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

[Will Deacon <will@kernel.org>]

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.

Barry -- why shouldn't we take this as-is?

Will