Re: PROBLEM: Poor wlan performance with lots of retries since kernel 5.9

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 [off-list ref]
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 [off-list ref]
      Signed-off-by: David S. Miller [off-list ref]

   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.


Thx
Rainer