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

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----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?

This flag shouldn't be global.

> +	}
> +
> +	/*
> +	 * 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