From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6D8534CEE51 for ; Mon, 12 Oct 2026 18:18:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791829132; cv=none; b=mTxRDWqP3U1CnGJXbCwpqLpJ9DWgpxr+tWuagIr6siaNmrnjkPR7HowtIm9xi3xH0q1DajkNJ1+4mYbSsBpEEOBL6T4ScKifOuUH5IHMmorDWJkYJjjrcdsLxIlg8S1p/BsBbNLluaA/vgVO1V+avhlBY0BXciZaSKO3EHGwlqw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791829132; c=relaxed/simple; bh=/5HdJP8iSJ48fNPsFLAJRZRS0HJynD08Wpk1N2ucp5M=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MMjUS6Y/uAM5U6hwJdE5qzhivDDs0oC92J6r7LWhroKo4XucCzfd25ebjfDqG9yPEa1nOH5RtF+3g4LPNe+6kGU6P28Ag9unyBcKuSwd832REbAaLhqsYNzJEphrMjQnV5l2AGceMWMH45AF+nteKb/Mey2MYjObc1GF8DGfHYM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hyzSJLFL; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="hyzSJLFL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CFBBD1F008A4; Mon, 12 Oct 2026 18:18:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791829131; bh=/d2FRlSLGGY8hw4wwZf+Q/0xEdR55VdWMYO+zuYbCyw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hyzSJLFL8ujbkodI/hzNuU34xM7m1LJomhXXELLD+hdG26/Yh1Vtu6/4vqn2uGzh1 MfnoKs7j0sMmeI3ZC6reR0ovcpkkG04eqI7oRMQs17afaqgTUcJDlZ2n390A/WEh1o lzQknNYgpmHCrf0PqHPLIcxCDCxm9odJTM4syQjxedDb7+DdWxbQH2coYFhklo7nMq Ifl5Gk8SaoFViWmsZZKFAZCA1TWZgxpXKASpxG6BKvK0Qgc4Oa2zyAGoyzcEhluzct EfvoC32duebL+MOi4MG6VRm32RrcUziwlm/kVTr13eG5gJESiUDE2vq3aCFAnvJ7t2 ZhGpSwc4K02nA== Message-ID: Date: Mon, 12 Oct 2026 20:18:45 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] powerpc/lib: prefetch ahead in __csum_partial() To: Rosen Penev , linuxppc-dev@lists.ozlabs.org Cc: Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , "Ritesh Harjani (IBM)" , Shrikanth Hegde , open list References: <20261012180052.55578-1-rosenp@gmail.com> Content-Language: fr-FR From: "Christophe Leroy (CS GROUP)" In-Reply-To: <20261012180052.55578-1-rosenp@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Le 12/10/2026 à 20:00, 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 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 csum_partial_copy_generic() does the same, so your patch makes sense. Wondering if you should use MAX_COPY_PREFETCH instead of hardcoding value 4. Reviewed-by: Christophe Leroy (CS GROUP) > --- > 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