Re: [PATCH v3] arm64: lib: accelerate do_csum with NEON instruction

On 2019/1/6 16:26, Ard Biesheuvel wrote:

On Sun, 6 Jan 2019 at 02:56, Lingyan Huang [off-list ref] wrote:

quoted

Function do_csum() in lib/checksum.c is used to compute checksum,
which is turned out to be slowly and costs a lot of resources.
Let's use neon instructions to accelerate the checksum computation
for arm64.

------
V2 ==> V3:
    only modify the arm64 codes instead of modifying headers
    under asm-generic and code in lib/checksum.c.
------
------
V1 ==> V2:
    Change NEON assembly code to NEON intrinsic code which is built
    on top of arm_neon.h to avoid dropping into assembly.
------

Please put the changelog between the --- below and the diffstat

ok

quoted

Here is the comparison results of function ip_compute_csum() between
general do_csum() and neon instruction do_csum(). The test platform
is HUAWEI 1620 server with TAISHAN cores.

        len(1000cycle)      general(ns)        do_csum_neon(ns)
          64B:                58060                 59460
         128B:                82930                 83930
         256B:               132480                 73570
         512B:               230100                 86230
        1024B:               426600                 98200

Again, very nice performance. How did you test for correctness?

I compare my computing results with the results of do_csum() function in lib/checksum.c to ensure its correctness.

quoted

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <redacted>
Cc: Ard Biesheuvel <redacted>
Signed-off-by: Lingyan Huang <redacted>
---
 arch/arm64/include/asm/checksum.h |   5 ++
 arch/arm64/lib/Makefile           |   8 +--
 arch/arm64/lib/checksum.c         |  26 ++++++++
 arch/arm64/lib/do_csum.c          | 136 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 171 insertions(+), 4 deletions(-)
 create mode 100644 arch/arm64/lib/checksum.c
 create mode 100644 arch/arm64/lib/do_csum.c

diff --git a/arch/arm64/include/asm/checksum.h b/arch/arm64/include/asm/checksum.h
index 0b6f5a7..7acd713 100644
--- a/arch/arm64/include/asm/checksum.h
+++ b/arch/arm64/include/asm/checksum.h

@@ -26,6 +26,10 @@ static inline __sum16 csum_fold(__wsum csum)
 }
 #define csum_fold csum_fold

+#define do_csum do_csum
+unsigned int do_csum(const unsigned char *buff, int len);
+extern unsigned int do_csum_arm(const unsigned char *buff, int len);
+
 static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
 {
        __uint128_t tmp;

@@ -46,6 +50,7 @@ static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
 }
 #define ip_fast_csum ip_fast_csum

+

Drop this whitespace-only change

OK

quoted

 #include <asm-generic/checksum.h>

 #endif /* __ASM_CHECKSUM_H */

diff --git a/arch/arm64/lib/Makefile b/arch/arm64/lib/Makefile
index 5540a16..c0b5b8c 100644
--- a/arch/arm64/lib/Makefile
+++ b/arch/arm64/lib/Makefile

@@ -3,12 +3,12 @@ lib-y         := clear_user.o delay.o copy_from_user.o                \
                   copy_to_user.o copy_in_user.o copy_page.o            \
                   clear_page.o memchr.o memcpy.o memmove.o memset.o    \
                   memcmp.o strcmp.o strncmp.o strlen.o strnlen.o       \
-                  strchr.o strrchr.o tishift.o
+                  strchr.o strrchr.o tishift.o checksum.o

 ifeq ($(CONFIG_KERNEL_MODE_NEON), y)
-obj-$(CONFIG_XOR_BLOCKS)       += xor-neon.o
-CFLAGS_REMOVE_xor-neon.o       += -mgeneral-regs-only
-CFLAGS_xor-neon.o              += -ffreestanding
+obj-y    += do_csum.o

Please indent aligned with the others

I'm sorry i was so careless

quoted

+CFLAGS_REMOVE_do_csum.o        += -mgeneral-regs-only
+CFLAGS_do_csum.o               += -ffreestanding
 endif

 # Tell the compiler to treat all general purpose registers (with the

diff --git a/arch/arm64/lib/checksum.c b/arch/arm64/lib/checksum.c
new file mode 100644
index 0000000..15a31bb
--- /dev/null
+++ b/arch/arm64/lib/checksum.c

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *
+ * Authors: Lingyan Huang <huanglingyan2@huawei.com>
+ * Copyright (C) 2018 Hisilicon, Inc. All Rights Reserved.
+ *
+ * Generic C or neon implementation of do_csum operations.
+ * Choose faster neon instructions when NEON is supported.
+ *
+ */
+
+#include <asm/neon.h>
+#include <asm/simd.h>
+#include <asm/checksum.h>
+
+#define CSUM_NEON_THRESHOLD 128
+
+unsigned int do_csum(const unsigned char *buff, int len)
+{
+#ifdef CONFIG_KERNEL_MODE_NEON
+       if (len >= CSUM_NEON_THRESHOLD)

Please change this into

if (IS_ENABLED(CONFIG_KERNEL_MODE_NEON) &&
    len >= CSUM_NEON_THRESHOLD &&
    may_use_simd()) {
        kernel_neon_begin();
        res = do_csum_neon(buff, len);
        kernel_neon_end();
    }

and drop the intermediate do_csum_arm()

quoted

+               return do_csum_arm(buff, len);
+#endif  /* CONFIG_KERNEL_MODE_NEON */

No else? What happens if len < CSUM_NEON_THRESHOLD ?

quoted

+#undef do_csum

Can we drop this?

Using NEON instructions will bring some costs. The spending maybe introduced when reservering/restoring
neon registers with kernel_neon_begin()/kernel_neon_end(). Therefore NEON code is Only used when
the length exceeds CSUM_NEON_THRESHOLD. General do csum() codes in lib/checksum.c will be used in
shorter length. To achieve this goal, I use the "#undef do_csum" in else clause to have the oppotunity to
utilize the general codes.

However, this solution may not be a standard way. Is there any other solutions to achieve this goal?

Codes in lib/checksum.c.

#ifndef do_csum
static unsigned int do_csum(const unsigned char *buff, int len)
{
……
}

quoted

+       return 0;
+}

diff --git a/arch/arm64/lib/do_csum.c b/arch/arm64/lib/do_csum.c
new file mode 100644
index 0000000..893583f
--- /dev/null
+++ b/arch/arm64/lib/do_csum.c

@@ -0,0 +1,136 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Authors: Lingyan Huang <huanglingyan2@huawei.com>
+ * Copyright (C) 2018 Hisilicon, Inc. All Rights Reserved.
+ *
+ * Generic C or neon implementation of do_csum operations.
+ * Choose faster neon instructions when NEON is supported.
+ *
+ */
+
+#include <asm/neon.h>
+#include <asm/simd.h>
+#include <asm/checksum.h>
+#include <asm/byteorder.h>
+#include <asm/neon-intrinsics.h>
+
+#define CSUM_NEON_THRESHOLD 128

Drop this - it is not used in this file

OK

quoted

+#ifdef CONFIG_KERNEL_MODE_NEON

This file is only built if KERNEL_MODE_NEON=y so the #ifdef can be dropped

OK

quoted

+static inline u32 from64to16(u64 x)
+{
+       /* add up 32-bit and 32-bit for 32+c bit */
+       x = (x & 0xffffffff) + (x >> 32);
+       /* add up carry.. */
+       x = (x & 0xffffffff) + (x >> 32);
+       /* add up 16-bit and 16-bit for 16+c bit */
+       x = ((u32)x & 0xffff) + ((u32)x >> 16);
+       /* add up carry.. */
+       x = ((u32)x & 0xffff) + ((u32)x >> 16);
+       return x;
+}
+
+unsigned int do_csum_neon(const unsigned char *buff, int len)
+{
+       unsigned int odd, count;
+       uint64_t result = 0;
+       unsigned int count64;
+       uint32x4_t vzero = (uint32x4_t){0, 0, 0, 0};
+
+       register uint32x4_t v0, v1, v2, v3;
+
+       if (unlikely(len <= 0))
+               return result;
+
+       odd = 1 & (unsigned long)buff;
+       if (unlikely(odd)) {
+               result = *buff;
+               len--;
+               buff++;
+       }
+
+       count = len >> 1;
+       if (count) {
+               if (2 & (unsigned long)buff) {
+                       result += *(unsigned short *)buff;
+                       count--;
+                       len -= 2;
+                       buff += 2;
+               }
+               count >>= 1;            /* nr of 32-bit words.. */
+               if (count) {
+                       if (4 & (unsigned long)buff) {
+                               result += *(unsigned int *)buff;
+                               count--;
+                               len -= 4;
+                               buff += 4;
+                       }
+                       count >>= 1;    /* nr of 64-bit words.. */
+
+                       v0 = vzero;
+                       v1 = vzero;
+                       v2 = vzero;
+                       v3 = vzero;
+
+                       count64 = count >> 3;  /* compute 64 Byte circle */
+                       while (count64) {
+                               v0 = vpadalq_u16(v0,
+                                       vld1q_u16((uint16_t *)buff + 0));
+                               v1 = vpadalq_u16(v1,
+                                       vld1q_u16((uint16_t *)buff + 8));
+                               v2 = vpadalq_u16(v2,
+                                       vld1q_u16((uint16_t *)buff + 16));
+                               v3 = vpadalq_u16(v3,
+                                       vld1q_u16((uint16_t *)buff + 24));
+                               buff += 64;
+                               count64--;
+                       }
+                       v0 = vaddq_u32(v0, v1);
+                       v2 = vaddq_u32(v2, v3);
+                       v0 = vaddq_u32(v0, v2);
+
+                       count %= 8;
+                       while (count >= 2) { /* compute 16 byte circle */
+                               v0 = vpadalq_u16(v0,
+                                       vld1q_u16((uint16_t *)buff + 0));
+                               buff += 16;
+                               count -= 2;
+                       }
+
+                       result += vgetq_lane_u32(v0, 0);
+                       result += vgetq_lane_u32(v0, 1);
+                       result += vgetq_lane_u32(v0, 2);
+                       result += vgetq_lane_u32(v0, 3);
+                       if (count & 1) {
+                               result += *(unsigned long long *)buff;
+                               buff += 8;
+                       }
+                       if (len & 4) {
+                               result += *(unsigned int *)buff;
+                               buff += 4;
+                       }
+               }
+               if (len & 2) {
+                       result += *(unsigned short *)buff;
+                       buff += 2;
+               }
+       }
+       if (len & 1)
+               result += *buff;
+       result = from64to16(result);
+       if (odd)
+               result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
+       return result;
+}
+#endif
+
+
+unsigned int do_csum_arm(const unsigned char *buff, int len)
+{
+       unsigned int res;
+
+       kernel_neon_begin();
+       res = do_csum_neon(buff, len);
+       kernel_neon_end();
+       return res;
+}

As I said above, please drop this intermediate function and fold the
logic into do_csum()

OK

.

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