Re: [PATCH v5 1/6] sched/fair: Do not skip CPUs of similar capacity with busy SMT siblings

[Vincent Guittot]

On Tue, 23 Jun 2026 at 01:55, Ricardo Neri
<ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
>
> 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.
>
> Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
> Changes in v5:
>  * Optimized logic to identify CPUs with busy SMT siblings only when
>    needed. (Prateek, Chen Yu)
>  * Added Reviewed-by tag from Prateek. Thanks!
>  * Christian also provided his Reviewed-by tag, but the patch changed
>    significantly since then. I did not think it was correct to keep it
>    without him reviewing the updated patch first.
>
> Changes in v4:
>  * Introduced this patch.
>
> Changes in v3:
>  * N/A
>
> Changes in v2:
>  * N/A
> ---
>  kernel/sched/fair.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d78467ec6ee1..892abd7fcc18 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12976,9 +12976,17 @@ 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
>
> --
> 2.43.0
>