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

[Darren Hart <darren@os.amperecomputing.com>]

On Wed, Feb 16, 2022 at 08:03:40PM +1300, Barry Song wrote:
> On Wed, Feb 16, 2022 at 7:30 PM Vincent Guittot
> <vincent.guittot@linaro.org> 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
> >
> > IIUC, the cluster is defined as the 2nd (no SMT) or 3rd (SMT) level in
> > the PPTT table whereas the MC level is defined as the number of cache
> > levels. So i would say that you should compare the level to know the
> > ordering
> >
> > Then, there is another point:
> > In your case, CLUSTER level still has the flag SD_SHARE_PKG_RESOURCES
> > which is used to define some scheduler internal variable like
> > sd_llc(sched domain last level of cache) which allows fast task
> > migration between this cpus in this level at wakeup. In your case the
> > sd_llc should not be the cluster but the MC with only one CPU. But I
> > would not be surprised that most of perf improvement comes from this
> > sd_llc wrongly set to cluster instead of the single CPU
> 
> I assume this "mistake" is actually what Ampere altra needs while it
> is wrong but getting
> right result? Ampere altra has already got both:

Hi Barry,

Generally yes - although I do think we're placing too much emphasis on
the "right" or "wrong" of a heuristic which are more fluid in
definition over time. (e.g. I expect this will look different in a year
based on what we learn from this and other non current default topologies).

> 1. Load Balance between clusters
> 2. wake_affine by select sibling cpu which is sharing SCU
> 
> I am not sure how much 1 and 2 are helping Darren's workloads respectively.

We definitely see improvements with load balancing between clusters.
We're running some tests with the wake_affine patchset you pointed me to
(thanks for that). My initial tbench runs resulted in higher average and
max latencies reported. I need to collect more results and see the
impact to other benchmarks of interest before I have more to share on
that.

Thanks,

-- 
Darren Hart
Ampere Computing / OS and Kernel

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