Re: [PATCH v2 1/7] sched/fair: Generalize asym_packing logic for SMT local sched group

[Dietmar Eggemann]

On 22/11/2022 21:35, Ricardo Neri wrote:
> When balancing load between two physical cores, an idle destination CPU can
> help another core only if all of its SMT siblings are also idle. Otherwise,
> there is not increase in throughput. It does not matter whether the other
> core is composed of SMT siblings.
> 
> Simply check if there are any tasks running on the local group and the
> other core has exactly one busy CPU before proceeding. Let
> find_busiest_group() handle the case of more than one busy CPU. This is
> true for SMT2, SMT4, SMT8, etc.

[...]

> Changes since v1:
>  * Reworded commit message and inline comments for clarity.
>  * Stated that this changeset does not impact STM4 or SMT8.
> ---
>  kernel/sched/fair.c | 29 +++++++++--------------------
>  1 file changed, 9 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e4a0b8bd941c..18c672ff39ef 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8900,12 +8900,10 @@ static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds,
>  				    struct sched_group *sg)

I'm not sure why you change asym_smt_can_pull_tasks() together with
removing SD_ASYM_PACKING from SMT level (patch 5/7)?

update_sg_lb_stats()

  ... && env->sd->flags & SD_ASYM_PACKING && .. && sched_asym()
                                                   ^^^^^^^^^^^^
    sched_asym()

      if ((sds->local->flags & SD_SHARE_CPUCAPACITY) ||
          (group->flags & SD_SHARE_CPUCAPACITY))
        return asym_smt_can_pull_tasks()
               ^^^^^^^^^^^^^^^^^^^^^^^^^

So x86 won't have a sched domain with SD_SHARE_CPUCAPACITY and
SD_ASYM_PACKING anymore. So sched_asym() would call sched_asym_prefer()
directly on MC. What do I miss here?

[...]