linux-kernel - Re: [PATCH] APICID: Avoid false sharing on the read mostly x86_cpu_to

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Message-ID: <4EA7AB7C.9080703@scalemp.com>
Date:	Wed, 26 Oct 2011 08:41:00 +0200
From:	Eial Czerwacki <eial@...lemp.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	linux-kernel@...r.kernel.org,
	"Shai Fultheim (Shai@...leMP.com)" <Shai@...lemp.com>
Subject: Re: [PATCH] APICID: Avoid false sharing on the read mostly x86_cpu_to_apicid

On 10/26/2011 08:21 AM, David Rientjes wrote:

> On Tue, 25 Oct 2011, Eial Czerwacki wrote:
> 
>> Avoid false sharing on the read mostly x86_cpu_to_apicid by moving it to
>> __read_mostly section.
> 
> x86_cpu_to_apicid used to be in .init.data and now it's never freed after 
> boot?
> 
>> The per-cpu area is write and read and this symbol shows up high on ipi
>> intensive/x86_cpu_to_apicid load.
>>
>> Signed-off-by: Eial Czerwacki<eial@...lemp.com>
>> Signed-off-by: Shai Fultheim<shai@...lemp.com>
>> Author: Ravikiran Thirumalai<kiran@...lex86.org>
> 
> If the author isn't you, then the patch should start with a single line 
> that says "From: name <email>" of the author followed by a blank line.  
> It's also strange that the author doesn't have a sign-off line.
> 


the author is mentioned after the last sign-off.

> Your email client also has corrupted the inline patch, please see 
> Documentation/email-clients.txt.
> 


thanks for the tips, I hope it is ok now.

> And, finally, please send this to the x86 maintainers once fixed.  See 
> ./scripts/get_maintainers.pl <your patch>.
> 


sure.

>> Index: b/arch/x86/include/asm/smp.h
>> ===================================================================
>> --- a/arch/x86/include/asm/smp.h	2010-06-01 09:56:03.000000000 -0700
>> +++ b/arch/x86/include/asm/smp.h	2010-06-02 15:59:21.000000000 -0700
>> @@ -36,7 +36,8 @@ static inline struct cpumask *cpu_core_m
>>   	return per_cpu(cpu_core_map, cpu);
>>   }
>>
>> -DECLARE_EARLY_PER_CPU(u16, x86_cpu_to_apicid);
>> +extern u16 x86_cpu_to_apicid[NR_CPUS];
>> +
>>   DECLARE_EARLY_PER_CPU(u16, x86_bios_cpu_apicid);
>>
>>   /* Static state in head.S used to set up a CPU */
>> @@ -142,7 +143,7 @@ void native_send_call_func_ipi(const str
>>   void native_send_call_func_single_ipi(int cpu);
>>
>>   void smp_store_cpu_info(int id);
>> -#define cpu_physical_id(cpu)	per_cpu(x86_cpu_to_apicid, cpu)
>> +#define cpu_physical_id(cpu)	x86_cpu_to_apicid[cpu]
>>
>>   /* We don't mark CPUs online until __cpu_up(), so we need another measure */
>>   static inline int num_booting_cpus(void)
>> Index: b/arch/x86/kernel/acpi/boot.c
>> ===================================================================
>> --- a/arch/x86/kernel/acpi/boot.c	2010-06-01 09:56:03.000000000 -0700
>> +++ b/arch/x86/kernel/acpi/boot.c	2010-06-02 15:59:21.000000000 -0700
>> @@ -568,7 +568,7 @@ EXPORT_SYMBOL(acpi_map_lsapic);
>>
>>   int acpi_unmap_lsapic(int cpu)
>>   {
>> -	per_cpu(x86_cpu_to_apicid, cpu) = -1;
>> +	x86_cpu_to_apicid[cpu] = -1;
>>   	set_cpu_present(cpu, false);
>>   	num_processors--;
>>
> 
> Shouldn't this be BAD_APICID?  Not sure where we check for -1.
> --
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> 


I'll check the rest of your comments.

Thanks,

Eial.
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