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

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Date: Fri, 7 Jun 2024 14:22:31 +0800
From: Jason Wang <jasowang@...hat.com>
To: Jiri Pirko <jiri@...nulli.us>
Cc: Jason Xing <kerneljasonxing@...il.com>, Heng Qi <hengqi@...ux.alibaba.com>, 
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, 
	mst@...hat.com, xuanzhuo@...ux.alibaba.com, virtualization@...ts.linux.dev, 
	ast@...nel.org, daniel@...earbox.net, hawk@...nel.org, 
	john.fastabend@...il.com, netdev@...r.kernel.org
Subject: Re: [patch net-next] virtio_net: add support for Byte Queue Limits

On Thu, Jun 6, 2024 at 9:41 PM Jiri Pirko <jiri@...nulli.us> wrote:
>
> Thu, Jun 06, 2024 at 06:25:15AM CEST, jasowang@...hat.com wrote:
> >On Thu, Jun 6, 2024 at 10:59 AM Jason Xing <kerneljasonxing@...il.com> wrote:
> >>
> >> Hello Jason,
> >>
> >> On Thu, Jun 6, 2024 at 8:21 AM Jason Wang <jasowang@...hat.com> wrote:
> >> >
> >> > On Wed, Jun 5, 2024 at 7:51 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
> >> > >
> >> > > On Wed, 5 Jun 2024 13:30:51 +0200, Jiri Pirko <jiri@...nulli.us> wrote:
> >> > > > Mon, May 20, 2024 at 02:48:15PM CEST, jiri@...nulli.us wrote:
> >> > > > >Fri, May 10, 2024 at 09:11:16AM CEST, hengqi@...ux.alibaba.com wrote:
> >> > > > >>On Thu,  9 May 2024 13:46:15 +0200, Jiri Pirko <jiri@...nulli.us> wrote:
> >> > > > >>> From: Jiri Pirko <jiri@...dia.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.

Maybe, care to post a patch?

The trick part is, a skb is queued when BQL is enabled but sent when
BQL is disabled as discussed here:

https://lore.kernel.org/netdev/21384cb5-99a6-7431-1039-b356521e1bc3@redhat.com/

Thanks

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