Re: [PATCH v5 16/19] arch_topology: Limit span of cpu_clustergroup_mask()

[Vincent Guittot]

On Mon, 27 Jun 2022 at 18:51, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
>
> From: Ionela Voinescu <ionela.voinescu@xxxxxxx>
>
> 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@xxxxxxx
> Cc: Darren Hart <darren@xxxxxxxxxxxxxxxxxxxxxx>
> Tested-by: Ionela Voinescu <ionela.voinescu@xxxxxxx>
> Signed-off-by: Ionela Voinescu <ionela.voinescu@xxxxxxx>
> Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxx>
> ---
>  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@xxxxxxxxxx>

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