linux-kernel - Re: [PATCH] x86-64: fix memset() to support sizes of 4Gb and above

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Date:	Fri, 06 Jan 2012 12:31:38 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	<tglx@...utronix.de>, "Andrew Morton" <akpm@...ux-foundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] x86-64: fix memset() to support sizes of 4Gb and
 above

>>> On 06.01.12 at 12:05, Ingo Molnar <mingo@...e.hu> wrote:

> * Jan Beulich <JBeulich@...e.com> wrote:
> 
>> While currently there doesn't appear to be any reachable 
>> in-tree case where such large memory blocks may be passed to 
>> memset() (alloc_bootmem() being the primary non-reachable one, 
>> as it gets called with suitably large sizes in FLATMEM 
>> configurations), we have recently hit the problem a second 
>> time in our Xen kernels. Rather than working around it a 
>> second time, prevent others from falling into the same trap by 
>> fixing this long standing limitation.
>> 
>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
> 
> Have you checked the before/after size of the hotpath?
> 
> The patch suggests that it got shorter by 3 instructions:
> 
>> -	movl %edx,%r8d
>> -	andl $7,%r8d
>> -	movl %edx,%ecx
>> -	shrl $3,%ecx
>> +	movq %rdx,%rcx
>> +	andl $7,%edx
>> +	shrq $3,%rcx
> 
> [...]
> 
>>  	movq %rdi,%r10
>> -	movq %rdx,%r11
>>  
> 
> [...]
> 
>> -	movl	%r11d,%ecx
>> -	andl	$7,%ecx
>> +	andl	$7,%edx
> 
> Is that quick impression correct? I have not tried building or 
> measuring it.

Yes, that's correct.

As the bodies of the individual flavors didn't change, I see no risk in
this change causing any performance degradation.

> Would be nice to add support for arch/x86/lib/memset_64.S as 
> well, and look at the before/after performance of it. In 
> userspace we can do a lot more accurate measurements of this 
> kind:

I'll see whether I can get this done, but I admit that I'm entirely
unfamiliar with this tool and its infrastructure. I hope doing this is
not going to be a requirement for acceptance of the patch.

Jan

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