Re: [PATCH] arm64: smp: Skip MC domain for SoCs without shared cache

[Barry Song <21cnbao@gmail.com>]

On Tue, Feb 15, 2022 at 12:17 AM Darren Hart
<darren@os.amperecomputing.com> 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,
>
> Hi Barry, thanks for the review.
>
> > What kind of resources are clusters sharing on Ampere Altra?
>
> The cluster pairs are DSU pairs (ARM DynamIQ Shared Unit). While there
> is no shared L3 cache, they do share an SCU (snoop control unit) and
> have a cache coherency latency advantage relative to non-DSU pairs.
>
> The Anandtech Altra review illustrates this advantage:
> https://www.anandtech.com/show/16315/the-ampere-altra-review/3
>
> Notably, the SCHED_CLUSTER change did result in marked improvements for
> some interactive workloads.

Thanks. there is a wake_affine patchset, i also wonder if your device can also
benefit from it:
https://lore.kernel.org/lkml/20220126080947.4529-1-yangyicong@hisilicon.com/

>
> > So on Altra, cpus are not sharing LLC? Each LLC is separate
> > for each cpu?
>
> Correct. On the processor side the last level is at each cpu, and then
> there is a memory side SLC (system level cache) that is shared by all
> cpus.

Thanks.

>
> >
> > > ---
> > >  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?
> >
> > This flag shouldn't be global.
>
> Please note that this patch is intended to maintain an identical final
> sched domain construction for a symmetric topology with no shared
> processor-side cache and with cache advantaged clusters and avoid the
> BUG messages since this topology is correct for this architecture.
>
> By using a sched topology without the MC layer, this more accurately
> describes this architecture and does not require changes to
> build_sched_domain(), in particular changes to the assumptions about
> what a valid topology is.
>
> The test above tests every cpu coregroup weight in order to limit the
> impact of this change to this specific kind of topology. It
> intentionally does not address, nor change existing behavior for, the
> assymetrical topology you describe.
>
> I felt this was the less invasive approach and consistent with how other
> architectures handled "non-default" topologies.
>
> If there is interest on working toward a more generic topology builder,
> I'd be interested in working on that too, but I think this change makes
> sense in the near term.

I do agree this patch makes sense for symmetric topology but
asymmetrical topology
is still breaking. it might be better to be more generic. we had a
similar fix over
here for smt before:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.16&id=55409ac5c3

>
> Thanks,
>
> >
> > > +   }
> > > +
> > > +   /*
> > > +    * SoCs with no shared processor-side cache will have cpu_coregroup_mask
> > > +    * weights=1. If they also define clusters with cpu_clustergroup_mask
> > > +    * weights > 1, build_sched_domain() will trigger a BUG as the CLS
> > > +    * cpu_mask will not be a subset of MC. It will extend the MC cpu_mask
> > > +    * to match CLS, and later discard the MC level. Avoid the bug by using
> > > +    * a topology without the MC if the cpu_coregroup_mask weights=1.
> > > +    */
> > > +   if (use_no_mc_topology) {
> > > +           pr_info("cpu_coregroup_mask weights=1, skipping MC topology level");
> > > +           set_sched_topology(arm64_no_mc_topology);
> > >     }
> > >  }
> > >
> > > --
> > > 2.31.1
> >
> >
> > Thanks
> > Barry
> >
>
> --
> Darren Hart
> Ampere Computing / OS and Kernel

Thanks
Barry