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

[Christophe Leroy (CS GROUP)]

Hi Rosen,

Le 08/10/2026 à 05:49, Rosen Penev a écrit :
> __csum_partial() reads every byte of the buffer once and never touches
> it again, so on cores without a hardware prefetcher every cache line is
> a demand miss. After a non-coherent DMA the received data is never in
> the cache, which makes the GRO checksum validation of forwarded TCP
> traffic the top entry in the profile on a 464FP (APM82181): 22% of all
> cycles were spent in __csum_partial() when routing with GRO enabled.

Very nice patch. What is the new % after the change ?

How do you test that ? I'd like to do some performance test on 8xx and 83xx.


When you say "never touches it again", do you mean the data is not used 
again after that ? In that case would it help to do a 'dcbi' after using 
the data in order to make the cache line available again and avoid 
possible writeback to free a new cache line ?

> Issue a dcbt four cache lines ahead in the main loop. dcbt is a hint and
> never faults, so prefetching past the end of the buffer is harmless.
>
> On a Meraki MX60 (APM82181 at 800 MHz, single TCP stream routed through
> a qca8k DSA switch, iperf3 median of 3, A/B in the same boot):
>
>    prefetch distance   LAN->WAN   WAN->LAN  (Mbit/s)
>    none                370        341
>    64 bytes            430        381
>    96 bytes            438        397
>    128 bytes           438        385
>    160 bytes           434        386
>    256 bytes           423        383

3x CACHE_SIZE distance seems the most efficient, why did you choose to 
implement 4x CACHE_SIZE ?

> Prefetching the first lines before the loop made no measurable
> difference and is not done.
>
> Assisted-by: LLM

In what way did LLM help ?

> Signed-off-by: Rosen Penev <rosenp@xxxxxxxxx>
> ---
>   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