netdev - Re: TCP performance problems - GSO/TSO, MSS, 8139cp related

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Date:   Fri, 11 Nov 2016 21:23:43 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>,
        netdev@...r.kernel.org
Cc:     qemu-devel@...gnu.org
Subject: Re: TCP performance problems - GSO/TSO, MSS, 8139cp related

On Fri, 2016-11-11 at 21:05 +0000, Russell King - ARM Linux wrote:
> 
> 18:59:38.782818 IP (tos 0x0, ttl 52, id 35619, offset 0, flags [DF], proto TCP (6), length 60)
>     84.xx.xxx.196.61236 > 195.92.253.2.http: Flags [S], cksum 0x88db (correct), seq 158975430, win 29200, options [mss 1452,sackOK,TS val 1377914597 ecr 0,nop,wscale 7], length 0

... (MSS 1452)

> 18:59:38.816371 IP (tos 0x0, ttl 64, id 25879, offset 0, flags [DF], proto TCP (6), length 1500)
>     195.92.253.2.http > 84.xx.xxx.196.61236: Flags [.], seq 1:1449, ack 154, win 235, options [nop,nop,TS val 3363137952 ecr 1377914627], length 1448: HTTP, length: 1448
> 18:59:38.816393 IP (tos 0x0, ttl 64, id 25880, offset 0, flags [DF], proto TCP (6), length 1484)
>     195.92.253.2.http > 84.xx.xxx.196.61236: Flags [.], seq 1449:2881, ack 154, win 235, options [nop,nop,TS val 3363137952 ecr 1377914627], length 1432: HTTP

Can you instrument cp_start_xmit() in 8139cp.c and get it to print the
value of 'mss' when this happens?

All we do is take that value from skb_shinfo(skb)->gso_size, shift it a
bit, and shove it in the descriptor ring. There's not much scope for a
driver-specific bug.

It's also *fairly* unlikely that the kernel in the guest has developed
a bug and isn't setting gso_size sanely. I'm more inclined to suspect
that qemu isn't properly emulating those bits. But at first glance at
the code, it looks like *that's* been there for the last decade too...

-- 
dwmw2
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