Re: [PATCH v3] arm64: lib: accelerate do_csum with NEON instruction

From: Will Deacon <will.deacon@arm.com>
To: "huanglingyan (A)" <huanglingyan2@huawei.com>
Cc: Zhangshaokun <zhangshaokun@hisilicon.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-arm-kernel@lists.infradead.org, ard.biesheuvel@linaro.org
Subject: Re: [PATCH v3] arm64: lib: accelerate do_csum with NEON instruction
Date: Wed, 16 Jan 2019 16:46:58 +0000	[thread overview]
Message-ID: <20190116164657.GA1910@brain-police> (raw)
In-Reply-To: <cd5bb83e-bb0e-b348-5365-095c5fcd9648@huawei.com>

On Wed, Jan 09, 2019 at 10:03:05AM +0800, huanglingyan (A) wrote:
> 
> On 2019/1/8 21:54, Will Deacon wrote:
> > [re-adding Ard and LAKML -- not sure why the headers are so munged]
> >
> > On Mon, Jan 07, 2019 at 10:38:55AM +0800, huanglingyan (A) wrote:
> >> On 2019/1/6 16:26, Ard Biesheuvel wrote:
> >>     Please change this into
> >>
> >>     if (IS_ENABLED(CONFIG_KERNEL_MODE_NEON) &&
> >>         len >= CSUM_NEON_THRESHOLD &&
> >>         may_use_simd()) {
> >>             kernel_neon_begin();
> >>             res = do_csum_neon(buff, len);
> >>             kernel_neon_end();
> >>         }
> >>
> >>     and drop the intermediate do_csum_arm()
> >>
> >>
> >>         +               return do_csum_arm(buff, len);
> >>         +#endif  /* CONFIG_KERNEL_MODE_NEON */
> >>
> >>     No else? What happens if len < CSUM_NEON_THRESHOLD ?
> >>
> >>
> >>         +#undef do_csum
> >>
> >>     Can we drop this?
> >>
> >> Using NEON instructions will bring some costs. The spending maybe introduced
> >> when reservering/restoring
> >> neon registers with kernel_neon_begin()/kernel_neon_end(). Therefore NEON code
> >> is Only used when
> >> the length exceeds CSUM_NEON_THRESHOLD. General do csum() codes in lib/
> >> checksum.c will be used in
> >> shorter length. To achieve this goal, I use the "#undef do_csum" in else clause
> >> to have the oppotunity to
> >> utilize the general codes.
> > I don't think that's how it works :/
> >
> > Before we get deeper into the implementation, please could you justify the
> > need for a CPU-optimised checksum implementation at all? I thought this was
> > usually offloaded to the NIC?
> >
> > Will
> >
> > .
> This problem is introduced when testing Intel x710 network card on my ARM server.
> Ip forward is set for ease of testing. Then send lots of packages to server by Tesgine
> machine and then receive.

In the marketing blurb, that card boasts:

  `Tx/Rx IP, SCTP, TCP, and UDP checksum offloading (IPv4, IPv6) capabilities'

so we shouldn't need to run this on the CPU. Again, I'm not keen to optimise
this given that it /really/ shouldn't be used on arm64 machines that care
about network performance.

Will

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel