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Date:   Mon, 21 Dec 2020 11:58:14 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Rainer Suhm <automat@...teo.de>
Cc:     "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>
Subject: Re: PROBLEM: Poor wlan performance with lots of retries since kernel 5.9

On Mon, Dec 21, 2020 at 12:19 AM Rainer Suhm <automat@...teo.de> wrote:
>
> Since kernel 5.9 I do have very poor wlan performance with one of my machines.
> The transmission rate is only about a tenth of normal speed (~60MB/s -> 6 MB/s).
> iperf3 -c <server> shows hundreds of retries per second
> --- snip ---
> [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
> [  5]   0.00-1.00   sec  8.55 MBytes  71.7 Mbits/sec  649   1.41 KBytes
> [  5]   1.00-2.00   sec  5.78 MBytes  48.5 Mbits/sec  366   17.0 KBytes
> [  5]   2.00-3.00   sec  5.97 MBytes  50.0 Mbits/sec  371   19.8 KBytes
> [  5]   3.00-4.00   sec  5.84 MBytes  49.0 Mbits/sec  390   17.0 KBytes
> [  5]   4.00-5.00   sec  6.21 MBytes  52.1 Mbits/sec  380   19.8 KBytes
> [  5]   5.00-6.00   sec  4.54 MBytes  38.1 Mbits/sec  299   1.41 KBytes
> [  5]   6.00-7.00   sec  6.34 MBytes  53.2 Mbits/sec  367   26.9 KBytes
> [  5]   7.00-8.00   sec  6.15 MBytes  51.6 Mbits/sec  390   19.8 KBytes
> [  5]   8.00-9.00   sec  5.65 MBytes  47.4 Mbits/sec  370   2.83 KBytes
> [  5]   9.00-10.00  sec  5.78 MBytes  48.5 Mbits/sec  377   21.2 KBytes
> - - - - - - - - - - - - - - - - - - - - - - - - -
> [ ID] Interval           Transfer     Bitrate         Retr
> [  5]   0.00-10.00  sec  60.8 MBytes  51.0 Mbits/sec  3959             sender
> [  5]   0.00-10.00  sec  60.1 MBytes  50.4 Mbits/sec                  receiver
> --- snap ---
> (this measurement was taken after booting from the current official arch iso)
>
> The problem is only when transmitting data. Receiving (iperf3 -c <server> -R) works ok.
> No related errors/warnings in dmesg.
> The problem exists since kernel v5.9 up to 10.0.2-rc1
> Using other (older) WiFi firmware doesn't help.
> Hardware: TUXEDO InfinityBook S 14 v5/L140CU,
> with Intel(R) Wi-Fi 6 AX200 160MHz, REV=0x340
> The machine runs Arch Linux, all updates installed.
> On all my other laptops this problem doesn't show up.
>
>
> Bisecting leads to:
> --- snip ---
> 3d5b459ba0e3788ab471e8cb98eee89964a9c5e8 is the first bad commit
> commit 3d5b459ba0e3788ab471e8cb98eee89964a9c5e8
> Author: Eric Dumazet <edumazet@...gle.com>
> Date:   Wed Jun 17 20:53:26 2020 -0700
>
>       net: tso: add UDP segmentation support
>
>       Note that like TCP, we do not support additional encapsulations,
>       and that checksums must be offloaded to the NIC.
>
>       Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>       Signed-off-by: David S. Miller <davem@...emloft.net>
>
>    net/core/tso.c | 29 ++++++++++++++++++-----------
>    1 file changed, 18 insertions(+), 11 deletions(-)
> --- snap ---
>
> After reverting e89964a9c5e8, everything seems ok. WLan works with expected speed, and iperf3 doesn't show retries any more.
> I confirm this for kernels 5.10.1 and 5.10.2-rc1 (00017-gc96cfd687a3f).
>
> If I should provide more information, let me know.
> Please note: I'm not on any list. So please make sure to address me explicitly if replying to this mail.
>
>
Thanks for the report.

A similar report has been sent a few days ago by Ben Greear, let's
keep the discussion there please.

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