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From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: "huanglingyan (A)" <huanglingyan2@huawei.com>
Cc: Zhangshaokun <zhangshaokun@hisilicon.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v3] arm64: lib: accelerate do_csum with NEON instruction
Date: Tue, 12 Feb 2019 08:07:35 +0100
Message-ID: <CAKv+Gu_jzU934k=SU-0bpF7NrqZ-KW2u_G3a+WDkxs+O6bzQow@mail.gmail.com> (raw)
In-Reply-To: <d97f1ba1-1b73-1bde-cd8f-de55115acd9e@huawei.com>

On Tue, 12 Feb 2019 at 03:25, huanglingyan (A) <huanglingyan2@huawei.com> wrote:
>
>
> On 2019/1/18 19:14, Ard Biesheuvel wrote:
> > On Fri, 18 Jan 2019 at 02:07, huanglingyan (A) <huanglingyan2@huawei.com> wrote:
> >>
> >> On 2019/1/17 0:46, Will Deacon wrote:
> >>> 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
> >>>
> >>> .
> >> Yeah, you are right. Checksum is usually done in network card which is told by
> >> someone familiar with NIC. However, it may be used in testing scenaries and
> >> some primary network cards. I think it's no harm to optimize this code while
> >> other ARCHs have their own optimized versions.
> > I disagree. If this code path is never exercised, we should not
> > include it. We can revisit this decision when there is a use case
> > where the checksumming performance is an actual bottleneck.
> >
> > .
> The mainstream network cards has an option to switch the csum pattern.
> Users can determine the one who calculate csum, hardware or software.
>
>         ethtool -K eth0 rx-checksum off
>         ethtool -K eth0 tx-checksum-ip-generic off
>
> What's more, there's some network features that may cause hardware
> checksum not work, like gso ( not so sure). Which means, the software
> checksum has its existing meaning.
>

This does not make any sense to me. Segmentation offload relies on the
hardware generating the actual packets, and I don't see how it would
be able to do that if it cannot generate the checksum as well.

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Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-06  1:55 Lingyan Huang
2019-01-06  8:26 ` Ard Biesheuvel
     [not found]   ` <9129b882-60f3-8046-0cb9-e0b2452a118d@huawei.com>
2019-01-08 13:54     ` Will Deacon
2019-01-09  2:03       ` huanglingyan (A)
2019-01-10  4:08         ` 胡海
2019-01-10  8:14           ` huanglingyan (A)
2019-01-16 16:46         ` Will Deacon
2019-01-18  1:07           ` huanglingyan (A)
2019-01-18 11:14             ` Ard Biesheuvel
2019-02-12  2:26               ` huanglingyan (A)
2019-02-12  7:07                 ` Ard Biesheuvel [this message]
2019-02-13  8:42                   ` huanglingyan (A)
2019-02-13  9:15                     ` Ard Biesheuvel
2019-02-13 17:55                       ` Ard Biesheuvel
2019-02-14  9:57                         ` huanglingyan (A)
2019-02-18  8:49                           ` huanglingyan (A)
2019-02-18  9:03                             ` Ard Biesheuvel
2019-01-09 14:58 ` Dave Martin
2019-01-10  8:03   ` huanglingyan (A)
2019-01-10 13:53     ` Dave Martin
     [not found] <1f065749-6676-6489-14ae-fdcfeeb3389c@huawei.com>
2019-01-07  6:11 ` huanglingyan (A)

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