Subject: [RFC PATCH] arm64: cpuinfo: reduce cache contention on update_{feature}_support

From: [email-masked] (David Daney)
Date: Fri, 04 Sep 2015 09:36:06 -0700

On 09/04/2015 09:04 AM, Yury Norov wrote:
> This patch is on top of https://lkml.org/lkml/2015/9/2/413
>
> In master, there's only a single function -
> 	update_mixed_endian_el0_support
> And similar function is on review mentioned above.
>
> The algorithm for them is like this:
>   - there's system-wide boolean marker for the feature that is
>     initially enabled;
>   - there's also updater for the feature that may disable it
>     system-widely if feature is not supported on current CPU.
>   - updater is called for each CPU on bootup.
>
> The problem is the way updater does its work. On each CPU, it
> unconditionally updates system-wide marker. For multi-core
> system it makes CPU issue invalidate message for a cache
> line containing marker. This invalidate increases cache
> contention for nothing, because there's a single marker reset
> that is really needed, and the others are useless.
>
> If the number of system-wide markers of this sort will grow,
> it may become a trouble on large-scale SOCs. The fix is trivial,
> though: do system-wide marker update conditionally, and preserve
> corresponding cache line in shared state for all update() calls,
> except, probably, one.
>
> Signed-off-by: Yury Norov <[email-masked]>
> ---
>   arch/arm64/kernel/cpuinfo.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
> index 4a6ae31..9972c1e 100644
> --- a/arch/arm64/kernel/cpuinfo.c
> +++ b/arch/arm64/kernel/cpuinfo.c
> @@ -87,12 +87,14 @@ bool system_supports_aarch32_el0(void)
>
>   static void update_mixed_endian_el0_support(struct cpuinfo_arm64 *info)
>   {
> -	mixed_endian_el0 &= id_aa64mmfr0_mixed_endian_el0(info->reg_id_aa64mmfr0);
> +	if (mixed_endian_el0 && !id_aa64mmfr0_mixed_endian_el0(info->reg_id_aa64mmfr0))
> +		mixed_endian_el0 = false;
>   }
>
>   static void update_aarch32_el0_support(struct cpuinfo_arm64 *info)
>   {
> -	aarch32_el0 &= id_aa64pfr0_aarch32_el0(info->reg_id_aa64pfr0);
> +	if (aarch32_el0 && !id_aa64pfr0_aarch32_el0(info->reg_id_aa64pfr0))
> +		aarch32_el0 = false;
>   }
>

How many times in the lifetime of the kernel are these functions called?

If it is just done at startup, then there is no "steady state" 
performance impact, and the burden of complicating the code may not be 
worthwhile.

David Daney

>   static void update_cpu_features(struct cpuinfo_arm64 *info)
>