On Thu, Jan 2, 2025 at 7:58 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: [...] > > > Last question, do I need the -rt kernel for preempt RCU? > > > > No, CONFIG_PREEMPT=y suffices. > > > > Note that CONFIG_PREEMPT_RT=y, AKA -rt, also makes spinlocks (but not > > raw spinlocks) be limited sleeplocks, and thus allows RCU read-side > > critical sections to block when acquiring these sleeping "spinlocks". > > But this is OK, because all of this is still subject to priority boosting. > > Should PREEMPT_RT kernels not throw warnings though when calling > rcu_note_context_switch() in RCU read-side sections? > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/rcu/tree_plugin.h#n331 > > I don't run a PREEMPT_RT kernel myself so I can't confirm if these > warnings somehow don't appear, but I figured it would be good to > double check in this discussion. Please ignore my noise, Steven clarified for me in a meeting today that preempt != 0 in that warning, when an a "spinlock-coverted-to-rtmutex for PREEMPT_RT" blocks. So even though the rtmutex blocks in an RCU read-side critical section. In other words, I believe rtmutex is the exception -- It can block in an RCU read-side critical section on any kernel (PREEMPT_RT or otherwise). I could stand corrected though but that's my current understanding. thanks, - Joel
