Subject: RFH, is it possible to set ndo_start_xmit() cpu affinity in ethernet driver?

From: Василий Умрихин <[email-masked]>
Date: Mon, 12 Sep 2022 19:48:43 +0400 (MSK)

Hi netdev, 

On the receiving side we have the opportunity to choose the CPU that will process the receive queue (RPS). 
On the sender side XPS selects the send queue for the given CPU, but there is no way to select the CPU on which ndo_start_xmit() will be launched. 
Taskset is able to bind user task, but in ndo_start_xmit() binding differs.
In my case CPU0 reserved for polling kthread, because our NIC have no interrupts, therefore it is necessary. I need nothing else to run on this CPU. 

For example, setting CPU1 for RPS on both nodes: 

host1: echo 0x2 > /sys/class/net//queues/rx-0/rps_cpus 
host2: echo 0x2 > /sys/class/net//queues/rx-0/rps_cpus 

Then run iperf on two nodes: 

host1: taskset -c 1 iperf -s 
host2: taskset -c 1 iperf -c host1 

After adding pr_info("cpu%d\n", smp_processor_id()); in my ndo_start_xmit() method, see in dmesg: 

host1: dmesg | grep cpu0 | wc -l 
0 
host2: dmesg | grep cpu0 | wc -l 
6512 

Is it possible to choose the CPU on which ndo_start_xmit() will be launched on the sender side? 

Kind regards, Vasiliy