[PATCHv4 12/12] sched/core: Disable SD_PREFER_SIBLING on asymmetric cpu capacity domains

[Morten Rasmussen]

The 'prefer sibling' sched_domain flag is intended to encourage
spreading tasks to sibling sched_domain to take advantage of more caches
and core for SMT systems. It has recently been changed to be on all
non-NUMA topology level. However, spreading across domains with cpu
capacity asymmetry isn't desirable, e.g. spreading from high capacity to
low capacity cpus even if high capacity cpus aren't overutilized might
give access to more cache but the cpu will be slower and possibly lead
to worse overall throughput.

To prevent this, we need to remove SD_PREFER_SIBLING on the sched_domain
level immediately below SD_ASYM_CPUCAPACITY.

cc: Ingo Molnar <mingo@xxxxxxxxxx>
cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Signed-off-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
---
 kernel/sched/topology.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 29c186961345..00c7a08c7f77 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1140,7 +1140,7 @@ 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
 					,
@@ -1186,17 +1186,21 @@ sd_init(struct sched_domain_topology_level *tl,
 	if (sd->flags & SD_ASYM_CPUCAPACITY) {
 		struct sched_domain *t = sd;
 
+		/*
+		 * Don't attempt to spread across cpus of different capacities.
+		 */
+		if (sd->child)
+			sd->child->flags &= ~SD_PREFER_SIBLING;
+
 		for_each_lower_domain(t)
 			t->flags |= SD_BALANCE_WAKE;
 	}
 
 	if (sd->flags & SD_SHARE_CPUCAPACITY) {
-		sd->flags |= SD_PREFER_SIBLING;
 		sd->imbalance_pct = 110;
 		sd->smt_gain = 1178; /* ~15% */
 
 	} else if (sd->flags & SD_SHARE_PKG_RESOURCES) {
-		sd->flags |= SD_PREFER_SIBLING;
 		sd->imbalance_pct = 117;
 		sd->cache_nice_tries = 1;
 		sd->busy_idx = 2;
@@ -1207,6 +1211,7 @@ 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 |
@@ -1216,7 +1221,6 @@ 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