From f2c2ba7219e535afdb2ae7d66e6e3df0332eab70 Mon Sep 17 00:00:00 2001 From: Ricardo Neri Date: Mon, 20 Jul 2026 19:43:22 -0700 Subject: [PATCH] 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 Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Vincent Guittot Tested-by: Christian Loehle Tested-by: Andrea Righi Link: https://patch.msgid.link/20260720-rneri-fix-cas-clusters-v6-6-bb500bf4afd4@linux.intel.com --- 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 42839cfa2778..dc3ec2452ee1 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 622e2e01974c..21e816ad23ee 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;