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

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Date:   Fri, 13 Jan 2017 07:35:34 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexey Kodanev <alexey.kodanev@...cle.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Vasily Isaenko <vasily.isaenko@...cle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: resend: tcp: performance issue with fastopen connections (mss >
 window)

On Fri, 2017-01-13 at 18:01 +0300, Alexey Kodanev wrote:
> Hi,
> 
> Got the issue when running LTP/netstress test on localhost with mss
> greater than the send window advertised by client (right after 3WHS).
> Here is the testscenario that can reproduce this:

Hi Alexey

So this is a combination of Fastopen + small window + large MSS ?

I would rather not force burning tons of coal or other fossil fuel,
by making each tcp_sendmsg() done by billions of linux devices more
expensive, only to accommodate for some LTP test doing something not
sensible ;)

Fact that you removed one condition in the BUG_ON() might hide another
issue later in the path.

I would suggest to clamp MSS to half the initial window, but I guess
this is impractical since window in SYN/SYNACK are not scaled.

Care to send a packetdrill test so that we have a clear picture of what
is going on ?

Thanks.