netdev - Re: [PATCH net-next 1/2] mptcp: don't need to check SKB_EXT_MPTCP in mptcp_reset

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Date: Fri, 5 Apr 2024 17:11:05 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	matttbe@...nel.org, martineau@...nel.org, geliang@...nel.org, 
	mptcp@...ts.linux.dev, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next 1/2] mptcp: don't need to check SKB_EXT_MPTCP in mptcp_reset_option()

Hello Paolo,

On Fri, Apr 5, 2024 at 4:34 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Fri, 2024-04-05 at 15:58 +0800, Jason Xing wrote:
> > Hello Paolo,
> >
> > On Fri, Apr 5, 2024 at 3:47 PM Paolo Abeni <pabeni@...hat.com> wrote:
> > >
> > > On Fri, 2024-04-05 at 10:39 +0800, Jason Xing wrote:
> > > > From: Jason Xing <kernelxing@...cent.com>
> > > >
> > > > Before this, what mptcp_reset_option() checks is totally the same as
> > > > mptcp_get_ext() does, so we could skip it.
> > >
> > > Note that the somewhat duplicate test is (a possibly not great)
> > > optimization to avoid jumping in the mptcp code (possible icache
> > > misses) for plain TCP sockets.
> > >
> > > I guess we want to maintain it.
> >
> > Okay, I just read code and found the duplication but may I ask why it
> > has something to do with icache misses?
>
> The first check/mptcp_get_ext() is in mptcp_reset_option() /
> tcp_v4_send_reset(). For plain TCP socket it will fail and the
> execution will continue inside the same compilation unit. The code
> locality should avoid icaches misses around there.
>
> Removing such check, even when processing plain TCP packets, the code
> execution will have to call into mptcp_get_reset_option() in the mptcp
> code, decreasing the code locality and increasing the chance of icache
> misses.

Interesting. Thanks for the explanation:)

>
> I don't have actual profile data, so this is an early optimization (and
> thus root of all evil), but sounds reasonable to me (yep, I'm biased!)

I'll drop this patch.

Thanks,
Jason

>
> Cheers,
>
> Paolo
>