[PATCH v7 05/12] sched/fair: Load balance only among preferred CPUs

[Shrikanth Hegde]

When cpu is marked as non preferred, any load pulled towards it is
pointless since in the next tick task will be pushed out again.
So, Consider only preferred CPUs for load balance.

This makes it not fight against the push task mechanism which happens
at tick. Also, this stops active balance to happen on non-preferred CPU
pulling the load.

This means there is no load balancing if the task is pinned only to
non-preferred CPUs. They will continue to run where they were previously
running before the CPUs was marked as non-preferred.

Bailout early for NEWIDLE and IDLE balance as load balancing is done
only on preferred CPUs.

Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
---
v6->v7:
- Merged two load balance related patches.

 kernel/sched/fair.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index df8c9c2c7918..53a34daf78d7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13399,7 +13399,7 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
 	};
 	bool need_unlock = false;
 
-	cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
+	cpumask_and(cpus, sched_domain_span(sd), cpu_preferred_mask);
 
 	schedstat_inc(sd->lb_count[idle]);
 
@@ -14308,6 +14308,10 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags)
 		if (!idle_cpu(balance_cpu))
 			continue;
 
+		/* There is no point in pulling the load, just to push it out next */
+		if (!cpu_preferred(balance_cpu))
+			continue;
+
 		/*
 		 * If this CPU gets work to do, stop the load balancing
 		 * work being done for other CPUs. Next load
@@ -14482,9 +14486,10 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
 	this_rq->idle_stamp = rq_clock(this_rq);
 
 	/*
-	 * Do not pull tasks towards !active CPUs...
+	 * Do not pull tasks towards !preferred CPUs...
+	 * preferred is always a subset of active.
 	 */
-	if (!cpu_active(this_cpu))
+	if (!cpu_preferred(this_cpu))
 		return 0;
 
 	/*
-- 
2.47.3