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sched/fair: Fix forced idle sibling starvation corner case
If there is only one long running local task and the sibling is forced idle, it might not get a chance to run until a schedule event happens on any cpu in the core. So we check for this condition during a tick to see if a sibling is starved and then give it a chance to schedule. Signed-off-by: Vineeth Pillai <viremana@linux.microsoft.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-by: Don Hiatt <dhiatt@digitalocean.com> Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com> Tested-by: Vincent Guittot <vincent.guittot@linaro.org> Link: https://lkml.kernel.org/r/20210422123308.617407840@infradead.org
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@ -5459,16 +5459,15 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
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/* reset state */
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rq->core->core_cookie = 0UL;
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if (rq->core->core_forceidle) {
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need_sync = true;
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rq->core->core_forceidle = false;
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}
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for_each_cpu(i, smt_mask) {
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struct rq *rq_i = cpu_rq(i);
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rq_i->core_pick = NULL;
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if (rq_i->core_forceidle) {
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need_sync = true;
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rq_i->core_forceidle = false;
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}
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if (i != cpu)
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update_rq_clock(rq_i);
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}
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@ -5588,8 +5587,10 @@ next_class:;
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if (!rq_i->core_pick)
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continue;
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if (is_task_rq_idle(rq_i->core_pick) && rq_i->nr_running)
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rq_i->core_forceidle = true;
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if (is_task_rq_idle(rq_i->core_pick) && rq_i->nr_running &&
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!rq_i->core->core_forceidle) {
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rq_i->core->core_forceidle = true;
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}
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if (i == cpu) {
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rq_i->core_pick = NULL;
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@ -10767,6 +10767,44 @@ static void rq_offline_fair(struct rq *rq)
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#endif /* CONFIG_SMP */
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#ifdef CONFIG_SCHED_CORE
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static inline bool
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__entity_slice_used(struct sched_entity *se, int min_nr_tasks)
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{
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u64 slice = sched_slice(cfs_rq_of(se), se);
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u64 rtime = se->sum_exec_runtime - se->prev_sum_exec_runtime;
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return (rtime * min_nr_tasks > slice);
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}
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#define MIN_NR_TASKS_DURING_FORCEIDLE 2
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static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
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{
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if (!sched_core_enabled(rq))
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return;
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/*
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* If runqueue has only one task which used up its slice and
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* if the sibling is forced idle, then trigger schedule to
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* give forced idle task a chance.
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*
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* sched_slice() considers only this active rq and it gets the
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* whole slice. But during force idle, we have siblings acting
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* like a single runqueue and hence we need to consider runnable
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* tasks on this cpu and the forced idle cpu. Ideally, we should
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* go through the forced idle rq, but that would be a perf hit.
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* We can assume that the forced idle cpu has atleast
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* MIN_NR_TASKS_DURING_FORCEIDLE - 1 tasks and use that to check
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* if we need to give up the cpu.
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*/
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if (rq->core->core_forceidle && rq->cfs.nr_running == 1 &&
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__entity_slice_used(&curr->se, MIN_NR_TASKS_DURING_FORCEIDLE))
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resched_curr(rq);
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}
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#else
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static inline void task_tick_core(struct rq *rq, struct task_struct *curr) {}
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#endif
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/*
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* scheduler tick hitting a task of our scheduling class.
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*
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@ -10790,6 +10828,8 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
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update_misfit_status(curr, rq);
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update_overutilized_status(task_rq(curr));
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task_tick_core(rq, curr);
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}
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/*
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@ -1083,12 +1083,12 @@ struct rq {
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unsigned int core_enabled;
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unsigned int core_sched_seq;
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struct rb_root core_tree;
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unsigned char core_forceidle;
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/* shared state */
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unsigned int core_task_seq;
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unsigned int core_pick_seq;
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unsigned long core_cookie;
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unsigned char core_forceidle;
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#endif
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};
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