linux-kernel - Re: [PATCH] x86/amd: Derive L3 shared_cpu_map from cpu_llc_shared

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Date:   Wed, 26 Jul 2017 05:51:23 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com
Subject: Re: [PATCH] x86/amd: Derive L3 shared_cpu_map from
 cpu_llc_shared_mask

On Thu, Jul 20, 2017 at 04:52:08AM -0500, Suravee Suthikulpanit wrote:
> With X86_FEATURE_TOPOEXT, current logic default to using APIC ID
> to calculate cpumask for shared_cpu_map. However, since APIC IDs
> are not guarantee to be contiguous for cores across different L3,
> such as in family17h system w/ downcore configuration.
> This results in incorrect L3 shared_cpu_map.

There are syntactic and formulation errors in this paragraph, it reads
funny. Please improve.

> Instead, it should be safe to always use the pre-calculated
> cpu_llc_shared_mask to derive L3 shared_cpu_map.

"should"?!?

What does that mean? You tested it on relevant machines or you're hoping
that nothing would go wrong?

> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> ---
>  arch/x86/kernel/cpu/intel_cacheinfo.c | 28 ++++++++++++++--------------
>  1 file changed, 14 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/intel_cacheinfo.c b/arch/x86/kernel/cpu/intel_cacheinfo.c
> index c55fb2c..10427bb 100644
> --- a/arch/x86/kernel/cpu/intel_cacheinfo.c
> +++ b/arch/x86/kernel/cpu/intel_cacheinfo.c
> @@ -811,7 +811,20 @@ static int __cache_amd_cpumap_setup(unsigned int cpu, int index,
>  	struct cacheinfo *this_leaf;
>  	int i, sibling;
>  
> -	if (boot_cpu_has(X86_FEATURE_TOPOEXT)) {
> +	if (index == 3) {
> +		for_each_cpu(i, cpu_llc_shared_mask(cpu)) {
> +			this_cpu_ci = get_cpu_cacheinfo(i);
> +			if (!this_cpu_ci->info_list)
> +				continue;
> +			this_leaf = this_cpu_ci->info_list + index;
> +			for_each_cpu(sibling, cpu_llc_shared_mask(cpu)) {
> +				if (!cpu_online(sibling))
> +					continue;
> +				cpumask_set_cpu(sibling,
> +						&this_leaf->shared_cpu_map);
> +			}
> +		}
> +	} else if (boot_cpu_has(X86_FEATURE_TOPOEXT)) {

So this leaves more questions open than before it:

* Is cpu_llc_shared_map now *the* mask for the L3?

* Is the X86_FEATURE_TOPOEXT thing still going to be used for the other cache
levels? Apparently yes, I guess.

IOW, what is the big picture here? I could use some more details in the
commit message about it.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--