linux-kernel - Re: [PATCH v3] topology: make core_mask include at least cluster

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Date:   Tue, 8 Mar 2022 12:04:53 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Will Deacon <will@...nel.org>
Cc:     Darren Hart <darren@...amperecomputing.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Arm <linux-arm-kernel@...ts.infradead.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Catalin Marinas <Catalin.Marinas@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Valentin Schneider <Valentin.Schneider@....com>,
        "D . Scott Phillips" <scott@...amperecomputing.com>,
        Ilkka Koskinen <ilkka@...amperecomputing.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v3] topology: make core_mask include at least cluster_siblings

On Tue, 8 Mar 2022 at 11:30, Will Deacon <will@...nel.org> wrote:
>
> On Fri, Mar 04, 2022 at 09:01:36AM -0800, Darren Hart wrote:
> > Ampere Altra defines CPU clusters in the ACPI PPTT. They share a Snoop
> > Control Unit, but have no shared CPU-side last level cache.
> >
> > cpu_coregroup_mask() will return a cpumask with weight 1, while
> > cpu_clustergroup_mask() will return a cpumask with weight 2.
> >
> > As a result, build_sched_domain() will BUG() once per CPU with:
> >
> > BUG: arch topology borken
> > the CLS domain not a subset of the MC domain
> >
> > The MC level cpumask is then extended to that of the CLS child, and is
> > later removed entirely as redundant. This sched domain topology is an
> > improvement over previous topologies, or those built without
> > SCHED_CLUSTER, particularly for certain latency sensitive workloads.
> > With the current scheduler model and heuristics, this is a desirable
> > default topology for Ampere Altra and Altra Max system.
> >
> > Rather than create a custom sched domains topology structure and
> > introduce new logic in arch/arm64 to detect these systems, update the
> > core_mask so coregroup is never a subset of clustergroup, extending it
> > to cluster_siblings if necessary.
> >
> > This has the added benefit over a custom topology of working for both
> > symmetric and asymmetric topologies. It does not address systems where
> > the cluster topology is above a populated mc topology, but these are not
> > considered today and can be addressed separately if and when they
> > appear.
> >
> > The final sched domain topology for a 2 socket Ampere Altra system is
> > unchanged with or without CONFIG_SCHED_CLUSTER, and the BUG is avoided:
> >
> > For CPU0:
> >
> > CONFIG_SCHED_CLUSTER=y
> > CLS  [0-1]
> > DIE  [0-79]
> > NUMA [0-159]
> >
> > CONFIG_SCHED_CLUSTER is not set
> > DIE  [0-79]
> > NUMA [0-159]
> >
> > Cc: Sudeep Holla <sudeep.holla@....com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> > Cc: Catalin Marinas <catalin.marinas@....com>
> > Cc: Will Deacon <will@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Vincent Guittot <vincent.guittot@...aro.org>
> > Cc: Barry Song <song.bao.hua@...ilicon.com>
> > Cc: Valentin Schneider <valentin.schneider@....com>
> > Cc: D. Scott Phillips <scott@...amperecomputing.com>
> > Cc: Ilkka Koskinen <ilkka@...amperecomputing.com>
> > Cc: <stable@...r.kernel.org> # 5.16.x
> > Suggested-by: Barry Song <song.bao.hua@...ilicon.com>
> > Signed-off-by: Darren Hart <darren@...amperecomputing.com>
> > ---
> > v1: Drop MC level if coregroup weight == 1
> > v2: New sd topo in arch/arm64/kernel/smp.c
> > v3: No new topo, extend core_mask to cluster_siblings
> >
> >  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 976154140f0b..a96f45db928b 100644
> > --- a/drivers/base/arch_topology.c
> > +++ b/drivers/base/arch_topology.c
> > @@ -628,6 +628,14 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
> >                       core_mask = &cpu_topology[cpu].llc_sibling;
> >       }
> >
> > +     /*
> > +      * For systems with no shared cpu-side LLC but with clusters defined,
> > +      * extend core_mask to cluster_siblings. The sched domain builder will
> > +      * then remove MC as redundant with CLS if SCHED_CLUSTER is enabled.
> > +      */
> > +     if (cpumask_subset(core_mask, &cpu_topology[cpu].cluster_sibling))
> > +             core_mask = &cpu_topology[cpu].cluster_sibling;
> > +
>
> Sudeep, Vincent, are you happy with this now?

I would not say that I'm happy because this solution skews the core
cpu mask in order to abuse the scheduler so that it will remove a
wrong but useless level when it will build its domains.
But this works so as long as the maintainer are happy, I'm fine

>
> Will