I've been running with this patch on ARM (dra7 SOC, which is a dual-core A15) with a 4.1.18-rt17 kernel for a while without any noticeable problems. Boosting the ktimersoftd processes on both cores to a higher priority fixed some timing problems my userspace application was seeing too.
The userspace application in question has a repeating timerfd in an epoll and spends most of its time in blocking epoll_wait calls. Under system load (most from other lower-RT-priority userspace processes), it was seeing several-ms delays between the timerfd interrupt happening and ksoftirqd running the HRTIMER softirq to wake it up. That was with ksoftirqd running as SCHED_OTHER, which is what it defaulted to. The priority inversion between the userspace processes is gone after making ktimersoftd the same priority as the highest-priority application. Tested-by: Brian Silverman <br...@peloton-tech.com>
