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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Michael Stapelberg , open list , "open list:ETHERNET PHY LIBRARY" References: <20261014194818.11640-1-rosenp@gmail.com> Content-Language: fr-FR From: "Christophe Leroy (CS GROUP)" In-Reply-To: <20261014194818.11640-1-rosenp@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Le 14/10/2026 à 21:48, Rosen Penev a écrit : > __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 MAX_COPY_PREFETCH cache lines ahead in the main loop, > the same distance csum_partial_copy_generic() already uses. That is > 128 bytes on the 44x. 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 Reviewed-by: Christophe Leroy (CS GROUP) > --- > v3: use MAX_COPY_PREFETCH > 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..5bb872e9e68f 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,MAX_COPY_PREFETCH*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