[RESEND RFC PATCH v2 28/29] [EXPERIMENTAL] sched/fair: Add a local counter to rate limit task push

[K Prateek Nayak]

Pushing tasks can fail for multitude of reasons - task affinity, the
unavailability of an idle CPUs by the time balance callback is executed,
etc.

Maintain a CPU local counter in sched_domain to rate limit push attempts
if the failures build up. This counter is reset at the time of periodic
balance to the value in "nr_idle_scan".

Since "nr_idle_scan" is only computed for SIS_UTIL, rate limiting has
been guarded behind the same sched_feat().

Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
 include/linux/sched/topology.h |  4 ++++
 kernel/sched/fair.c            | 23 +++++++++++++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index 074ee2980cdf..ebe26ce82c1a 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -122,6 +122,10 @@ struct sched_domain {
 	unsigned int alb_failed;
 	unsigned int alb_pushed;
 
+	/* Push load balancing */
+	unsigned long last_nr_push_update;
+	int nr_push_attempt;
+
 	/* SD_BALANCE_EXEC stats */
 	unsigned int sbe_count;
 	unsigned int sbe_balanced;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 34aeb8e58e0b..46d33ab63336 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12356,6 +12356,16 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
 		rq->max_idle_balance_cost =
 			max((u64)sysctl_sched_migration_cost, max_cost);
 	}
+	if (sched_feat(SIS_UTIL)) {
+		sd = rcu_dereference(per_cpu(sd_llc, cpu));
+
+		if (sd && sd->shared &&
+		    time_after_eq(jiffies, sd->last_nr_push_update + sd->min_interval)) {
+			sd->nr_push_attempt = READ_ONCE(sd->shared->nr_idle_scan);
+			sd->last_nr_push_update = jiffies;
+		}
+	}
+
 	rcu_read_unlock();
 
 	/*
@@ -13110,8 +13120,6 @@ static inline bool should_push_tasks(struct rq *rq)
 	struct sched_domain *sd;
 	int cpu = cpu_of(rq);
 
-	/* TODO: Add a CPU local failure counter. */
-
 	/* CPU doesn't have any fair task to push. */
 	if (!has_pushable_tasks(rq))
 		return false;
@@ -13126,6 +13134,10 @@ static inline bool should_push_tasks(struct rq *rq)
 	if (!sd)
 		return false;
 
+	/* We've failed to push task too many times. */
+	if (sched_feat(SIS_UTIL) && sd->nr_push_attempt <= 0)
+		return false;
+
 	/*
 	 * We may not be able to find a push target.
 	 * Skip for this tick and depend on the periodic
@@ -13176,6 +13188,13 @@ static bool push_fair_task(struct rq *rq)
 		return true;
 	}
 
+	/*
+	 * If the push failed after a full search, decrement the
+	 * attempt counter to dicourage further attempts. Periodic
+	 * balancer will reset the "nr_push_attempt" after a while.
+	 */
+	sd->nr_push_attempt--;
+
 	return false;
 }
 
-- 
2.43.0