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>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 56362ac036b23976fe616013a1fb3f66f2f23213..2dc28766cf9a3ab3c0b00ba4089b8aeda635f7f3 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5650,6 +5650,7 @@ 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;
 
@@ -6612,6 +6613,8 @@ 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)
@@ -7240,7 +7243,8 @@ 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)) {
                        raw_spin_lock_irqsave(&busiest->lock, flags);