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sched/fair: Allow load balancing between CPUs of identical capacity

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sched_balance_find_src_rq() avoids selecting a runqueue with a single
running task as busiest if doing so results in migrating the task to a
CPU with less than ~5% of extra capacity. It also unintentionally
prevents migrations between CPUs of identical capacity.

When CONFIG_SCHED_CLUSTER is enabled, load should be balanced across
clusters of CPUs with the same capacity. Allowing migration between CPUs
of identical capacity is necessary to meet this goal.

Use get_actual_cpu_capacity() to reflect architectural capacity as well
as diminished capacity due to hardware or cpufreq pressure. Guard this
check with the sched_cluster_active static key so that systems without
cluster topology are unaffected.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: Christian Loehle <christian.loehle@arm.com>
Tested-by: Andrea Righi <arighi@nvidia.com>
Link: https://patch.msgid.link/20260720-rneri-fix-cas-clusters-v6-5-bb500bf4afd4@linux.intel.com
This commit is contained in:
Ricardo Neri 2026-07-20 19:43:21 -07:00 committed by Peter Zijlstra
parent 0fbd428d07
commit 7fd540b1bc
1 changed files with 8 additions and 1 deletions
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9
kernel/sched/fair.c
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@ -13102,13 +13102,20 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
*/
if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
nr_running == 1) {
bool cluster_equal_cap = static_branch_unlikely(&sched_cluster_active) &&
(get_actual_cpu_capacity(env->dst_cpu) ==
get_actual_cpu_capacity(i));
bool smt_degraded_cap = sched_smt_active() && !is_core_idle(i);
/*
* Busy SMT siblings reduce the capacity of CPU @i. Do
* not skip it in this case.
*
* CONFIG_SCHED_CLUSTER requires balancing load across
* clusters of identical capacity, accounting for
* hardware and cpufreq pressure.
*/
if (!smt_degraded_cap &&
if (!smt_degraded_cap && !cluster_equal_cap &&
!capacity_greater(capacity_of(env->dst_cpu), capacity))
continue;
}