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Subject[PATCH] sched/topology: Set SD_PREFER_SIBLING consistently on non-NUMA levels
Date
SD_PREFER_SIBLING adds an additional bias towards spreading tasks on the
_parent_ sched_domain even if a sched_group isn't overloaded. It is
currently set on:

1. SMT level to promote spreading to sibling cores rather than using
sibling HW-threads (caff37ef96eac7fe96a).

2. Non-NUMA levels which don't have the SD_SHARE_PKG_RESOURCES flag
set (= DIE level in the default topology) as it was found to
improve benchmarks on certain NUMA systems (6956dc568f34107f1d02b).

3. Any non-NUMA level that inherits the flag due to elimination of
its parent sched_domain level in the de-generate step of the
sched_domain hierarchy set up (= MC level in the default
topology).

Preferring siblings seems to be a useful tweak for all non-NUMA levels,
so we should enable it on all non-NUMA levels. As it is, it is possible
to have it SMT and DIE, but not MC in between when using the default
topology.

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
---
kernel/sched/topology.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 6798276d29af..7f70806bfa0f 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1122,7 +1122,7 @@ sd_init(struct sched_domain_topology_level *tl,
| 0*SD_SHARE_CPUCAPACITY
| 0*SD_SHARE_PKG_RESOURCES
| 0*SD_SERIALIZE
- | 0*SD_PREFER_SIBLING
+ | 1*SD_PREFER_SIBLING
| 0*SD_NUMA
| sd_flags
,
@@ -1153,7 +1153,6 @@ sd_init(struct sched_domain_topology_level *tl,
}

if (sd->flags & SD_SHARE_CPUCAPACITY) {
- sd->flags |= SD_PREFER_SIBLING;
sd->imbalance_pct = 110;
sd->smt_gain = 1178; /* ~15% */

@@ -1168,6 +1167,7 @@ sd_init(struct sched_domain_topology_level *tl,
sd->busy_idx = 3;
sd->idle_idx = 2;

+ sd->flags &= ~SD_PREFER_SIBLING;
sd->flags |= SD_SERIALIZE;
if (sched_domains_numa_distance[tl->numa_level] > RECLAIM_DISTANCE) {
sd->flags &= ~(SD_BALANCE_EXEC |
@@ -1177,7 +1177,6 @@ sd_init(struct sched_domain_topology_level *tl,

#endif
} else {
- sd->flags |= SD_PREFER_SIBLING;
sd->cache_nice_tries = 1;
sd->busy_idx = 2;
sd->idle_idx = 1;
--
2.7.4
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