linux-kernel - Re: [PATCH v5 16/19] arch_topology: Limit span of cpu_clustergroup

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Message-ID: <CAKfTPtAxNKV=L0DHuBGmBiUww5tDhyAiw-6N3cun11iQLALYHA@mail.gmail.com>
Date:   Tue, 28 Jun 2022 12:28:05 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Atish Patra <atishp@...shpatra.org>,
        Atish Patra <atishp@...osinc.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Qing Wang <wangqing@...o.com>,
        Rob Herring <robh+dt@...nel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Pierre Gondois <pierre.gondois@....com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org,
        Darren Hart <darren@...amperecomputing.com>
Subject: Re: [PATCH v5 16/19] arch_topology: Limit span of cpu_clustergroup_mask()

On Mon, 27 Jun 2022 at 18:51, Sudeep Holla <sudeep.holla@....com> wrote:
>
> From: Ionela Voinescu <ionela.voinescu@....com>
>
> Currently the cluster identifier is not set on DT based platforms.
> The reset or default value is -1 for all the CPUs. Once we assign the
> cluster identifier values correctly, the cluster_sibling mask will be
> populated and returned by cpu_clustergroup_mask() to contribute in the
> creation of the CLS scheduling domain level, if SCHED_CLUSTER is
> enabled.
>
> To avoid topologies that will result in questionable or incorrect
> scheduling domains, impose restrictions regarding the span of clusters,
> as presented to scheduling domains building code: cluster_sibling should
> not span more or the same CPUs as cpu_coregroup_mask().
>
> This is needed in order to obtain a strict separation between the MC and
> CLS levels, and maintain the same domains for existing platforms in
> the presence of CONFIG_SCHED_CLUSTER, where the new cluster information
> is redundant and irrelevant for the scheduler.
>
> While previously the scheduling domain builder code would have removed MC
> as redundant and kept CLS if SCHED_CLUSTER was enabled and the
> cpu_coregroup_mask() and cpu_clustergroup_mask() spanned the same CPUs,
> now CLS will be removed and MC kept.
>
> Link: https://lore.kernel.org/r/20220621192034.3332546-18-sudeep.holla@arm.com
> Cc: Darren Hart <darren@...amperecomputing.com>
> Tested-by: Ionela Voinescu <ionela.voinescu@....com>
> Signed-off-by: Ionela Voinescu <ionela.voinescu@....com>
> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> ---
>  drivers/base/arch_topology.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 46fa1b70b02b..277b65cf3306 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -686,6 +686,14 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
>
>  const struct cpumask *cpu_clustergroup_mask(int cpu)
>  {
> +       /*
> +        * Forbid cpu_clustergroup_mask() to span more or the same CPUs as
> +        * cpu_coregroup_mask().
> +        */
> +       if (cpumask_subset(cpu_coregroup_mask(cpu),
> +                          &cpu_topology[cpu].cluster_sibling))
> +               return get_cpu_mask(cpu);

AFAICT, this will change the Altra scheduling topology which will now
have a MC level instead of the CLS but it's probably not a problem as
the flags are the same for now

Acked-by: Vincent Guittot <vincent.guittot@...aro.org>

> +
>         return &cpu_topology[cpu].cluster_sibling;
>  }
>
> --
> 2.36.1
>