4156e20 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -11323,17 +11323,16 @@ void sched_change_end(struct sched_change_ctx *ctx) if (p->sched_class->switched_to) p->sched_class->switched_to(rq, p); + /* + * Check if the task can preempt the current in case the class + * was upgraded. Note that none of the switch*_from() methods + * know the new class and none of the switch*_to() methods know + * the old class so we must resched here. + */ + if (ctx->queued) + wakeup_preempt(rq, p, 0); + if (ctx->running) { - /* - * If this was a class promotion; let the old class - * know it got preempted. Note that none of the - * switch*_from() methods know the new class and none - * of the switch*_to() methods know the old class. - */ - if (sched_class_above(p->sched_class, ctx->class)) { - rq->next_class->wakeup_preempt(rq, p, 0); - rq->next_class = p->sched_class; - } /* * If this was a degradation in class; make sure to * reschedule. -- 2.53.0