On Thu, Jul 12, 2018 at 09:17:04AM -0700, Paul E. McKenney wrote: > On Thu, Jul 12, 2018 at 05:53:51AM -0700, Paul E. McKenney wrote: > > On Thu, Jul 12, 2018 at 01:00:42PM +0100, David Woodhouse wrote: > > > > > > > > > On Wed, 2018-07-11 at 14:08 -0700, Paul E. McKenney wrote: > > > > > > > > > Also... why in $DEITY's name was the existing > > > > > rcu_virt_note_context_switch() not actually sufficient? If we had that > > > > > there, why did we need an additional explicit calls to rcu_all_qs() in > > > > > the KVM loop, or the more complex fixes to need_resched() which > > > > > ultimately had the same effect, to avoid ten-second latencies? > > > > > > > > My guess is that this was because control passed through the > > > > rcu_virt_note_context_switch() only once, and then subsequent > > > > scheduling-clock interrupts bypassed this code. > > > > Gah! My guess was instead that the code did a rcu_kvm_enter() going in, > > but somehow managed to miss the rcu_kvm_exit() going out. But that makes > > absolutely no sense -- had that happened, rcutorture would likely have > > screamed bloody murder, loudly and often. No mere near misses! > > > > And besides, thus far, -ENOREPRODUCE. :-/ > > OK, one close call in 63 hours of rcutorture, this one on scenario TREE03 > (yesterday hit TREE01 and TREE03). Time for probabilitistic long-runtime > bisection. Plus thought about how to get more information out of the near > misses. Fun! ;-) Most of the weekend was devoted to testing today's upcoming pull request, but I did get a bit more testing done on this. I was able to make this happen more often by tweaking rcutorture a bit, but I still do not yet have statistically significant results. Nevertheless, I have thus far only seen failures with David's patch or with both David's and my patch. And I actually got a full-up rcutorture failure (a too-short grace period) in addition to the aforementioned close calls. Over this coming week I expect to devote significant testing time to the commit just prior to David's in my stack. If I don't see failures on that commit, we will need to spent some quality time with the KVM folks on whether or not kvm_x86_ops->run() and friends have the option of failing to return, but instead causing control to pop up somewhere else. Or someone could tell me how I am being blind to some obvious bug in the two commits that allow RCU to treat KVM guest-OS execution as an extended quiescent state. ;-) Thanx, Paul
