RCU stall warnings even with rcu_nocbs and rcu_nocb_poll

Florent Carli
2022-10-05T16:01:52Z

Hello,

I'm trying to isolate some cores to run a CPU-bound real-time task and
even though I'm using rcu_nocbs and rcu_nocb_poll, I can see the rcuc
threads wake up, and I get RCU stall warnings on the isolated core.
As per the documentation, if I use rcutree.kthread_prio with a
priority > my RT task, then the rcu stall does not happen.

However I find it confusing: why are the rcuc threads woken up on the
isolated cores despite using rcu_nocbs and rcu_nocb_poll? In my (very
likely erroneous) understanding, I shouldn't have to fiddle with rcu
priority... In other words, how come I get rcu stalls on cores that
have no rcu callbacks?

Thanks.