Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for BALANCE_WAKE

[Peter Zijlstra <peterz@infradead.org>]

So maybe you want something like the below; that cures the thing Morten
raised, and we continue looking for sd, even after we found affine_sd.

It also avoids the pointless idle_cpu() check Mike raised by making
select_idle_sibling() return -1 if it doesn't find anything.

Then it continues doing the full balance IFF sd was set, which is keyed
off of sd->flags.

And note (as Mike already said), BALANCE_WAKE does _NOT_ look for idle
CPUs, it looks for the least loaded CPU. And its damn expensive.


Rewriting this entire thing is somewhere on the todo list :/

---
 kernel/sched/fair.c | 36 +++++++++++++++++++++++-------------
 1 file changed, 23 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d597aea43030..7330fb4fea9c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1385,8 +1385,13 @@ static void task_numa_compare(struct task_numa_env *env,
 	 * One idle CPU per node is evaluated for a task numa move.
 	 * Call select_idle_sibling to maybe find a better one.
 	 */
-	if (!cur)
-		env->dst_cpu = select_idle_sibling(env->p, env->dst_cpu);
+	if (!cur) {
+		int cpu = select_idle_sibling(env->p, env->dst_cpu);
+		if (cpu < 0)
+			cpu = env->dst_cpu;
+
+		env->dst_cpu = cpu;
+	}
 
 assign:
 	task_numa_assign(env, cur, imp);
@@ -4951,16 +4956,15 @@ static int select_idle_sibling(struct task_struct *p, int target)
 					goto next;
 			}
 
-			target = cpumask_first_and(sched_group_cpus(sg),
+			return cpumask_first_and(sched_group_cpus(sg),
 					tsk_cpus_allowed(p));
-			goto done;
 next:
 			sg = sg->next;
 		} while (sg != sd->groups);
 	}
-done:
-	return target;
+	return -1;
 }
+
 /*
  * get_cpu_usage returns the amount of capacity of a CPU that is used by CFS
  * tasks. The unit of the return value must be the one of capacity so we can
@@ -5022,22 +5026,28 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
 		 * If both cpu and prev_cpu are part of this domain,
 		 * cpu is a valid SD_WAKE_AFFINE target.
 		 */
-		if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
-		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
+		if (want_affine && !affine_sd &&
+		    (tmp->flags & SD_WAKE_AFFINE) &&
+		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
 			affine_sd = tmp;
-			break;
-		}
 
 		if (tmp->flags & sd_flag)
 			sd = tmp;
+		else if (!want_affine || (want_affine && affine_sd))
+			break;
 	}
 
-	if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync))
+	if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync)) {
 		prev_cpu = cpu;
+		sd = NULL; /* WAKE_AFFINE trumps BALANCE_WAKE */
+	}
 
 	if (sd_flag & SD_BALANCE_WAKE) {
-		new_cpu = select_idle_sibling(p, prev_cpu);
-		goto unlock;
+		int tmp = select_idle_sibling(p, prev_cpu);
+		if (tmp >= 0) {
+			new_cpu = tmp;
+			goto unlock;
+		}
 	}
 
 	while (sd) {