Subject: Re: [PATCH v2 2/3] powerpc/64: enhance memcmp() with VMX instruction for long bytes comparision

From: Cyril Bur <[email-masked]>
Date: Sat, 23 Sep 2017 00:06:48 +1000

On Thu, 2017-09-21 at 07:34 +0800, [email-masked] wrote:
> From: Simon Guo <[email-masked]>
> 
> This patch add VMX primitives to do memcmp() in case the compare size
> exceeds 4K bytes.
> 

Hi Simon,

Sorry I didn't see this sooner, I've actually been working on a kernel
version of glibc commit dec4a7105e (powerpc: Improve memcmp performance
for POWER8) unfortunately I've been distracted and it still isn't done.
I wonder if we can consolidate our efforts here. One thing I did come
across in my testing is that for memcmp() that will fail early (I
haven't narrowed down the the optimal number yet) the cost of enabling
VMX actually turns out to be a performance regression, as such I've
added a small check of the first 64 bytes to the start before enabling
VMX to ensure the penalty is worth taking.

Also, you should consider doing 4K and greater, KSM (Kernel Samepage
Merging) uses PAGE_SIZE which can be as small as 4K.

Cyril

> Test result with following test program(replace the "^>" with ""):
> ------
> > # cat tools/testing/selftests/powerpc/stringloops/memcmp.c
> > #include <malloc.h>
> > #include <stdlib.h>
> > #include <string.h>
> > #include <time.h>
> > #include "utils.h"
> > #define SIZE (1024 * 1024 * 900)
> > #define ITERATIONS 40
> 
> int test_memcmp(const void *s1, const void *s2, size_t n);
> 
> static int testcase(void)
> {
>         char *s1;
>         char *s2;
>         unsigned long i;
> 
>         s1 = memalign(128, SIZE);
>         if (!s1) {
>                 perror("memalign");
>                 exit(1);
>         }
> 
>         s2 = memalign(128, SIZE);
>         if (!s2) {
>                 perror("memalign");
>                 exit(1);
>         }
> 
>         for (i = 0; i < SIZE; i++)  {
>                 s1[i] = i & 0xff;
>                 s2[i] = i & 0xff;
>         }
>         for (i = 0; i < ITERATIONS; i++) {
> 		int ret = test_memcmp(s1, s2, SIZE);
> 
> 		if (ret) {
> 			printf("return %d at[%ld]! should have returned zero\n", ret, i);
> 			abort();
> 		}
> 	}
> 
>         return 0;
> }
> 
> int main(void)
> {
>         return test_harness(testcase, "memcmp");
> }
> ------
> Without VMX patch:
>        7.435191479 seconds time elapsed                                          ( +- 0.51% )
> With VMX patch:
>        6.802038938 seconds time elapsed                                          ( +- 0.56% )
> 		There is ~+8% improvement.
> 
> However I am not aware whether there is use case in kernel for memcmp on
> large size yet.
> 
> Signed-off-by: Simon Guo <[email-masked]>
> ---
>  arch/powerpc/include/asm/asm-prototypes.h |  2 +-
>  arch/powerpc/lib/copypage_power7.S        |  2 +-
>  arch/powerpc/lib/memcmp_64.S              | 82 +++++++++++++++++++++++++++++++
>  arch/powerpc/lib/memcpy_power7.S          |  2 +-
>  arch/powerpc/lib/vmx-helper.c             |  2 +-
>  5 files changed, 86 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/asm-prototypes.h b/arch/powerpc/include/asm/asm-prototypes.h
> index 7330150..e6530d8 100644
> --- a/arch/powerpc/include/asm/asm-prototypes.h
> +++ b/arch/powerpc/include/asm/asm-prototypes.h
> @@ -49,7 +49,7 @@ void __trace_hcall_exit(long opcode, unsigned long retval,
>  /* VMX copying */
>  int enter_vmx_usercopy(void);
>  int exit_vmx_usercopy(void);
> -int enter_vmx_copy(void);
> +int enter_vmx_ops(void);
>  void * exit_vmx_copy(void *dest);
>  
>  /* Traps */
> diff --git a/arch/powerpc/lib/copypage_power7.S b/arch/powerpc/lib/copypage_power7.S
> index ca5fc8f..9e7729e 100644
> --- a/arch/powerpc/lib/copypage_power7.S
> +++ b/arch/powerpc/lib/copypage_power7.S
> @@ -60,7 +60,7 @@ _GLOBAL(copypage_power7)
>  	std	r4,-STACKFRAMESIZE+STK_REG(R30)(r1)
>  	std	r0,16(r1)
>  	stdu	r1,-STACKFRAMESIZE(r1)
> -	bl	enter_vmx_copy
> +	bl	enter_vmx_ops
>  	cmpwi	r3,0
>  	ld	r0,STACKFRAMESIZE+16(r1)
>  	ld	r3,STK_REG(R31)(r1)
> diff --git a/arch/powerpc/lib/memcmp_64.S b/arch/powerpc/lib/memcmp_64.S
> index 6dccfb8..40218fc 100644
> --- a/arch/powerpc/lib/memcmp_64.S
> +++ b/arch/powerpc/lib/memcmp_64.S
> @@ -162,6 +162,13 @@ _GLOBAL(memcmp)
>  	blr
>  
>  .Llong:
> +#ifdef CONFIG_ALTIVEC
> +	/* Try to use vmx loop if length is larger than 4K */
> +	cmpldi  cr6,r5,4096
> +	bgt	cr6,.Lvmx_cmp
> +
> +.Llong_novmx_cmp:
> +#endif
>  	li	off8,8
>  	li	off16,16
>  	li	off24,24
> @@ -319,4 +326,79 @@ _GLOBAL(memcmp)
>  8:
>  	blr
>  
> +#ifdef CONFIG_ALTIVEC
> +.Lvmx_cmp:
> +	mflr    r0
> +	std     r3,-STACKFRAMESIZE+STK_REG(R31)(r1)
> +	std     r4,-STACKFRAMESIZE+STK_REG(R30)(r1)
> +	std     r5,-STACKFRAMESIZE+STK_REG(R29)(r1)
> +	std     r0,16(r1)
> +	stdu    r1,-STACKFRAMESIZE(r1)
> +	bl      enter_vmx_ops
> +	cmpwi   cr1,r3,0
> +	ld      r0,STACKFRAMESIZE+16(r1)
> +	ld      r3,STK_REG(R31)(r1)
> +	ld      r4,STK_REG(R30)(r1)
> +	ld      r5,STK_REG(R29)(r1)
> +	addi	r1,r1,STACKFRAMESIZE
> +	mtlr    r0
> +	beq     cr1,.Llong_novmx_cmp
> +
> +3:
> +	/* Enter with src/dst address 8 bytes aligned, and len is
> +	 * no less than 4KB. Need to align with 16 bytes further.
> +	 */
> +	andi.	rA,r3,8
> +	beq	4f
> +	LD	rA,0,r3
> +	LD	rB,0,r4
> +	cmpld	cr0,rA,rB
> +	bne	cr0,.LcmpAB_lightweight
> +
> +	addi	r3,r3,8
> +	addi	r4,r4,8
> +	addi	r5,r5,-8
> +
> +4:
> +	/* compare 32 bytes for each loop */
> +	srdi	r0,r5,5
> +	mtctr	r0
> +	andi.	r5,r5,31
> +	li	off16,16
> +
> +.balign 16
> +5:
> +	lvx 	v0,0,r3
> +	lvx 	v1,0,r4
> +	vcmpequd. v0,v0,v1
> +	bf	24,7f
> +	lvx 	v0,off16,r3
> +	lvx 	v1,off16,r4
> +	vcmpequd. v0,v0,v1
> +	bf	24,6f
> +	addi	r3,r3,32
> +	addi	r4,r4,32
> +	bdnz	5b
> +
> +	cmpdi	r5,0
> +	beq	.Lzero
> +	b	.L8bytes_aligned
> +
> +6:
> +	addi	r3,r3,16
> +	addi	r4,r4,16
> +
> +7:
> +	LD	rA,0,r3
> +	LD	rB,0,r4
> +	cmpld	cr0,rA,rB
> +	bne	cr0,.LcmpAB_lightweight
> +
> +	li	off8,8
> +	LD	rA,off8,r3
> +	LD	rB,off8,r4
> +	cmpld	cr0,rA,rB
> +	bne	cr0,.LcmpAB_lightweight
> +	b	.Lzero
> +#endif
>  EXPORT_SYMBOL(memcmp)
> diff --git a/arch/powerpc/lib/memcpy_power7.S b/arch/powerpc/lib/memcpy_power7.S
> index 193909a..682e386 100644
> --- a/arch/powerpc/lib/memcpy_power7.S
> +++ b/arch/powerpc/lib/memcpy_power7.S
> @@ -230,7 +230,7 @@ _GLOBAL(memcpy_power7)
>  	std	r5,-STACKFRAMESIZE+STK_REG(R29)(r1)
>  	std	r0,16(r1)
>  	stdu	r1,-STACKFRAMESIZE(r1)
> -	bl	enter_vmx_copy
> +	bl	enter_vmx_ops
>  	cmpwi	cr1,r3,0
>  	ld	r0,STACKFRAMESIZE+16(r1)
>  	ld	r3,STK_REG(R31)(r1)
> diff --git a/arch/powerpc/lib/vmx-helper.c b/arch/powerpc/lib/vmx-helper.c
> index bf925cd..923a9ab 100644
> --- a/arch/powerpc/lib/vmx-helper.c
> +++ b/arch/powerpc/lib/vmx-helper.c
> @@ -53,7 +53,7 @@ int exit_vmx_usercopy(void)
>  	return 0;
>  }
>  
> -int enter_vmx_copy(void)
> +int enter_vmx_ops(void)
>  {
>  	if (in_interrupt())
>  		return 0;