Subject: Re: [PATCH 10/11] powerpc/smp: Implement cpu_to_coregroup_id

From: Gautham R Shenoy <[email-masked]>
Date: Fri, 17 Jul 2020 13:56:53 +0530

On Tue, Jul 14, 2020 at 10:06:23AM +0530, Srikar Dronamraju wrote:
> Lookup the coregroup id from the associativity array.
> 
> If unable to detect the coregroup id, fallback on the core id.
> This way, ensure sched_domain degenerates and an extra sched domain is
> not created.
> 
> Ideally this function should have been implemented in
> arch/powerpc/kernel/smp.c. However if its implemented in mm/numa.c, we
> don't need to find the primary domain again.
> 
> If the device-tree mentions more than one coregroup, then kernel
> implements only the last or the smallest coregroup, which currently
> corresponds to the penultimate domain in the device-tree.
> 
> Cc: linuxppc-dev <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Nick Piggin <[email-masked]>
> Cc: Oliver OHalloran <[email-masked]>
> Cc: Nathan Lynch <[email-masked]>
> Cc: Michael Neuling <[email-masked]>
> Cc: Anton Blanchard <[email-masked]>
> Cc: Gautham R Shenoy <[email-masked]>
> Cc: Vaidyanathan Srinivasan <[email-masked]>
> Signed-off-by: Srikar Dronamraju <[email-masked]>
> ---
>  arch/powerpc/mm/numa.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index d9ab9da85eab..4e85564ef62a 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -1697,6 +1697,23 @@ static const struct proc_ops topology_proc_ops = {
> 
>  int cpu_to_coregroup_id(int cpu)
>  {
> +	__be32 associativity[VPHN_ASSOC_BUFSIZE] = {0};
> +	int index;
> +

It would be good to have an assert here to ensure that we are calling
this function only when coregroups are enabled.

Else, we may end up returning the penultimate index which maps to the
chip-id.

> +	if (cpu < 0 || cpu > nr_cpu_ids)
> +		return -1;
> +
> +	if (!firmware_has_feature(FW_FEATURE_VPHN))
> +		goto out;
> +
> +	if (vphn_get_associativity(cpu, associativity))
> +		goto out;
> +
> +	index = of_read_number(associativity, 1);
> +	if ((index > min_common_depth + 1) && coregroup_enabled)
> +		return of_read_number(&associativity[index - 1], 1);
> +
> +out:
>  	return cpu_to_core_id(cpu);
>  }
> 
> -- 
> 2.17.1
>