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Subject[tip: x86/core] x86/csum: Rewrite/optimize csum_partial()
The following commit has been merged into the x86/core branch of tip:

Commit-ID: d31c3c683ee668ba5d87c0730610442fd672525f
Gitweb: https://git.kernel.org/tip/d31c3c683ee668ba5d87c0730610442fd672525f
Author: Eric Dumazet <edumazet@google.com>
AuthorDate: Fri, 12 Nov 2021 08:19:50 -08:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 17 Nov 2021 14:48:54 +01:00

x86/csum: Rewrite/optimize csum_partial()

With more NIC supporting CHECKSUM_COMPLETE, and IPv6 being widely
used. csum_partial() is heavily used with small amount of bytes, and
is consuming many cycles.

IPv6 header size for instance is 40 bytes.

Another thing to consider is that NET_IP_ALIGN is 0 on x86, meaning
that network headers are not word-aligned, unless the driver forces
this.

This means that csum_partial() fetches one u16 to 'align the buffer',
then perform three u64 additions with carry in a loop, then a
remaining u32, then a remaining u16.

With this new version, we perform a loop only for the 64 bytes blocks,
then the remaining is bisected.

Tested on various cpus, all of them show a big reduction in
csum_partial() cost (by 50 to 80 %)

Before:
4.16% [kernel] [k] csum_partial
After:
0.83% [kernel] [k] csum_partial

If run in a loop 1,000,000 times:

Before:
26,922,913 cycles # 3846130.429 GHz
80,302,961 instructions # 2.98 insn per cycle
21,059,816 branches # 3008545142.857 M/sec
2,896 branch-misses # 0.01% of all branches
After:
17,960,709 cycles # 3592141.800 GHz
41,292,805 instructions # 2.30 insn per cycle
11,058,119 branches # 2211623800.000 M/sec
2,997 branch-misses # 0.03% of all branches

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://lore.kernel.org/r/20211112161950.528886-1-eric.dumazet@gmail.com
---
arch/x86/lib/csum-partial_64.c | 162 +++++++++++++-------------------
1 file changed, 67 insertions(+), 95 deletions(-)

diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
index e7925d6..5ec3562 100644
--- a/arch/x86/lib/csum-partial_64.c
+++ b/arch/x86/lib/csum-partial_64.c
@@ -9,6 +9,7 @@
#include <linux/compiler.h>
#include <linux/export.h>
#include <asm/checksum.h>
+#include <asm/word-at-a-time.h>

static inline unsigned short from32to16(unsigned a)
{
@@ -21,120 +22,92 @@ static inline unsigned short from32to16(unsigned a)
}

/*
- * Do a 64-bit checksum on an arbitrary memory area.
+ * Do a checksum on an arbitrary memory area.
* Returns a 32bit checksum.
*
* This isn't as time critical as it used to be because many NICs
* do hardware checksumming these days.
- *
- * Things tried and found to not make it faster:
- * Manual Prefetching
- * Unrolling to an 128 bytes inner loop.
- * Using interleaving with more registers to break the carry chains.
+ *
+ * Still, with CHECKSUM_COMPLETE this is called to compute
+ * checksums on IPv6 headers (40 bytes) and other small parts.
+ * it's best to have buff aligned on a 64-bit boundary
*/
-static unsigned do_csum(const unsigned char *buff, unsigned len)
+__wsum csum_partial(const void *buff, int len, __wsum sum)
{
- unsigned odd, count;
- unsigned long result = 0;
+ u64 temp64 = (__force u64)sum;
+ unsigned odd, result;

- if (unlikely(len == 0))
- return result;
odd = 1 & (unsigned long) buff;
if (unlikely(odd)) {
- result = *buff << 8;
+ if (unlikely(len == 0))
+ return sum;
+ temp64 += (*(unsigned char *)buff << 8);
len--;
buff++;
}
- count = len >> 1; /* nr of 16-bit words.. */
- 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) {
- unsigned long zero;
- unsigned count64;
- if (4 & (unsigned long) buff) {
- result += *(unsigned int *) buff;
- count--;
- len -= 4;
- buff += 4;
- }
- count >>= 1; /* nr of 64-bit words.. */

- /* main loop using 64byte blocks */
- zero = 0;
- count64 = count >> 3;
- while (count64) {
- asm("addq 0*8(%[src]),%[res]\n\t"
- "adcq 1*8(%[src]),%[res]\n\t"
- "adcq 2*8(%[src]),%[res]\n\t"
- "adcq 3*8(%[src]),%[res]\n\t"
- "adcq 4*8(%[src]),%[res]\n\t"
- "adcq 5*8(%[src]),%[res]\n\t"
- "adcq 6*8(%[src]),%[res]\n\t"
- "adcq 7*8(%[src]),%[res]\n\t"
- "adcq %[zero],%[res]"
- : [res] "=r" (result)
- : [src] "r" (buff), [zero] "r" (zero),
- "[res]" (result));
- buff += 64;
- count64--;
- }
+ while (unlikely(len >= 64)) {
+ asm("addq 0*8(%[src]),%[res]\n\t"
+ "adcq 1*8(%[src]),%[res]\n\t"
+ "adcq 2*8(%[src]),%[res]\n\t"
+ "adcq 3*8(%[src]),%[res]\n\t"
+ "adcq 4*8(%[src]),%[res]\n\t"
+ "adcq 5*8(%[src]),%[res]\n\t"
+ "adcq 6*8(%[src]),%[res]\n\t"
+ "adcq 7*8(%[src]),%[res]\n\t"
+ "adcq $0,%[res]"
+ : [res] "+r" (temp64)
+ : [src] "r" (buff)
+ : "memory");
+ buff += 64;
+ len -= 64;
+ }
+
+ if (len & 32) {
+ asm("addq 0*8(%[src]),%[res]\n\t"
+ "adcq 1*8(%[src]),%[res]\n\t"
+ "adcq 2*8(%[src]),%[res]\n\t"
+ "adcq 3*8(%[src]),%[res]\n\t"
+ "adcq $0,%[res]"
+ : [res] "+r" (temp64)
+ : [src] "r" (buff)
+ : "memory");
+ buff += 32;
+ }
+ if (len & 16) {
+ asm("addq 0*8(%[src]),%[res]\n\t"
+ "adcq 1*8(%[src]),%[res]\n\t"
+ "adcq $0,%[res]"
+ : [res] "+r" (temp64)
+ : [src] "r" (buff)
+ : "memory");
+ buff += 16;
+ }
+ if (len & 8) {
+ asm("addq 0*8(%[src]),%[res]\n\t"
+ "adcq $0,%[res]"
+ : [res] "+r" (temp64)
+ : [src] "r" (buff)
+ : "memory");
+ buff += 8;
+ }
+ if (len & 7) {
+ unsigned int shift = (8 - (len & 7)) * 8;
+ unsigned long trail;

- /* last up to 7 8byte blocks */
- count %= 8;
- while (count) {
- asm("addq %1,%0\n\t"
- "adcq %2,%0\n"
- : "=r" (result)
- : "m" (*(unsigned long *)buff),
- "r" (zero), "0" (result));
- --count;
- buff += 8;
- }
- result = add32_with_carry(result>>32,
- result&0xffffffff);
+ trail = (load_unaligned_zeropad(buff) << shift) >> shift;

- if (len & 4) {
- result += *(unsigned int *) buff;
- buff += 4;
- }
- }
- if (len & 2) {
- result += *(unsigned short *) buff;
- buff += 2;
- }
+ asm("addq %[trail],%[res]\n\t"
+ "adcq $0,%[res]"
+ : [res] "+r" (temp64)
+ : [trail] "r" (trail));
}
- if (len & 1)
- result += *buff;
- result = add32_with_carry(result>>32, result & 0xffffffff);
+ result = add32_with_carry(temp64 >> 32, temp64 & 0xffffffff);
if (unlikely(odd)) {
result = from32to16(result);
result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
}
- return result;
-}
-
-/*
- * computes the checksum of a memory block at buff, length len,
- * and adds in "sum" (32-bit)
- *
- * returns a 32-bit number suitable for feeding into itself
- * or csum_tcpudp_magic
- *
- * this function must be called with even lengths, except
- * for the last fragment, which may be odd
- *
- * it's best to have buff aligned on a 64-bit boundary
- */
-__wsum csum_partial(const void *buff, int len, __wsum sum)
-{
- return (__force __wsum)add32_with_carry(do_csum(buff, len),
- (__force u32)sum);
+ return (__force __wsum)result;
}
EXPORT_SYMBOL(csum_partial);

@@ -147,4 +120,3 @@ __sum16 ip_compute_csum(const void *buff, int len)
return csum_fold(csum_partial(buff,len,0));
}
EXPORT_SYMBOL(ip_compute_csum);
-
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