On Mon, Jul 09, 2018 at 01:42:48PM -0700, Paul E. McKenney wrote: > On Mon, Jul 09, 2018 at 09:35:38PM +0100, David Woodhouse wrote: > > > > > > 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? > > The scheduler invokes rcu_note_context_switch() before looking to see > if there really will or won't be a context switch.
Correct. Just getting to __schedule() means we can schedule and thus is a valid point for RCU to progress. Even if we then end up selecting the very same task and not switching at all.
