From 0fbd428d078876cefe908efdbee47a4d5adc5f24 Mon Sep 17 00:00:00 2001 From: Ricardo Neri Date: Mon, 20 Jul 2026 19:43:20 -0700 Subject: [PATCH] sched/fair: Skip misfit load accounting when the destination CPU cannot help In domains with asymmetric capacity, identifying misfit load in a scheduling group is not useful when the destination CPU cannot help (i.e., its capacity exceeds the group's maximum CPU capacity by less than ~5%). In such cases, it also prevents load balance among clusters of equal capacity when CONFIG_SCHED_CLUSTER is enabled. This happens because update_sd_pick_busiest() skips candidate groups of type misfit_task if the destination CPU has similar capacity. Skipping misfit load accounting in this situation allows the group to be classified as has_spare or fully_busy and lets load balancing proceed. Keep marking scheduling groups as overloaded when misfit tasks are present. The sg_overloaded flag propagates to the root domain and allows bigger CPUs in it to help via newly idle balance. Signed-off-by: Ricardo Neri Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Christian Loehle Reviewed-by: Chen Yu Reviewed-by: Vincent Guittot Tested-by: Christian Loehle Tested-by: Andrea Righi Link: https://patch.msgid.link/20260720-rneri-fix-cas-clusters-v6-4-bb500bf4afd4@linux.intel.com --- kernel/sched/fair.c | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index f9d0edd6828d..f1b4db36c23b 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -11970,12 +11970,25 @@ static inline void update_sg_lb_stats(struct lb_env *env, continue; if (sd_flags & SD_ASYM_CPUCAPACITY) { - /* Check for a misfit task on the cpu */ - if (sgs->group_misfit_task_load < rq->misfit_task_load) { - sgs->group_misfit_task_load = rq->misfit_task_load; - + if (rq->misfit_task_load) { + /* + * Always mark the root domain overloaded so big + * CPUs can pick up misfit tasks via newly idle + * balance. + */ if (balancing_at_rd) *sg_overloaded = 1; + + /* + * Only account misfit load if @dst_cpu can + * help; otherwise, the group may be classified + * as misfit_task and update_sd_pick_busiest() + * will skip it. + */ + if (capacity_greater(capacity_of(env->dst_cpu), + group->sgc->max_capacity) && + (sgs->group_misfit_task_load < rq->misfit_task_load)) + sgs->group_misfit_task_load = rq->misfit_task_load; } } else if (env->idle && sched_reduced_capacity(rq, env->sd)) { /* Check for a task running on a CPU with reduced capacity */