linux-kernel - Re: [PATCH 2/2] x86: Move x86_cpu_to_apicid to the __read

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Message-Id: <201205071641.14946.vlad@scalemp.com>
Date:	Mon, 7 May 2012 16:41:14 +0300
From:	Vlad Zolotarov <vlad@...lemp.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"linux-kernel" <linux-kernel@...r.kernel.org>,
	alan@...rguk.ukuu.org.uk,
	"Shai Fultheim (Shai@...leMP.com)" <Shai@...lemp.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 2/2] x86: Move x86_cpu_to_apicid to the __read_mostly section

On Monday, May 07, 2012 01:52:47 pm Ingo Molnar wrote:
> * Vlad Zolotarov <vlad@...lemp.com> wrote:
> > From: Shai Fultheim <shai@...lemp.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@...lemp.com>
> > Signed-off-by: Vlad Zolotarov <vlad@...lemp.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?

U r right. At least x86_bios_cpu_apicid is read mostly as well. I'll fix and 
respin.

Did u have a chance to look at the other "__read_mostly" patch for fs/buffer.c 
I've sent?

thanks,
vlad

> 
> Thanks,
> 
> 	Ingo
> --
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