sched/fair: Defer calculation of 'prev_eff_load' in wake_affine_weight() until needed

On sync wakeups, the previous CPU effective load may not be used so delay
the calculation until it's needed.
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Giovanni Gherdovich <ggherdovich@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180213133730.24064-3-mgorman@techsingularity.netSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -5724,7 +5724,6 @@ wake_affine_weight(struct sched_domain *sd, struct task_struct *p,
 	unsigned long task_load;
 
 	this_eff_load = target_load(this_cpu, sd->wake_idx);
-	prev_eff_load = source_load(prev_cpu, sd->wake_idx);
 
 	if (sync) {
 		unsigned long current_load = task_h_load(current);
@@ -5742,6 +5741,7 @@ wake_affine_weight(struct sched_domain *sd, struct task_struct *p,
 		this_eff_load *= 100;
 	this_eff_load *= capacity_of(prev_cpu);
 
+	prev_eff_load = source_load(prev_cpu, sd->wake_idx);
 	prev_eff_load -= task_load;
 	if (sched_feat(WA_BIAS))
 		prev_eff_load *= 100 + (sd->imbalance_pct - 100) / 2;