Subject: Re: [PATCH v4 07/10] Powerpc/numa: Detect support for coregroup

From: Michael Ellerman <[email-masked]>
Date: Fri, 31 Jul 2020 17:49:55 +1000

Srikar Dronamraju <[email-masked]> writes:
> Add support for grouping cores based on the device-tree classification.
> - The last domain in the associativity domains always refers to the
> core.
> - If primary reference domain happens to be the penultimate domain in
> the associativity domains device-tree property, then there are no
> coregroups. However if its not a penultimate domain, then there are
> coregroups. There can be more than one coregroup. For now we would be
> interested in the last or the smallest coregroups.

This still doesn't tell me what a coregroup actually represents.

I get that it's a grouping of cores, and that the device tree specifies
it for us, but grouping based on what?

I think the answer is we aren't being told by firmware, it's just a
grouping based on some opaque performance characteristic and we just
have to take that as given.

But please explain that clearly in the change log and the code comments.

cheers

> Cc: linuxppc-dev <[email-masked]>
> Cc: LKML <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Nicholas Piggin <[email-masked]>
> Cc: Anton Blanchard <[email-masked]>
> Cc: Oliver O'Halloran <[email-masked]>
> Cc: Nathan Lynch <[email-masked]>
> Cc: Michael Neuling <[email-masked]>
> Cc: Gautham R Shenoy <[email-masked]>
> Cc: Ingo Molnar <[email-masked]>
> Cc: Peter Zijlstra <[email-masked]>
> Cc: Valentin Schneider <[email-masked]>
> Cc: Jordan Niethe <[email-masked]>
> Reviewed-by: Gautham R. Shenoy <[email-masked]>
> Signed-off-by: Srikar Dronamraju <[email-masked]>
> ---
> Changelog v1 -> v2:
> 	Explained Coregroup in commit msg (Michael Ellerman)
>
>  arch/powerpc/include/asm/smp.h |  1 +
>  arch/powerpc/kernel/smp.c      |  1 +
>  arch/powerpc/mm/numa.c         | 34 +++++++++++++++++++++-------------
>  3 files changed, 23 insertions(+), 13 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
> index 49a25e2400f2..5bdc17a7049f 100644
> --- a/arch/powerpc/include/asm/smp.h
> +++ b/arch/powerpc/include/asm/smp.h
> @@ -28,6 +28,7 @@
>  extern int boot_cpuid;
>  extern int spinning_secondaries;
>  extern u32 *cpu_to_phys_id;
> +extern bool coregroup_enabled;
>  
>  extern void cpu_die(void);
>  extern int cpu_to_chip_id(int cpu);
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 3c5ccf6d2b1c..698000c7f76f 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -74,6 +74,7 @@ static DEFINE_PER_CPU(int, cpu_state) = { 0 };
>  
>  struct task_struct *secondary_current;
>  bool has_big_cores;
> +bool coregroup_enabled;
>  
>  DEFINE_PER_CPU(cpumask_var_t, cpu_sibling_map);
>  DEFINE_PER_CPU(cpumask_var_t, cpu_smallcore_map);
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index 2298899a0f0a..51cb672f113b 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -886,7 +886,9 @@ static void __init setup_node_data(int nid, u64 start_pfn, u64 end_pfn)
>  static void __init find_possible_nodes(void)
>  {
>  	struct device_node *rtas;
> -	u32 numnodes, i;
> +	const __be32 *domains;
> +	int prop_length, max_nodes;
> +	u32 i;
>  
>  	if (!numa_enabled)
>  		return;
> @@ -895,25 +897,31 @@ static void __init find_possible_nodes(void)
>  	if (!rtas)
>  		return;
>  
> -	if (of_property_read_u32_index(rtas, "ibm,current-associativity-domains",
> -				min_common_depth, &numnodes)) {
> -		/*
> -		 * ibm,current-associativity-domains is a fairly recent
> -		 * property. If it doesn't exist, then fallback on
> -		 * ibm,max-associativity-domains. Current denotes what the
> -		 * platform can support compared to max which denotes what the
> -		 * Hypervisor can support.
> -		 */
> -		if (of_property_read_u32_index(rtas, "ibm,max-associativity-domains",
> -				min_common_depth, &numnodes))
> +	/*
> +	 * ibm,current-associativity-domains is a fairly recent property. If
> +	 * it doesn't exist, then fallback on ibm,max-associativity-domains.
> +	 * Current denotes what the platform can support compared to max
> +	 * which denotes what the Hypervisor can support.
> +	 */
> +	domains = of_get_property(rtas, "ibm,current-associativity-domains",
> +					&prop_length);
> +	if (!domains) {
> +		domains = of_get_property(rtas, "ibm,max-associativity-domains",
> +					&prop_length);
> +		if (!domains)
>  			goto out;
>  	}
>  
> -	for (i = 0; i < numnodes; i++) {
> +	max_nodes = of_read_number(&domains[min_common_depth], 1);
> +	for (i = 0; i < max_nodes; i++) {
>  		if (!node_possible(i))
>  			node_set(i, node_possible_map);
>  	}
>  
> +	prop_length /= sizeof(int);
> +	if (prop_length > min_common_depth + 2)
> +		coregroup_enabled = 1;
> +
>  out:
>  	of_node_put(rtas);
>  }
> -- 
> 2.17.1