[RFC PATCH 07/16] sched/fair: Fix forced idle sibling starvation corner case

[Vineeth Remanan Pillai]

From: vpillai <vpillai@xxxxxxxxxxxxxxxx>

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 Remanan Pillai <vpillai@xxxxxxxxxxxxxxxx>
Signed-off-by: Julien Desfossez <jdesfossez@xxxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ae17507533a0..49fb93296e35 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10613,6 +10613,40 @@ static void rq_offline_fair(struct rq *rq)
 
 #endif /* CONFIG_SMP */
 
+#ifdef CONFIG_SCHED_CORE
+static inline bool
+__entity_slice_used(struct sched_entity *se)
+{
+	return (se->sum_exec_runtime - se->prev_sum_exec_runtime) >
+		sched_slice(cfs_rq_of(se), se);
+}
+
+/*
+ * If runqueue has only one task which used up its slice and if the sibling
+ * is forced idle, then trigger schedule to give forced idle task a chance.
+ */
+static void resched_forceidle_sibling(struct rq *rq, struct sched_entity *se)
+{
+	int cpu = cpu_of(rq), sibling_cpu;
+
+	if (rq->cfs.nr_running > 1 || !__entity_slice_used(se))
+		return;
+
+	for_each_cpu(sibling_cpu, cpu_smt_mask(cpu)) {
+		struct rq *sibling_rq;
+		if (sibling_cpu == cpu)
+			continue;
+		if (cpu_is_offline(sibling_cpu))
+			continue;
+
+		sibling_rq = cpu_rq(sibling_cpu);
+		if (sibling_rq->core_forceidle) {
+			resched_curr(sibling_rq);
+		}
+	}
+}
+#endif
+
 /*
  * scheduler tick hitting a task of our scheduling class.
  *
@@ -10636,6 +10670,11 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
 
 	update_misfit_status(curr, rq);
 	update_overutilized_status(task_rq(curr));
+
+#ifdef CONFIG_SCHED_CORE
+	if (sched_core_enabled(rq))
+		resched_forceidle_sibling(rq, &curr->se);
+#endif
 }
 
 /*
-- 
2.17.1