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

[Will Deacon <will@kernel.org>]

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?

Will