lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Fri, 04 Sep 2015 09:36:06 -0700
From:	David Daney <ddaney.cavm@...il.com>
To:	Yury Norov <ynorov@...iumnetworks.com>
CC:	mark.rutland@....com, Catalin.Marinas@....com,
	ard.biesheuvel@...aro.org, yury.norov@...il.com,
	Suzuki.Poulose@....com, will.deacon@....com,
	linux-kernel@...r.kernel.org, klimov.linux@...il.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH] arm64: cpuinfo: reduce cache contention on update_{feature}_support

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 <ynorov@...iumnetworks.com>
> ---
>   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)
>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists