sched/fair: Don't move tasks to lower capacity cpus unless necessary

When lower capacity CPUs are load balancing and considering to pull
something from a higher capacity group, we should not pull tasks from a
cpu with only one task running as this is guaranteed to impede progress
for that task. If there is more than one task running, load balance in
the higher capacity group would have already made any possible moves to
resolve imbalance and we should make better use of system compute
capacity by moving a task if we still have more than one running.

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b7fbfebbc024cc503d1d94046044f1fcdf0136e9..9270aa5f1ec0d9fb320437015a2ca0786d519be7 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8645,6 +8645,17 @@ static struct rq *find_busiest_queue(struct lb_env *env,
 
                capacity = capacity_of(i);
 
+               /*
+                * For ASYM_CPUCAPACITY domains, don't pick a cpu that could
+                * eventually lead to active_balancing high->low capacity.
+                * Higher per-cpu capacity is considered better than balancing
+                * average load.
+                */
+               if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
+                       capacity_of(env->dst_cpu) < capacity &&
+                       rq->nr_running == 1)
+                       continue;
+
                wl = weighted_cpuload(rq);
 
                /*