On Mon, 2018-07-09 at 13:34 -0700, Paul E. McKenney wrote: > > So here are the possible code paths when .rcu_urgent_qs is set to true: > > 1.      A context switch will record the quiescent state and clear >         .rcu_urgent_qs.  (The failure to do the clearing in current -rcu >         for PREEMPT builds is a performance bug that I need to fix.) What if there's nothing else runnable and there is no actual context switch? > 2.      A cond_resched() will cause rcu_all_qs() to be invoked, which >         will record a quiescent state and clear .rcu_urgent_qs. > > 3.      With the patch below, a scheduling-clock interrupt of a >         non-idle non-userspace task will force a reschedule, which >         will result in #1 above happening.