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sched/fair: Do not skip CPUs of similar capacity with busy SMT siblings

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When picking a busiest CPU with only one running task, the function
sched_balance_find_src_rq() skips candidate CPUs if the destination CPU has
less than ~5% extra capacity. This condition only holds if all the SMT
siblings of a CPU are idle.

SMT siblings share the computing resources of a physical core and this
results in reduced capacity if more than one sibling is busy.

Skipping a CPU as described would prevent the load balancer from pulling
tasks from a scheduling group previously and correctly identified as
group_smt_balance (i.e., one with more than one task running).

Do not skip a candidate CPU of similar capacity if it has busy SMT
siblings.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: Andrea Righi <arighi@nvidia.com>
Link: https://patch.msgid.link/20260720-rneri-fix-cas-clusters-v6-1-bb500bf4afd4@linux.intel.com
This commit is contained in:
Ricardo Neri 2026-07-20 19:43:17 -07:00 committed by Peter Zijlstra
parent 293f9611ae
commit 180ff97c18
1 changed files with 11 additions and 3 deletions
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14
kernel/sched/fair.c
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@ -13086,9 +13086,17 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
* average load.
*/
if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
!capacity_greater(capacity_of(env->dst_cpu), capacity) &&
nr_running == 1)
continue;
nr_running == 1) {
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.
*/
if (!smt_degraded_cap &&
!capacity_greater(capacity_of(env->dst_cpu), capacity))
continue;
}
/*
* Make sure we only pull tasks from a CPU of lower priority