sched/fair: Make the use of prev_cpu consistent in the wakeup path

In commit:

  ac66f5477239 ("sched/numa: Introduce migrate_swap()")

select_task_rq() got a 'cpu' argument to enable overriding of prev_cpu
in special cases (NUMA task swapping).

However, the select_task_rq_fair() helper functions: wake_affine() and
select_idle_sibling(), still use task_cpu(p) directly to work out
prev_cpu, which leads to inconsistencies.

This patch passes prev_cpu (potentially overridden by NUMA code) into
the helper functions to ensure prev_cpu is indeed the same CPU
everywhere in the wakeup path.

cc: Ingo Molnar <mingo@redhat.com>
cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dietmar.eggemann@arm.com
Cc: linux-kernel@vger.kernel.org
Cc: mgalbraith@suse.de
Cc: vincent.guittot@linaro.org
Cc: yuyang.du@intel.com
Link: http://lkml.kernel.org/r/1466615004-3503-3-git-send-email-morten.rasmussen@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -656,7 +656,7 @@ static u64 sched_vslice(struct cfs_rq *cfs_rq, struct sched_entity *se)
 }
 
 #ifdef CONFIG_SMP
-static int select_idle_sibling(struct task_struct *p, int cpu);
+static int select_idle_sibling(struct task_struct *p, int prev_cpu, int cpu);
 static unsigned long task_h_load(struct task_struct *p);
 
 /*
@@ -1512,7 +1512,8 @@ balance:
 	 * Call select_idle_sibling to maybe find a better one.
 	 */
 	if (!cur)
-		env->dst_cpu = select_idle_sibling(env->p, env->dst_cpu);
+		env->dst_cpu = select_idle_sibling(env->p, env->src_cpu,
+						   env->dst_cpu);
 
 assign:
 	task_numa_assign(env, cur, imp);
@@ -5101,18 +5102,18 @@ static int wake_wide(struct task_struct *p)
 	return 1;
 }
 
-static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
+static int wake_affine(struct sched_domain *sd, struct task_struct *p,
+		       int prev_cpu, int sync)
 {
 	s64 this_load, load;
 	s64 this_eff_load, prev_eff_load;
-	int idx, this_cpu, prev_cpu;
+	int idx, this_cpu;
 	struct task_group *tg;
 	unsigned long weight;
 	int balanced;
 
 	idx	  = sd->wake_idx;
 	this_cpu  = smp_processor_id();
-	prev_cpu  = task_cpu(p);
 	load	  = source_load(prev_cpu, idx);
 	this_load = target_load(this_cpu, idx);
 
@@ -5277,11 +5278,10 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
 /*
  * Try and locate an idle CPU in the sched_domain.
  */
-static int select_idle_sibling(struct task_struct *p, int target)
+static int select_idle_sibling(struct task_struct *p, int prev, int target)
 {
 	struct sched_domain *sd;
 	struct sched_group *sg;
-	int i = task_cpu(p);
 
 	if (idle_cpu(target))
 		return target;
@@ -5289,8 +5289,8 @@ static int select_idle_sibling(struct task_struct *p, int target)
 	/*
 	 * If the prevous cpu is cache affine and idle, don't be stupid.
 	 */
-	if (i != target && cpus_share_cache(i, target) && idle_cpu(i))
-		return i;
+	if (prev != target && cpus_share_cache(prev, target) && idle_cpu(prev))
+		return prev;
 
 	/*
 	 * Otherwise, iterate the domains and find an eligible idle cpu.
@@ -5311,6 +5311,8 @@ static int select_idle_sibling(struct task_struct *p, int target)
 	for_each_lower_domain(sd) {
 		sg = sd->groups;
 		do {
+			int i;
+
 			if (!cpumask_intersects(sched_group_cpus(sg),
 						tsk_cpus_allowed(p)))
 				goto next;
@@ -5419,13 +5421,13 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
 
 	if (affine_sd) {
 		sd = NULL; /* Prefer wake_affine over balance flags */
-		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
+		if (cpu != prev_cpu && wake_affine(affine_sd, p, prev_cpu, sync))
 			new_cpu = cpu;
 	}
 
 	if (!sd) {
 		if (sd_flag & SD_BALANCE_WAKE) /* XXX always ? */
-			new_cpu = select_idle_sibling(p, new_cpu);
+			new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
 
 	} else while (sd) {
 		struct sched_group *group;