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Date:   Tue, 15 Feb 2022 09:32:38 -0800
From:   Darren Hart <darren@os.amperecomputing.com>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Will Deacon <will@kernel.org>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Arm <linux-arm-kernel@lists.infradead.org>,
        Catalin Marinas <Catalin.Marinas@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Valentin Schneider <Valentin.Schneider@arm.com>,
        "D . Scott Phillips" <scott@os.amperecomputing.com>,
        Ilkka Koskinen <ilkka@os.amperecomputing.com>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: [PATCH] arm64: smp: Skip MC domain for SoCs without shared cache
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On Tue, Feb 15, 2022 at 06:09:08PM +0100, Vincent Guittot wrote:
> On Tue, 15 Feb 2022 at 17:46, Will Deacon <will@kernel.org> wrote:
> >
> > On Tue, Feb 15, 2022 at 08:44:23AM -0800, Darren Hart wrote:
> > > On Tue, Feb 15, 2022 at 04:38:59PM +0000, Will Decon wrote:
> > > > On Fri, Feb 11, 2022 at 03:20:51AM +0000, Song Bao Hua (Barry Song) wrote:
> > > > >
> > > > >
> > > > > > -----Original Message-----
> > > > > > From: Darren Hart [mailto:darren@os.amperecomputing.com]
> > > > > > Sent: Friday, February 11, 2022 2:43 PM
> > > > > > To: LKML <linux-kernel@vger.kernel.org>; Linux Arm
> > > > > > <linux-arm-kernel@lists.infradead.org>
> > > > > > Cc: Catalin Marinas <catalin.marinas@arm.com>; Will Deacon <will@kernel.org>;
> > > > > > Peter Zijlstra <peterz@infradead.org>; Vincent Guittot
> > > > > > <vincent.guittot@linaro.org>; Song Bao Hua (Barry Song)
> > > > > > <song.bao.hua@hisilicon.com>; Valentin Schneider
> > > > > > <valentin.schneider@arm.com>; D . Scott Phillips
> > > > > > <scott@os.amperecomputing.com>; Ilkka Koskinen
> > > > > > <ilkka@os.amperecomputing.com>; stable@vger.kernel.org
> > > > > > Subject: [PATCH] arm64: smp: Skip MC domain for SoCs without shared cache
> > > > > >
> > > > > > SoCs such as the Ampere Altra define clusters but have no shared
> > > > > > processor-side cache. As of v5.16 with CONFIG_SCHED_CLUSTER and
> > > > > > CONFIG_SCHED_MC, build_sched_domain() will BUG() with:
> > > > > >
> > > > > > BUG: arch topology borken
> > > > > >      the CLS domain not a subset of the MC domain
> > > > > >
> > > > > > for each CPU (160 times for a 2 socket 80 core Altra system). The MC
> > > > > > level cpu mask is then extended to that of the CLS child, and is later
> > > > > > removed entirely as redundant.
> > > > > >
> > > > > > This change detects when all cpu_coregroup_mask weights=1 and uses an
> > > > > > alternative sched_domain_topology equivalent to the default if
> > > > > > CONFIG_SCHED_MC were disabled.
> > > > > >
> > > > > > The final resulting sched domain topology is unchanged with or without
> > > > > > CONFIG_SCHED_CLUSTER, and the BUG is avoided:
> > > > > >
> > > > > > For CPU0:
> > > > > >
> > > > > > With CLS:
> > > > > > CLS  [0-1]
> > > > > > DIE  [0-79]
> > > > > > NUMA [0-159]
> > > > > >
> > > > > > Without CLS:
> > > > > > DIE  [0-79]
> > > > > > NUMA [0-159]
> > > > > >
> > > > > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > > > > Cc: Will Deacon <will@kernel.org>
> > > > > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > > > > Cc: Vincent Guittot <vincent.guittot@linaro.org>
> > > > > > Cc: Barry Song <song.bao.hua@hisilicon.com>
> > > > > > Cc: Valentin Schneider <valentin.schneider@arm.com>
> > > > > > Cc: D. Scott Phillips <scott@os.amperecomputing.com>
> > > > > > Cc: Ilkka Koskinen <ilkka@os.amperecomputing.com>
> > > > > > Cc: <stable@vger.kernel.org> # 5.16.x
> > > > > > Signed-off-by: Darren Hart <darren@os.amperecomputing.com>
> > > > >
> > > > > Hi Darrent,
> > > > > What kind of resources are clusters sharing on Ampere Altra?
> > > > > So on Altra, cpus are not sharing LLC? Each LLC is separate
> > > > > for each cpu?
> > > > >
> > > > > > ---
> > > > > >  arch/arm64/kernel/smp.c | 32 ++++++++++++++++++++++++++++++++
> > > > > >  1 file changed, 32 insertions(+)
> > > > > >
> > > > > > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> > > > > > index 27df5c1e6baa..0a78ac5c8830 100644
> > > > > > --- a/arch/arm64/kernel/smp.c
> > > > > > +++ b/arch/arm64/kernel/smp.c
> > > > > > @@ -715,9 +715,22 @@ void __init smp_init_cpus(void)
> > > > > >         }
> > > > > >  }
> > > > > >
> > > > > > +static struct sched_domain_topology_level arm64_no_mc_topology[] = {
> > > > > > +#ifdef CONFIG_SCHED_SMT
> > > > > > +       { cpu_smt_mask, cpu_smt_flags, SD_INIT_NAME(SMT) },
> > > > > > +#endif
> > > > > > +
> > > > > > +#ifdef CONFIG_SCHED_CLUSTER
> > > > > > +       { cpu_clustergroup_mask, cpu_cluster_flags, SD_INIT_NAME(CLS) },
> > > > > > +#endif
> > > > > > +       { cpu_cpu_mask, SD_INIT_NAME(DIE) },
> > > > > > +       { NULL, },
> > > > > > +};
> > > > > > +
> > > > > >  void __init smp_prepare_cpus(unsigned int max_cpus)
> > > > > >  {
> > > > > >         const struct cpu_operations *ops;
> > > > > > +       bool use_no_mc_topology = true;
> > > > > >         int err;
> > > > > >         unsigned int cpu;
> > > > > >         unsigned int this_cpu;
> > > > > > @@ -758,6 +771,25 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
> > > > > >
> > > > > >                 set_cpu_present(cpu, true);
> > > > > >                 numa_store_cpu_info(cpu);
> > > > > > +
> > > > > > +               /*
> > > > > > +                * Only use no_mc topology if all cpu_coregroup_mask weights=1
> > > > > > +                */
> > > > > > +               if (cpumask_weight(cpu_coregroup_mask(cpu)) > 1)
> > > > > > +                       use_no_mc_topology = false;
> > > > >
> > > > > This seems to be wrong? If you have 5 cpus,
> > > > > Cpu0 has cpu_coregroup_mask(cpu)== 1, cpu1-4
> > > > > has cpu_coregroup_mask(cpu)== 4, for cpu0, you still
> > > > > need to remove MC, but for cpu1-4, you will need
> > > > > CLS and MC both?
> > > >
> > > > What is the *current* behaviour on such a system?
> > > >
> > >
> > > As I understand it, any system that uses the default topology which has
> > > a cpus_coregroup weight of 1 and a child (cluster, smt, ...) weight > 1
> > > will behave as described above by printing the following for each CPU
> > > matching this criteria:
> > >
> > >   BUG: arch topology borken
> > >         the [CLS,SMT,...] domain not a subset of the MC domain
> > >
> > > And then extend the MC domain cpumask to match that of the child and continue
> > > on.
> > >
> > > That would still be the behavior for this type of system after this
> > > patch is applied.
> >
> > That's what I thought, but in that case applying your patch is a net
> > improvement: systems either get current or better behaviour.
> 
> CLUSTER level is normally defined as a intermediate group of the MC
> level and both levels have the scheduler flag SD_SHARE_PKG_RESOURCES
> flag
> 
> In the case of Ampere altra, they consider that CPUA have a CLUSTER
> level which SD_SHARE_PKG_RESOURCES with another CPUB but the next and
> larger MC level then says that CPUA doesn't SD_SHARE_PKG_RESOURCES
> with CPUB which seems to be odd because the SD_SHARE_PKG_RESOURCES has
> not disappeared Looks like there is a mismatch in topology description

Hi Vincent,

Agree. Where do you think this mismatch exists?

I'd describe this as a mismatch between the default assumptions of
the sched domains construction code (that SD_SHARE_PKG_RESOURCES implies
a shared cpu-side cache) and SoCs without a shared cpu-side cache. This
is encoded in properties of the MC level and the requirement that child
domains be a subset of the parent domain cpumask.

The MC-less topology addresses this in a consistent way with other
architectures using the provided API for non-default topologies without
changing these fundamental assumptions and without changing the final
resulting topology which is correct and matches the topology supplied in
the ACPI PPTT.

-- 
Darren Hart
Ampere Computing / OS and Kernel