ANDROID: sched: Prevent unnecessary active balance of single task in sched group

Scenarios with the busiest group having just one task and the local
being idle on topologies with sched groups with different numbers of
cpus manage to dodge all load-balance bailout conditions resulting the
nr_balance_failed counter to be incremented. This eventually causes a
pointless active migration of the task. This patch prevents this by not
incrementing the counter when the busiest group only has one task.
ASYM_PACKING migrations and migrations due to reduced capacity should
still take place as these are explicitly captured by
need_active_balance().

A better solution would be to not attempt the load-balance in the first
place, but that requires significant changes to the order of bailout
conditions and statistics gathering.

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Change-Id: I6a2f51017c49614d5e4224f0e16f240ad8af6d0f
Signed-off-by: Chris Redpath <chris.redpath@arm.com>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5c09ddf8c8321ca1aa18a6ff7f78d3d2937ec92e..803eca831bb8c8fbd12de6503ce3c0adbd5f9eb1 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6580,6 +6580,7 @@ struct lb_env {
        int                     new_dst_cpu;
        enum cpu_idle_type      idle;
        long                    imbalance;
+       unsigned int            src_grp_nr_running;
        /* The set of CPUs under consideration for load-balancing */
        struct cpumask          *cpus;
 
@@ -7629,6 +7630,8 @@ next_group:
        if (env->sd->flags & SD_NUMA)
                env->fbq_type = fbq_classify_group(&sds->busiest_stat);
 
+       env->src_grp_nr_running = sds->busiest_stat.sum_nr_running;
+
        if (!env->sd->parent) {
                /* update overload indicator if we are at root domain */
                if (env->dst_rq->rd->overload != overload)
@@ -8251,7 +8254,8 @@ more_balance:
                 * excessive cache_hot migrations and active balances.
                 */
                if (idle != CPU_NEWLY_IDLE)
-                       sd->nr_balance_failed++;
+                       if (env.src_grp_nr_running > 1)
+                               sd->nr_balance_failed++;
 
                if (need_active_balance(&env)) {
                        unsigned long flags;