On Thu, Jul 19, 2018 at 08:16:47AM +0200, David Woodhouse wrote: > > > On Wed, 2018-07-18 at 20:11 -0700, Paul E. McKenney wrote: > > > > > That is interesting. As I replied to Paul, we are already calling > > > rcu_user_enter/exit() on guest_enter/exit_irqsoff(). So I'm wondering why > > > you're seeing such an optimization by repeating those calls. > > > > > > Perhaps the rcu_user_* somehow aren't actually called from > > > __context_tracking_enter()...? Some bug in context tracking? > > > Otherwise it's a curious side effect. > > > > David is working with v4.15. Is this maybe something that has changed > > since then? > > To clarify: in 4.15 without CONFIG_PREEMPT and without NO_HZ_FULL I was > seeing RCU stalls because a thread in vcpu_run() was *never* seen to go > through a quiescent state. Hence the change to need_resched() in the > first patch in this thread, which fixed the problem at hand and seemed > to address the general case. > > It then seemed by *inspection* that the NO_HZ_FULL case was probably > broken, because we'd failed to spot the rcu_user_* calls. But > rcu_user_enter() does nothing in the !NO_HZ_FULL case, so wouldn't have > helped in the testing that we were doing anyway. Oh ok, so the optimization you saw is likely unrelated to the rcu_user* things.
