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

["huanglingyan (A)" <huanglingyan2@huawei.com>]

On 2019/2/12 15:07, Ard Biesheuvel wrote:
> 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.
I test on my platform of  IP-forward scenery.  The network card has checksum capability.
The hardware do checksum when gro feature is off. However, checksum is done by
software when gro is on. In this sceney, do_csum function has 60% percentage of CPU load
and the performance decreases 20% due to software checksum.

The command I use is
        ethtool -K eth0 gro off

> .
>


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