Re: [patch net-next] virtio_net: add support for Byte Queue Limits

Jiri Pirko
2024-06-06T13:41:18Z

Thu, Jun 06, 2024 at 06:25:15AM CEST, jasowang@redhat.com wrote:
>On Thu, Jun 6, 2024 at 10:59 AM Jason Xing <kerneljasonxing@gmail.com> wrote:
>>
>> Hello Jason,
>>
>> On Thu, Jun 6, 2024 at 8:21 AM Jason Wang <jasowang@redhat.com> wrote:
>> >
>> > On Wed, Jun 5, 2024 at 7:51 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
>> > >
>> > > On Wed, 5 Jun 2024 13:30:51 +0200, Jiri Pirko <jiri@resnulli.us> wrote:
>> > > > Mon, May 20, 2024 at 02:48:15PM CEST, jiri@resnulli.us wrote:
>> > > > >Fri, May 10, 2024 at 09:11:16AM CEST, hengqi@linux.alibaba.com wrote:
>> > > > >>On Thu, 9 May 2024 13:46:15 +0200, Jiri Pirko <jiri@resnulli.us> wrote:
>> > > > >>> From: Jiri Pirko <jiri@nvidia.com>
>> > > > >>>
>> > > > >>> Add support for Byte Queue Limits (BQL).
>> > > > >>
>> > > > >>Historically both Jason and Michael have attempted to support BQL
>> > > > >>for virtio-net, for example:
>> > > > >>
>> > > > >>https://lore.kernel.org/netdev/21384cb5-99a6-7431-1039-b356521e1bc3@redhat.com/
>> > > > >>
>> > > > >>These discussions focus primarily on:
>> > > > >>
>> > > > >>1. BQL is based on napi tx. Therefore, the transfer of statistical information
>> > > > >>needs to rely on the judgment of use_napi. When the napi mode is switched to
>> > > > >>orphan, some statistical information will be lost, resulting in temporary
>> > > > >>inaccuracy in BQL.
>> > > > >>
>> > > > >>2. If tx dim is supported, orphan mode may be removed and tx irq will be more
>> > > > >>reasonable. This provides good support for BQL.
>> > > > >
>> > > > >But when the device does not support dim, the orphan mode is still
>> > > > >needed, isn't it?
>> > > >
>> > > > Heng, is my assuption correct here? Thanks!
>> > > >
>> > >
>> > > Maybe, according to our cloud data, napi_tx=on works better than orphan mode in
>> > > most scenarios. Although orphan mode performs better in specific benckmark,
>> >
>> > For example pktgen (I meant even if the orphan mode can break pktgen,
>> > it can finish when there's a new packet that needs to be sent after
>> > pktgen is completed).
>> >
>> > > perf of napi_tx can be enhanced through tx dim. Then, there is no reason not to
>> > > support dim for devices that want the best performance.
>> >
>> > Ideally, if we can drop orphan mode, everything would be simplified.
>>
>> Please please don't do this. Orphan mode still has its merits. In some
>> cases which can hardly be reproduced in production, we still choose to
>> turn off the napi_tx mode because the delay of freeing a skb could
>> cause lower performance in the tx path,
>
>Well, it's probably just a side effect and it depends on how to define
>performance here.
>
>> which is, I know, surely
>> designed on purpose.
>
>I don't think so and no modern NIC uses that. It breaks a lot of things.
>
>>
>> If the codes of orphan mode don't have an impact when you enable
>> napi_tx mode, please keep it if you can.
>
>For example, it complicates BQL implementation.

Well, bql could be disabled when napi is not used. It is just a matter
of one "if" in the xmit path.

>
>Thanks
>
>>
>> Thank you.
>>
>