sched/fair: ignore backup CPU when not valid

The find_best_target can sometimes not return a valid backup CPU, either
because it cannot find one or just becasue it returns prev_cpu as a backup.
In these cases we should skip the energy_diff evaluation for the backup CPU.

Change-Id: I3787dbdfe74122348dd7a7485b88c4679051bd32
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>

Author: derkling

kernel/sched/fair.c

@@ -6686,7 +6686,9 @@ static int select_energy_cpu_brute(struct task_struct *p, int prev_cpu, int sync
 			/* No energy saving for target_cpu, try backup */
 			target_cpu = tmp_backup;
 			eenv.dst_cpu = target_cpu;
-			if (tmp_backup < 0 || energy_diff(&eenv) >= 0) {
+			if (tmp_backup < 0 ||
+			    tmp_backup == prev_cpu ||
+			    energy_diff(&eenv) >= 0) {
 				schedstat_inc(p, se.statistics.nr_wakeups_secb_no_nrg_sav);
 				schedstat_inc(this_rq(), eas_stats.secb_no_nrg_sav);
 				target_cpu = prev_cpu;