[PATCH net-next 0/2] net: Use SMP threads for backlog NAPI (or optional).

[Sebastian Andrzej Siewior]

The RPS code and "deferred skb free" both send IPI/ function call
to a remote CPU in which a softirq is raised. This leads to a warning on
PREEMPT_RT because raising softiqrs from function call led to undesired
behaviour in the past. I had duct tape in RT for the "deferred skb free"
and Wander Lairson Costa reported the RPS case.

Changes:
- RFC…v1 https://lore.kernel.org/all/20230818092111.5d86e351@xxxxxxxxxx

   - Patch #2 has been removed. Removing the warning is still an option.

   - There are two patches in the series:
     - Patch #1 always creates backlog threads
     - Patch #2 creates the backlog threads if requested at boot time,
       mandatory on PREEMPT_RT.
     So it is either or and I wanted to show how both look like.

   - The kernel test robot reported a performance regression with
     loopback (stress-ng --udp X --udp-ops Y) against the RFC version.
     The regression is now avoided by using local-NAPI if backlog
     processing is requested on the local CPU.

Sebastian