[PATCH v10 06/12] sched/fair: Load balance only among preferred CPUs

[Shrikanth Hegde <sshegde@linux.ibm.com>]

When a CPU is marked as non-preferred, any load pulled towards it is
pointless since the task will be pushed out again in the next tick.
So, consider only preferred CPUs for load balancing.

This ensures load balancing does not fight against the push task mechanism
which happens at the tick. Also, this stops active balancing from happening
on a non-preferred CPU pulling the load.

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

Bail out early for NEWIDLE balancing, as load balancing is done only on
preferred CPUs. Note that idle balancing is allowed to go through, since
that naturally updates nohz.next_balance when all the idle CPUs are
non-preferred.

Also, optimization in find_new_ilb() is skipped. The steal governor driver,
which is introduced in later patches, updates the preferred CPUs state in
descending order. find_new_ilb() checks for idle CPUs in ascending order.
Hence, in most common scenarios, the idle CPU found by find_new_ilb() will
already be a preferred CPU. When all idle CPUs are non-preferred, the first
idle CPU has to be chosen anyway. All of this is naturally handled in
find_new_ilb() currently. Adding additional complexity to it for rare
edge cases is not necessary.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 kernel/sched/fair.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index dcf860c59a14..04c2b2e17120 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13429,7 +13429,7 @@ 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]);
 
@@ -14544,10 +14544,8 @@ 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...
-	 */
-	if (!cpu_active(this_cpu))
+	/* Do not pull tasks towards !preferred CPUs */
+	if (!cpu_preferred(this_cpu))
 		return 0;
 
 	/*
-- 
2.47.3