Try enabling "Explicit Tx rule" mode if possible.
I was able to achieve 137 Mpps @ 64B with the following command:

dpdk-testpmd -a 21:00.0 -a c1:00.0 --in-memory -- \
    -i --rxq=1 --txq=1 --hairpinq=8 --hairpin-mode=0x10
Based o this I was able to achieve 142 Mpps(96.08 Gbps) @ 64B with the following command:

sudo dpdk-testpmd -l 0-1 -n 4 -a 0000:c4:00.0,hp_buf_log_sz=13 \
    --in-memory -- --rxq=1 --txq=1 --hairpinq=12 --hairpin-mode=0x10 -i
    
flow create 0 ingress pattern eth src is 00:10:94:00:00:02 / end actions rss queues 1 2 3 4 5 6 7 8 9 10 11 12 end / end

Almost full speed :).
Any other value of "hp_buf_log_sz" or more queues does not get better results, but instead makes them worse.

RxQ pinned in device memory requires firmware configuration [1]:

mlxconfig -y -d $pci_addr set MEMIC_SIZE_LIMIT=0 HAIRPIN_DATA_BUFFER_LOCK=1
mlxfwreset -y -d $pci_addr reset

[1]: https://doc.dpdk.org/guides/platform/mlx5.html?highlight=hairpin_data_buffer_lock

However, pinned RxQ didn't improve anything for me.
I tried it, but it didn't improve anything for me either.

Mário

Hi Mário,

2024-06-19 08:45 (UTC+0200), Mário Kuka:

Hello,

I want to use hairpin queues to forward high priority traffic (such as 
LACP).
My goal is to ensure that this traffic is not dropped in case the 
software pipeline is overwhelmed.
But during testing with dpdk-testpmd I can't achieve full throughput for 
hairpin queues.

For maintainers: I'd like to express interest in this use case too.

The best result I have been able to achieve for 64B packets is 83 Gbps 
in this configuration:
$ sudo dpdk-testpmd -l 0-1 -n 4 -a 0000:17:00.0,hp_buf_log_sz=19 -- 
--rxq=1 --txq=1 --rxd=4096 --txd=4096 --hairpinq=2
testpmd> flow create 0 ingress pattern eth src is 00:10:94:00:00:03 /   
end actions rss queues 1 2 end / end

Try enabling "Explicit Tx rule" mode if possible.
I was able to achieve 137 Mpps @ 64B with the following command:

dpdk-testpmd -a 21:00.0 -a c1:00.0 --in-memory -- \
    -i --rxq=1 --txq=1 --hairpinq=8 --hairpin-mode=0x10

You might get even better speed, because my flow rules were more complicated
(RTE Flow based "router on-a-stick"):

flow create 0 ingress group 1 pattern eth / vlan vid is 721 / end actions of_set_vlan_vid vlan_vid 722 / rss queues 1 2 3 4 5 6 7 8 end / end
flow create 1 ingress group 1 pattern eth / vlan vid is 721 / end actions of_set_vlan_vid vlan_vid 722 / rss queues 1 2 3 4 5 6 7 8 end / end
flow create 0 ingress group 1 pattern eth / vlan vid is 722 / end actions of_set_vlan_vid vlan_vid 721 / rss queues 1 2 3 4 5 6 7 8 end / end
flow create 1 ingress group 1 pattern eth / vlan vid is 722 / end actions of_set_vlan_vid vlan_vid 721 / rss queues 1 2 3 4 5 6 7 8 end / end
flow create 0 ingress group 0 pattern end actions jump group 1 / end
flow create 1 ingress group 0 pattern end actions jump group 1 / end

For packets in the range 68-80B I measured even lower throughput.
Full throughput I measured only from packets larger than 112B

For only one queue, I didn't get more than 55Gbps:
$ sudo dpdk-testpmd -l 0-1 -n 4 -a 0000:17:00.0,hp_buf_log_sz=19 -- 
--rxq=1 --txq=1 --rxd=4096 --txd=4096 --hairpinq=1 -i
testpmd> flow create 0 ingress pattern eth src is 00:10:94:00:00:03 /   
end actions queue index 1 / end

I tried to use locked device memory for TX and RX queues, but it seems 
that this is not supported:
"--hairpin-mode=0x011000" (bit 16 - hairpin TX queues will use locked 
device memory, bit 12 - hairpin RX queues will use locked device memory)

RxQ pinned in device memory requires firmware configuration [1]:

mlxconfig -y -d $pci_addr set MEMIC_SIZE_LIMIT=0 HAIRPIN_DATA_BUFFER_LOCK=1
mlxfwreset -y -d $pci_addr reset

[1]: https://doc.dpdk.org/guides/platform/mlx5.html?highlight=hairpin_data_buffer_lock

However, pinned RxQ didn't improve anything for me.

TxQ pinned in device memory is not supported by net/mlx5.
TxQ pinned to DPDK memory made performance awful (predictably).

I was expecting that achieving full throughput with hairpin queues would 
not be a problem.
Is my expectation too optimistic?

What other parameters besides 'hp_buf_log_sz' can I use to achieve full 
throughput?

In my experiments, default "hp_buf_log_sz" of 16 is optimal.
The most influential parameter appears to be the number of hairpin queues.

I tried combining the following parameters: mprq_en=, rxqs_min_mprq=, 
mprq_log_stride_num=, txq_inline_mpw=, rxq_pkt_pad_en=,
but with no positive impact on throughput.