sched/fair: Fix the sd_parent_degenerate() code

I found that on my WSM box I had a redundant domain:

[    0.949769] CPU0 attaching sched-domain:
[    0.953765]  domain 0: span 0,12 level SIBLING
[    0.958335]   groups: 0 (cpu_power = 587) 12 (cpu_power = 588)
[    0.964548]   domain 1: span 0-5,12-17 level MC
[    0.969206]    groups: 0,12 (cpu_power = 1175) 1,13 (cpu_power = 1176) 2,14 (cpu_power = 1176) 3,15 (cpu_power = 1176) 4,16 (cpu_power = 1176) 5,17 (cpu_power = 1176)
[    0.984993]    domain 2: span 0-5,12-17 level CPU
[    0.989822]     groups: 0-5,12-17 (cpu_power = 7055)
[    0.995049]     domain 3: span 0-23 level NUMA
[    0.999620]      groups: 0-5,12-17 (cpu_power = 7055) 6-11,18-23 (cpu_power = 7056)

Note how domain 2 has only a single group and spans the same CPUs as
domain 1. We should not keep such domains and do in fact have code to
prune these.

It turns out that the 'new' SD_PREFER_SIBLING flag causes this, it
makes sd_parent_degenerate() fail on the CPU domain. We can easily
fix this by 'ignoring' the SD_PREFER_SIBLING bit and transfering it
to whatever domain ends up covering the span.

With this patch the domains now look like this:

[    0.950419] CPU0 attaching sched-domain:
[    0.954454]  domain 0: span 0,12 level SIBLING
[    0.959039]   groups: 0 (cpu_power = 587) 12 (cpu_power = 588)
[    0.965271]   domain 1: span 0-5,12-17 level MC
[    0.969936]    groups: 0,12 (cpu_power = 1175) 1,13 (cpu_power = 1176) 2,14 (cpu_power = 1176) 3,15 (cpu_power = 1176) 4,16 (cpu_power = 1176) 5,17 (cpu_power = 1176)
[    0.985737]    domain 2: span 0-23 level NUMA
[    0.990231]     groups: 0-5,12-17 (cpu_power = 7055) 6-11,18-23 (cpu_power = 7056)

Reviewed-by: Paul Turner <pjt@google.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-ys201g4jwukj0h8xcamakxq1@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -4914,7 +4914,8 @@ sd_parent_degenerate(struct sched_domain *sd, struct sched_domain *parent)
 				SD_BALANCE_FORK |
 				SD_BALANCE_EXEC |
 				SD_SHARE_CPUPOWER |
-				SD_SHARE_PKG_RESOURCES);
+				SD_SHARE_PKG_RESOURCES |
+				SD_PREFER_SIBLING);
 		if (nr_node_ids == 1)
 			pflags &= ~SD_SERIALIZE;
 	}
@@ -5118,6 +5119,13 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 			tmp->parent = parent->parent;
 			if (parent->parent)
 				parent->parent->child = tmp;
+			/*
+			 * Transfer SD_PREFER_SIBLING down in case of a
+			 * degenerate parent; the spans match for this
+			 * so the property transfers.
+			 */
+			if (parent->flags & SD_PREFER_SIBLING)
+				tmp->flags |= SD_PREFER_SIBLING;
 			destroy_sched_domain(parent, cpu);
 		} else
 			tmp = tmp->parent;