netdev - Re: [PATCH net-next] net: stmmac: propagate feature flags to vlan

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Message-ID: <20230417121146.654b980d@kernel.org>
Date:   Mon, 17 Apr 2023 12:11:46 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Corinna Vinschen <vinschen@...hat.com>
Cc:     netdev@...r.kernel.org,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        alexandre.torgue@...s.st.com, Jose Abreu <joabreu@...opsys.com>
Subject: Re: [PATCH net-next] net: stmmac: propagate feature flags to vlan

On Mon, 17 Apr 2023 21:03:12 +0200 Corinna Vinschen wrote:
> Yes, that was a good idea.  Turns out, TSO doesn't really work well
> with VLANs.  The speed is... suboptimal.  Here are some results
> with iperf, showing only the summary lines.

Hah, good to find out before users did :)

IIRC there is something in the TCP stack which retries sending 
as non-TSO if TSO keeps failing so "TSO is very slow" may in fact 
mean TSO is completely broken but TCP is managing to deliver a little
bit with just retransmissions or super tiny cwnd or some such.