linux-kernel - Re: [PATCH 12/29] x86, tsx: Add a per thread transaction disable count

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Date:	Sat, 23 Mar 2013 16:52:56 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, x86@...nel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 12/29] x86, tsx: Add a per thread transaction disable
 count

On Sat, Mar 23, 2013 at 02:51:56PM +0100, Andi Kleen wrote:
> Bit fields are slower and larger in code and unlike the others this is
> on hot paths.

Really? Let's see:

unsigned:
=========

	.file 8 "/w/kernel/linux-2.6/arch/x86/include/asm/thread_info.h"
	.loc 8 211 0
#APP
# 211 "/w/kernel/linux-2.6/arch/x86/include/asm/thread_info.h" 1
	movq %gs:kernel_stack,%rax	#, pfo_ret__
# 0 "" 2
.LVL238:
#NO_APP

...									# AMD F10h			SNB

disable:
	incl    -8056(%rax)     # ti_25->notxn				# INC mem: 4			; 6

test:
        cmpl    $0, -8056(%rax) #, ti_24->notxn				# CMP mem, imm: 4		; 1

reenable:
	decl    -8056(%rax)     # ti_25->notxn				# DEC mem: 4			; 6


bitfield:
=========

	.file 8 "/w/kernel/linux-2.6/arch/x86/include/asm/thread_info.h"
	.loc 8 211 0
#APP
# 211 "/w/kernel/linux-2.6/arch/x86/include/asm/thread_info.h" 1
	movq %gs:kernel_stack,%rax	#, pfo_ret__
# 0 "" 2
.LVL238:
#NO_APP

disable:
	xorb    $4, -8056(%rax) #,					# XOR mem, imm: 1		; 0

test:
        testb   $4, -8056(%rax) #,					# TEST mem, imm: 4		; -

reenable:
        xorb    $4, -8056(%rax) #,					# XOR mem, imm: 1		; 0


So let's explain. The AMD F10h column shows the respective instruction
latencies on AMD F10h. All instructions are DirectPath single.

The SNB column is something similar which I could find for Intel
Sandybridge: http://www.agner.org/optimize/instruction_tables.pdf. I'm
assuming Agner Fog's measurements are more or less accurate.

And wow, the XOR is *actually* faster. That's whopping three cycles on
AMD. Similar observation on SNB.

Now let's look at decoding bandwidth:

unsigned:
=========

disable:
  13:   ff 80 88 e0 ff ff       incl   -0x1f78(%rax)

test:
   9:   83 b8 88 e0 ff ff 00    cmpl   $0x0,-0x1f78(%rax)

reenable:
  13:   ff 88 88 e0 ff ff       decl   -0x1f78(%rax)


bitfield:
=========

disable:
  13:   80 b0 88 e0 ff ff 04    xorb   $0x4,-0x1f78(%rax)

test:
   9:   f6 80 88 e0 ff ff 04    testb  $0x4,-0x1f78(%rax)

reenable:
  13:   80 b0 88 e0 ff ff 04    xorb   $0x4,-0x1f78(%rax)

This particular XOR encoding is 1 byte longer, the rest is on-par.

Oh, and compiler is gcc (Debian 4.7.2-5) 4.7.2.

So you were saying?

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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