Re: [PATCH v2 1/7] sched/fair: Generalize asym_packing logic for SMT local sched group
From: Dietmar Eggemann
Date: Tue Dec 06 2022 - 12:23:10 EST
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?
[...]