[tip: sched/core] sched/topology: Restore SD_PREFER_SIBLING in domains with asymmetric capacity

[tip-bot2 for Ricardo Neri]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     f2c2ba7219e535afdb2ae7d66e6e3df0332eab70
Gitweb:        https://git.kernel.org/tip/f2c2ba7219e535afdb2ae7d66e6e3df0332eab70
Author:        Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
AuthorDate:    Mon, 20 Jul 2026 19:43:22 -07:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 07 Aug 2026 18:27:11 +02:00

sched/topology: Restore SD_PREFER_SIBLING in domains with asymmetric capacity

Commit 9c63e84db29b ("sched/core: Disable SD_PREFER_SIBLING on asymmetric
CPU capacity domains") removed the SD_PREFER_SIBLING from the domains with
asymmetric capacity. This was done to avoid spreading tasks to sibling
scheduling groups with less capacity, but this does not happen: checks for
capacity in update_sd_pick_busiest(), sched_balance_find_src_group(), and
sched_balance_find_src_rq() prevent migrations from high- to low-capacity
CPUs if the busiest group is not overloaded.

The cluster topology is a notable example: some systems have scheduling
domains spanning CPUs of asymmetric capacity, grouped into two or more
equal-capacity clusters sharing an L2 cache. When CONFIG_SCHED_CLUSTER is
enabled, SD_PREFER_SIBLING is needed in the domain to spread load across
these clusters.

CPUs with spare capacity, big or small, have always helped overloaded
groups. Once the overloading condition disappears, misfit load will still
be used to move high-utilization tasks to bigger CPUs if they have spare
capacity.

Adding the SD_PREFER_SIBLING flag shifts load balancing in shared-LLC
domains from equalizing the number of idle CPUs to equalizing the number
of running tasks. This enables migrations among clusters from newly-idle
load balance, where the outgoing task is already dequeued but the CPU
has not yet transitioned to idle.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Tested-by: Christian Loehle <christian.loehle@xxxxxxx>
Tested-by: Andrea Righi <arighi@xxxxxxxxxx>
Link: https://patch.msgid.link/20260720-rneri-fix-cas-clusters-v6-6-bb500bf4afd4@xxxxxxxxxxxxxxx
---
 include/linux/sched/sd_flags.h | 3 +--
 kernel/sched/topology.c        | 4 ----
 2 files changed, 1 insertion(+), 6 deletions(-)

diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
index 42839cf..dc3ec24 100644
--- a/include/linux/sched/sd_flags.h
+++ b/include/linux/sched/sd_flags.h
@@ -146,8 +146,7 @@ SD_FLAG(SD_ASYM_PACKING, SDF_NEEDS_GROUPS)
 /*
  * Prefer to place tasks in a sibling domain
  *
- * Set up until domains start spanning NUMA nodes. Close to being a SHARED_CHILD
- * flag, but cleared below domains with SD_ASYM_CPUCAPACITY.
+ * Set up until domains start spanning NUMA nodes.
  *
  * NEEDS_GROUPS: Load balancing flag.
  */
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 622e2e0..21e816a 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1995,10 +1995,6 @@ sd_init(struct sched_domain_topology_level *tl,
 	/*
 	 * Convert topological properties into behaviour.
 	 */
-	/* Don't attempt to spread across CPUs of different capacities. */
-	if ((sd->flags & SD_ASYM_CPUCAPACITY) && sd->child)
-		sd->child->flags &= ~SD_PREFER_SIBLING;
-
 	if (sd->flags & SD_SHARE_CPUCAPACITY) {
 		sd->imbalance_pct = 110;