Commit 10866e62 authored Aug 19, 2013 by Peter Zijlstra Committed by Ingo Molnar Sep 02, 2013

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>

parent 30ce5dab

kernel/sched/core.c

+9 −1

Original line number	Diff line number	Diff line
		@@ -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;