linux-kernel - [PATCH] sched/topology: Set SD_PREFER_SIBLING consistently on non-NUMA levels

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Date:   Mon, 27 Nov 2017 17:29:27 +0000
From:   Morten Rasmussen <morten.rasmussen@....com>
To:     peterz@...radead.org, mingo@...hat.com
Cc:     dietmar.eggemann@....com, vincent.guittot@...aro.org,
        mgalbraith@...e.de, linux-kernel@...r.kernel.org,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: [PATCH] sched/topology: Set SD_PREFER_SIBLING consistently on non-NUMA levels

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@....com>
cc: Ingo Molnar <mingo@...hat.com>
cc: Peter Zijlstra <peterz@...radead.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