Re: Softirq priority inversion from "softirq: reduce latencies"

From: Eric Dumazet
Date: Mon Feb 29 2016 - 10:28:30 EST

On lun., 2016-02-29 at 07:03 -0800, Peter Hurley wrote:

> The reason why Eric's change is so effective for Eric's workload is
> that it fixes the problem where NET_RX keeps getting new network packets
> so it keeps looping, servicing more NET_RX softirq.

You have very little idea of what is happening in networking land.

Once hard irq for RX has triggered, we arm a NAPI (NET_RX softirq), and
no more irq will come unless the napi handler ran. Then when NAPI is
complete, we re-allow interrupt to be delivered when a new packet is

Yes, ksoftirqd runs under load, and this is _wanted_.

Sure, it might add a latency if some high prio task is wanting the same
cpu, but this is exactly the purpose of having multi tasking.