Cause: A possible livelock in the NAPI polling and busy polling routines could lead the system to a livelock on threads running at high, realtime, priorities.
Consequence: threads running at priorities lower than the ones of the threads involved in the livelock would be prevented from running on the CPUs affected by the livelock. Among those threads are the rcuc/ threads.
Fix: right before (4 jiffies) a RCU stall is detected, the rcuc/ threads on the CPUs facing the livelock have their priorities boosted above the priority of the threads involved in the livelock. Also, the softirqs are now handled in a way closer to the behavior of the upstream kernel.
Result: Even under pressure thre rcuc/ threads are able to run, mitigating the RCU stalls.