[PATCH v2] powerpc/lib: prefetch ahead in __csum_partial()

[Rosen Penev <rosenp@gmail.com>]

__csum_partial() walks the buffer linearly and reads each cache line
exactly once. On cores without a hardware prefetcher, such as the 464FP
in the APM82181, every new line is therefore a demand miss that stalls
the loop. Packets received by non-coherent DMA are never in the cache,
so this is the common case for GRO checksum validation: when routing
TCP with GRO enabled on a Meraki MX60, __csum_partial() takes about 30%
of all cycles and is the top entry in the profile.

Issue a dcbt four cache lines (128 bytes) ahead in the main loop.
dcbt is only a hint and never faults, so prefetching past the end of
the buffer is harmless. Lines that are already cached, for example when
checksumming data the CPU just wrote, are left untouched.

Measured on a Meraki MX60 (APM82181 at 800 MHz), single TCP stream
routed through a qca8k DSA switch, iperf3 median of 5. Only the dcbt
was toggled at runtime, in the same boot:

              LAN->WAN               WAN->LAN
              Mbit/s  csum cycles    Mbit/s  csum cycles
  before      495     30.5%          439     27.2%
  after       599     15.3%          520     13.7%

That is 58% less time spent in __csum_partial() per byte.

The prefetch distance was chosen with an earlier, slower driver
configuration (median of 3, Mbit/s):

  distance    LAN->WAN   WAN->LAN
  none        370        341
  64 bytes    430        381
  96 bytes    438        397
  128 bytes   438        385
  160 bytes   434        386
  256 bytes   423        383

Prefetching the first lines before entering the loop made no
measurable difference and is not done.

Assisted-by: LLM
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 v2: reword commit message and retest numbers.
 arch/powerpc/lib/checksum_32.S | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/lib/checksum_32.S b/arch/powerpc/lib/checksum_32.S
index cd00b9bdd772..5a14d45c7b9d 100644
--- a/arch/powerpc/lib/checksum_32.S
+++ b/arch/powerpc/lib/checksum_32.S
@@ -43,6 +43,7 @@ _GLOBAL(__csum_partial)
 	bdnz	2b
 21:	srwi.	r6,r4,4		/* # blocks of 4 words to do */
 	beq	3f
+	li	r9,4*L1_CACHE_BYTES	/* prefetch distance */
 	lwz	r0,4(r3)
 	mtctr	r6
 	lwz	r6,8(r3)
@@ -52,7 +53,8 @@ _GLOBAL(__csum_partial)
 	lwzu	r8,16(r3)
 	adde	r5,r5,r7
 	bdz	23f
-22:	lwz	r0,4(r3)
+22:	dcbt	r3,r9
+	lwz	r0,4(r3)
 	adde	r5,r5,r8
 	lwz	r6,8(r3)
 	adde	r5,r5,r0
-- 
2.56.0