Bug report: UDP ~20% degradation

[Tariq Toukan]

Hi all,

Our performance verification team spotted a degradation of up to ~20% in 
UDP performance, for a specific combination of parameters.

Our matrix covers several parameters values, like:
IP version: 4/6
MTU: 1500/9000
Msg size: 64/1452/8952 (only when applicable while avoiding ip 
fragmentation).
Num of streams: 1/8/16/24.
Num of directions: unidir/bidir.

Surprisingly, the issue exists only with this specific combination:
8 streams,
MTU 9000,
Msg size 8952,
both ipv4/6,
bidir.
(in unidir it repros only with ipv4)

The reproduction is consistent on all the different setups we tested with.

Bisect [2] was done between these two points, v5.19 (Good), and v6.0-rc1 
(Bad), with ConnectX-6DX NIC.

c82a69629c53eda5233f13fc11c3c01585ef48a2 is the first bad commit [1].

We couldn't come up with a good explanation how this patch causes this 
issue. We also looked for related changes in the networking/UDP stack, 
but nothing looked suspicious.

Maybe someone here can help with this.
We can provide more details or do further tests/experiments to progress 
with the debug.

Thanks,
Tariq

[1]
commit c82a69629c53eda5233f13fc11c3c01585ef48a2
Author: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Date:   Fri Jul 8 17:44:01 2022 +0200

    sched/fair: fix case with reduced capacity CPU

    The capacity of the CPU available for CFS tasks can be reduced 
because of
    other activities running on the latter. In such case, it's worth 
trying to
    move CFS tasks on a CPU with more available capacity.




    The rework of the load balance has filtered the case when the CPU 
is 


    classified to be fully busy but its capacity is reduced. 







    Check if CPU's capacity is reduced while gathering load balance 
statistic 


    and classify it group_misfit_task instead of group_fully_busy so we 
can 


    try to move the load on another CPU. 







    Reported-by: David Chen <david.chen@xxxxxxxxxxx> 



    Reported-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx> 



    Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx> 



    Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> 



    Tested-by: David Chen <david.chen@xxxxxxxxxxx> 



    Tested-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx> 



    Link: 
https://lkml.kernel.org/r/20220708154401.21411-1-vincent.guittot@xxxxxxxxxx 




[2]

Detailed bisec steps:

+--------------+--------+-----------+-----------+
| Commit       | Status | BW (Gbps) | BW (Gbps) |
|              |        | run1      | run2      |
+--------------+--------+-----------+-----------+
| 526942b8134c | Bad    | ---       | ---       |
+--------------+--------+-----------+-----------+
| 2e7a95156d64 | Bad    | ---       | ---       |
+--------------+--------+-----------+-----------+
| 26c350fe7ae0 | Good   | 279.8     | 281.9     |
+--------------+--------+-----------+-----------+
| 9de1f9c8ca51 | Bad    | 257.243   | ---       |
+--------------+--------+-----------+-----------+
| 892f7237b3ff | Good   | 285       | 300.7     |
+--------------+--------+-----------+-----------+
| 0dd1cabe8a4a | Good   | 305.599   | 290.3     |
+--------------+--------+-----------+-----------+
| dfea84827f7e | Bad    | 250.2     | 258.899   |
+--------------+--------+-----------+-----------+
| 22a39c3d8693 | Bad    | 236.8     | 245.399   |
+--------------+--------+-----------+-----------+
| e2f3e35f1f5a | Good   | 277.599   | 287       |
+--------------+--------+-----------+-----------+
| 401e4963bf45 | Bad    | 250.149   | 248.899   |
+--------------+--------+-----------+-----------+
| 3e8c6c9aac42 | Good   | 299.09    | 294.9     |
+--------------+--------+-----------+-----------+
| 1fcf54deb767 | Good   | 292.719   | 301.299   |
+--------------+--------+-----------+-----------+
| c82a69629c53 | Bad    | 254.7     | 246.1     |
+--------------+--------+-----------+-----------+
| c02d5546ea34 | Good   | 276.4     | 294       |
+--------------+--------+-----------+-----------+