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) Signed-off-by: Peter Zijlstra --- kernel/sched/core.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4948,7 +4948,8 @@ sd_parent_degenerate(struct sched_domain 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; } @@ -5157,6 +5158,13 @@ cpu_attach_domain(struct sched_domain *s 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; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/