sched/fair: Don't set LBF_ALL_PINNED unnecessarily

Setting LBF_ALL_PINNED during active load balance is only valid when there
is only 1 running task on the rq otherwise this ends up increasing the
balance interval whereas other tasks could migrate after the next interval
once they become cache-cold as an example.

LBF_ALL_PINNED flag is now always set it by default. It is then cleared
when we find one task that can be pulled when calling detach_tasks() or
during active migration.
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20210107103325.30851-3-vincent.guittot@linaro.org

kernel/sched/fair.c

@@ -9639,6 +9639,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 	env.src_rq = busiest;
 
 	ld_moved = 0;
+	/* Clear this flag as soon as we find a pullable task */
+	env.flags |= LBF_ALL_PINNED;
 	if (busiest->nr_running > 1) {
 		/*
 		 * Attempt to move tasks. If find_busiest_group has found
@@ -9646,7 +9648,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		 * still unbalanced. ld_moved simply stays zero, so it is
 		 * correctly treated as an imbalance.
 		 */
-		env.flags |= LBF_ALL_PINNED;
 		env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
 
 more_balance:
@@ -9772,10 +9773,12 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 			if (!cpumask_test_cpu(this_cpu, busiest->curr->cpus_ptr)) {
 				raw_spin_unlock_irqrestore(&busiest->lock,
 							    flags);
-				env.flags |= LBF_ALL_PINNED;
 				goto out_one_pinned;
 			}
 
+			/* Record that we found at least one task that could run on this_cpu */
+			env.flags &= ~LBF_ALL_PINNED;
+
 			/*
 			 * ->active_balance synchronizes accesses to
 			 * ->active_balance_work.  Once set, it's cleared