netdev - Re: resend: tcp: performance issue with fastopen connections (mss

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Date:   Wed, 18 Jan 2017 13:33:11 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Yuchung Cheng <ycheng@...gle.com>,
        Alexey Kodanev <alexey.kodanev@...cle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Vasily Isaenko <vasily.isaenko@...cle.com>
Subject: Re: resend: tcp: performance issue with fastopen connections (mss > window)

On Wed, Jan 18, 2017 at 1:16 PM, Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Jan 18, 2017 at 10:13 AM, Yuchung Cheng <ycheng@...gle.com> wrote:
> > On Wed, Jan 18, 2017 at 9:35 AM, Eric Dumazet <edumazet@...gle.com> wrote:
> >>
> >> On Wed, Jan 18, 2017 at 9:32 AM, Alexey Kodanev
> >> <alexey.kodanev@...cle.com> wrote:
> >> > Hi Eric,
> >> >
> >> > On 01/13/2017 08:07 PM, Alexey Kodanev wrote:
> >> >
> >>
> >> > Looks like max_window not correctly initialized for tfo sockets.
> >> > On my test machine it has set to '5592320' in tcp_fastopen_create_child().
> >> >
> >> > This diff fixes the issue, the question: is this the right place to do it?
> >> >
> >> > diff --git a/net/ipv4/tcp_fastopen.c b/net/ipv4/tcp_fastopen.c
> >> > index 4e777a3..33ed508 100644
> >> > --- a/net/ipv4/tcp_fastopen.c
> >> > +++ b/net/ipv4/tcp_fastopen.c
> >> > @@ -206,6 +206,8 @@ static struct sock *tcp_fastopen_create_child(struct
> >> > sock *sk,
> >> >          */
> >> >         tp->snd_wnd = ntohs(tcp_hdr(skb)->window);
> >> >
> >> > +       tp->max_window = tp->snd_wnd;
> >> > +
> >>
> >> Excellent catch. Let me test our regression tests with this.
> > Indeed nice catch. Thanks for the investigative work!
> >
>
> We do have 2 failures, but tests might have depended on undocumented behavior
>
> (For googlers :
> Ran 211 tests: 209 passing, 0 flaky 2 failing
> Sponge: http://sponge/f1575065-6e1c-4514-bced-9167ce56d2ee
> )

Yes, exactly, those test failures look fine. Looks like the test was
implicitly expecting the old buggy behavior (and it wasn't noticed
because the 10*MSS output matches IW10, so it looked fine. But now
with the fix, I believe we are seeing the correct new behavior because
tcp_xmit_size_goal() is calling tcp_bound_to_half_wnd(), and now with
max_window fixed, the outgoing TSO skb is now the correct 5*MSS (half
of the rwin of 10000). So the test results look good to me.

Thanks, Alexey, for tracking this down! :-)

neal