linux-kernel - Re: [PATCH v2 1/7] sched/fair: Generalize asym

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Message-ID: <76e23104-a8c0-a5fc-b8c6-27de79df2372@arm.com>
Date:   Tue, 6 Dec 2022 18:22:41 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Valentin Schneider <vschneid@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [PATCH v2 1/7] sched/fair: Generalize asym_packing logic for SMT
 local sched group

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?

[...]