Re: [PATCH 2/2] x86: Move x86_cpu_to_apicid to the __read_mostly section

From: Ingo Molnar <mingo@kernel.org>
To: Vlad Zolotarov <vlad@scalemp.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
	alan@lxorguk.ukuu.org.uk,
	"Shai Fultheim (Shai@ScaleMP.com)" <Shai@scalemp.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH 2/2] x86: Move x86_cpu_to_apicid to the __read_mostly section
Date: Mon, 7 May 2012 12:52:47 +0200	[thread overview]
Message-ID: <20120507105247.GJ23002@gmail.com> (raw)
In-Reply-To: <201204241429.54089.vlad@scalemp.com>

* Vlad Zolotarov <vlad@scalemp.com> wrote:

> From: Shai Fultheim <shai@scalemp.com>
> 
> x86_cpu_to_apicid is only written to during initialization. Benchmarks
> show that moving it to the __read_mostly section helps avoid false
> sharing on large multiprocessing systems.
> 
> Signed-off-by: Shai Fultheim <shai@scalemp.com>
> Signed-off-by: Vlad Zolotarov <vlad@scalemp.com>
> ---
>  arch/x86/include/asm/smp.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
> index 0434c40..b9d6c1e 100644
> --- a/arch/x86/include/asm/smp.h
> +++ b/arch/x86/include/asm/smp.h
> @@ -53,7 +53,7 @@ static inline struct cpumask *cpu_llc_shared_mask(int cpu)
>  	return per_cpu(cpu_llc_shared_map, cpu);
>  }
>  
> -DECLARE_EARLY_PER_CPU(u16, x86_cpu_to_apicid);
> +DECLARE_EARLY_PER_CPU_READ_MOSTLY(u16, x86_cpu_to_apicid);
>  DECLARE_EARLY_PER_CPU(u16, x86_bios_cpu_apicid);
>  #if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86_32)
>  DECLARE_EARLY_PER_CPU(int, x86_cpu_to_logical_apicid);

Looks like all of the surrounding fields are read mostly as 
well, only initialized very rarely, during cpu init?

Thanks,

	Ingo