netdev - Re: [PATCH net-next v3 3/7] net: ethtool: add support for configuring tcp-data-split-thresh

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Message-ID: <CAMArcTUDDw5+AmydzmERpxWofWKPhF5mw2Ch9T0g3a6E9LgYUg@mail.gmail.com>
Date: Sat, 5 Oct 2024 15:11:24 +0900
From: Taehee Yoo <ap420073@...il.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
	edumazet@...gle.com, netdev@...r.kernel.org, linux-doc@...r.kernel.org, 
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	aleksander.lobakin@...el.com, dw@...idwei.uk, sridhar.samudrala@...el.com, 
	bcreeley@....com
Subject: Re: [PATCH net-next v3 3/7] net: ethtool: add support for configuring tcp-data-split-thresh

On Fri, Oct 4, 2024 at 10:47 AM Mina Almasry <almasrymina@...gle.com> wrote:
>
> On Thu, Oct 3, 2024 at 12:33 PM Taehee Yoo <ap420073@...il.com> wrote:
> >
> > On Fri, Oct 4, 2024 at 3:25 AM Mina Almasry <almasrymina@...gle.com> wrote:
> > >
> > > On Thu, Oct 3, 2024 at 9:07 AM Taehee Yoo <ap420073@...il.com> wrote:
> > > >
> > > > The tcp-data-split-thresh option configures the threshold value of
> > > > the tcp-data-split.
> > > > If a received packet size is larger than this threshold value, a packet
> > > > will be split into header and payload.
> > >
> > > Why do you need this? devmem TCP will always not work with unsplit
> > > packets. Seems like you always want to set thresh to 0 to support
> > > something like devmem TCP.
> > >
> > > Why would the user ever want to configure this? I can't think of a
> > > scenario where the user wouldn't want packets under X bytes to be
> > > unsplit.
> >
> > I totally understand what you mean,
> > Yes, tcp-data-split is zerocopy friendly option but as far as I know,
> > this option is not only for the zerocopy usecase.
> > So, If users enable tcp-data-split, they would assume threshold is 0.
> > But there are already NICs that have been supporting tcp-data-split
> > enabled by default.
> > bnxt_en's default value is 256bytes.
> > If we just assume the tcp-data-split-threshold to 0 for all cases,
> > it would change the default behavior of bnxt_en driver(maybe other drivers too)
> > for the not zerocopy case.
> > Jakub pointed out the generic case, not only for zerocopy usecase
> > in the v1 and I agree with that opinion.
> > https://lore.kernel.org/netdev/20240906183844.2e8226f3@kernel.org/
>
> I see, thanks. The ability to tune the threshold to save some pcie
> bandwidth is interesting. Not sure how much it would matter in
> practice. I guess if you're receiving _lots_ of small packets then it
> could be critical.
>
> Sounds good then, please consider adding Jakub's reasoning for why
> tuning this could be valuable to the commit message for future
> userspace readers that wonder why to set this.

Okay, I will add an explanation of this feature to commit message in v4 patch.

Thanks a lot!
Taehee Yoo

>
> --
> Thanks,
> Mina