From c2a4e54e8b6a89dde574608c47e460a0371e44be Mon Sep 17 00:00:00 2001 From: Simon Guo Date: Thu, 7 Jun 2018 09:57:54 +0800 Subject: [PATCH] powerpc/64: add 32 bytes prechecking before using VMX optimization on memcmp() This patch is based on the previous VMX patch on memcmp(). To optimize ppc64 memcmp() with VMX instruction, we need to think about the VMX penalty brought with: If kernel uses VMX instruction, it needs to save/restore current thread's VMX registers. There are 32 x 128 bits VMX registers in PPC, which means 32 x 16 = 512 bytes for load and store. The major concern regarding the memcmp() performance in kernel is KSM, who will use memcmp() frequently to merge identical pages. So it will make sense to take some measures/enhancement on KSM to see whether any improvement can be done here. Cyril Bur indicates that the memcmp() for KSM has a higher possibility to fail (unmatch) early in previous bytes in following mail. https://patchwork.ozlabs.org/patch/817322/#1773629 And I am taking a follow-up on this with this patch. Per some testing, it shows KSM memcmp() will fail early at previous 32 bytes. More specifically: - 76% cases will fail/unmatch before 16 bytes; - 83% cases will fail/unmatch before 32 bytes; - 84% cases will fail/unmatch before 64 bytes; So 32 bytes looks a better choice than other bytes for pre-checking. The early failure is also true for memcmp() for non-KSM case. With a non-typical call load, it shows ~73% cases fail before first 32 bytes. This patch adds a 32 bytes pre-checking firstly before jumping into VMX operations, to avoid the unnecessary VMX penalty. It is not limited to KSM case. And the testing shows ~20% improvement on memcmp() average execution time with this patch. And note the 32B pre-checking is only performed when the compare size is long enough (>=4K currently) to allow VMX operation. The detail data and analysis is at: https://github.com/justdoitqd/publicFiles/blob/master/memcmp/README.md Signed-off-by: Simon Guo Signed-off-by: Michael Ellerman --- arch/powerpc/lib/memcmp_64.S | 57 +++++++++++++++++++++++++++++------- 1 file changed, 46 insertions(+), 11 deletions(-) diff --git a/arch/powerpc/lib/memcmp_64.S b/arch/powerpc/lib/memcmp_64.S index be2f7925926b..844d8e774492 100644 --- a/arch/powerpc/lib/memcmp_64.S +++ b/arch/powerpc/lib/memcmp_64.S @@ -404,8 +404,27 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S) #ifdef CONFIG_ALTIVEC .Lsameoffset_vmx_cmp: /* Enter with src/dst addrs has the same offset with 8 bytes - * align boundary + * align boundary. + * + * There is an optimization based on following fact: memcmp() + * prones to fail early at the first 32 bytes. + * Before applying VMX instructions which will lead to 32x128bits + * VMX regs load/restore penalty, we compare the first 32 bytes + * so that we can catch the ~80% fail cases. */ + + li r0,4 + mtctr r0 +.Lsameoffset_prechk_32B_loop: + LD rA,0,r3 + LD rB,0,r4 + cmpld cr0,rA,rB + addi r3,r3,8 + addi r4,r4,8 + bne cr0,.LcmpAB_lightweight + addi r5,r5,-8 + bdnz .Lsameoffset_prechk_32B_loop + ENTER_VMX_OPS beq cr1,.Llong_novmx_cmp @@ -482,16 +501,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S) #endif .Ldiffoffset_8bytes_make_align_start: -#ifdef CONFIG_ALTIVEC -BEGIN_FTR_SECTION - /* only do vmx ops when the size equal or greater than 4K bytes */ - cmpdi cr5,r5,VMX_THRESH - bge cr5,.Ldiffoffset_vmx_cmp -END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S) - -.Ldiffoffset_novmx_cmp: -#endif - /* now try to align s1 with 8 bytes */ rlwinm r6,r3,3,26,28 beq .Ldiffoffset_align_s1_8bytes @@ -515,6 +524,17 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S) .Ldiffoffset_align_s1_8bytes: /* now s1 is aligned with 8 bytes. */ +#ifdef CONFIG_ALTIVEC +BEGIN_FTR_SECTION + /* only do vmx ops when the size equal or greater than 4K bytes */ + cmpdi cr5,r5,VMX_THRESH + bge cr5,.Ldiffoffset_vmx_cmp +END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S) + +.Ldiffoffset_novmx_cmp: +#endif + + cmpdi cr5,r5,31 ble cr5,.Lcmp_lt32bytes @@ -526,6 +546,21 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S) #ifdef CONFIG_ALTIVEC .Ldiffoffset_vmx_cmp: + /* perform a 32 bytes pre-checking before + * enable VMX operations. + */ + li r0,4 + mtctr r0 +.Ldiffoffset_prechk_32B_loop: + LD rA,0,r3 + LD rB,0,r4 + cmpld cr0,rA,rB + addi r3,r3,8 + addi r4,r4,8 + bne cr0,.LcmpAB_lightweight + addi r5,r5,-8 + bdnz .Ldiffoffset_prechk_32B_loop + ENTER_VMX_OPS beq cr1,.Ldiffoffset_novmx_cmp