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Date:   Wed, 16 Feb 2022 08:24:48 -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 Wed, Feb 16, 2022 at 09:30:49AM +0100, Vincent Guittot wrote:
> On Tue, 15 Feb 2022 at 21:05, Darren Hart <darren@os.amperecomputing.com> wrote:
> >
> > On Tue, Feb 15, 2022 at 07:19:45PM +0100, Vincent Guittot wrote:
> > > On Tue, 15 Feb 2022 at 18:32, Darren Hart <darren@os.amperecomputing.com> wrote:
> > > >
> > > > 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 think that the problem comes from that the default topology order is
> > > assumed to be :
> > > SMT
> > > CLUSTER shares pkg resources i.e. cache
> > > MC
> > > DIE
> > > NUMA
> > >
> > > but in your case, you want a topology order like :
> > > SMT
> > > MC
> > > CLUSTER shares SCU
> > > DIE
> > > NUMA
> >
> > Given the fairly loose definition of some of these domains and the
> > freedom to adjust flags with custom topologies, I think it's difficult
> > to say it needs to be this or that. As you point out, this stems from an
> > assumption in the default topology, so eliding the MC level within the
> > current set of abstractions for a very targeted topology still seems
> > reasonable to address the BUG in the very near term in a contained way.
> 
> But if another SoC comes with a valid MC then a CLUSTER, this proposal
> will not work.
> 
> Keep in mind that the MC level will be removed/degenerate when
> building because it is useless in your case so the scheduler topology
> will still be the same at the end but it will support more case. That
> why I think you should keep MC level

Hi Vincent,

Thanks for reiterating, I don't think I quite understood what you were
suggesting before. Is the following in line with what you were thinking?

I am testing a version of this patch which uses a topology like this instead:

MC
CLS
DIE
NUMA

(I tested without an SMT domain since the trigger is still MC weight==1, so
there is no valid topology that includes an SMT level under these conditions).

Which results in no BUG output and a final topology on Altra of:

CLS
DIE
NUMA

Which so far seems right (I still need to do some testing and review the sched
debug data).

If we take this approach, I think to address your concern about other systems
with valid MCs, we would need a different trigger that MC weight == 1 to use
this alternate topology. Do you have a suggestion on what to trigger this on?

Thanks,

-- 
Darren Hart
Ampere Computing / OS and Kernel