[tip: sched/core] sched/fair: Check CPU capacity before comparing group types during load balance

[tip-bot2 for Ricardo Neri]

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

Commit-ID:     50b101f6e586b4417d060a976fd831cd87e86e2b
Gitweb:        https://git.kernel.org/tip/50b101f6e586b4417d060a976fd831cd87e86e2b
Author:        Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
AuthorDate:    Mon, 20 Jul 2026 19:43:19 -07:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 07 Aug 2026 18:27:10 +02:00

sched/fair: Check CPU capacity before comparing group types during load balance

update_sd_pick_busiest() may incorrectly select a fully_busy group as the
busiest group when its per-CPU capacity exceeds that of the destination
CPU. This happens because the type of busiest group is initialized to
group_has_spare and allows the fully_busy group to win the type comparison.

update_sd_pick_busiest() should not choose a candidate scheduling group
with at most one runnable task if its per-CPU capacity is greater than that
of the destination CPU. Such a check already exists, but it is done too
late: after the type comparison, preventing a subsequent fully_busy group
of equal per-CPU capacity from being correctly selected.

Move this check to occur before comparing group types.

Fixes: 0b0695f2b34a ("sched/fair: Rework load_balance()")
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Christian Loehle <christian.loehle@xxxxxxx>
Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
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-v6-3-bb500bf4afd4@xxxxxxxxxxxxxxx
---
 kernel/sched/fair.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2c5cfec..f9d0edd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12054,6 +12054,17 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 	     sds->local_stat.group_type != group_has_spare))
 		return false;
 
+	/*
+	 * Candidate sg has no more than one task per CPU and has higher
+	 * per-CPU capacity. Migrating tasks to less capable CPUs may harm
+	 * throughput. Maximize throughput, power/energy consequences are not
+	 * considered.
+	 */
+	if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
+	    (sgs->group_type <= group_fully_busy) &&
+	    (capacity_greater(sg->sgc->min_capacity, capacity_of(env->dst_cpu))))
+		return false;
+
 	if (sgs->group_type > busiest->group_type)
 		return true;
 
@@ -12160,17 +12171,6 @@ has_spare:
 		break;
 	}
 
-	/*
-	 * Candidate sg has no more than one task per CPU and has higher
-	 * per-CPU capacity. Migrating tasks to less capable CPUs may harm
-	 * throughput. Maximize throughput, power/energy consequences are not
-	 * considered.
-	 */
-	if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
-	    (sgs->group_type <= group_fully_busy) &&
-	    (capacity_greater(sg->sgc->min_capacity, capacity_of(env->dst_cpu))))
-		return false;
-
 	return true;
 }