On Wed, Jul 11, 2018 at 06:03:42PM +0100, David Woodhouse wrote: > On Wed, 2018-07-11 at 09:49 -0700, Paul E. McKenney wrote: > > And here is an updated v4.15 patch with Marius's Reported-by and David's > > fix to my lost exclamation point. > > Thanks. Are you sending the original version of that to Linus? It'd be > useful to have the commit ID so that we can watch for it landing, and > chase this one up to Greg. > > As discussed on IRC, this patch reduces synchronize_sched() latency for > us from ~4600s to ~160ms, which is nice. > > However, it isn't going to be sufficient in the NO_HZ_FULL case. For > that you want a patch like the one below, which happily reduces the > latency in our (!NO_HZ_FULL) case still further to ~40ms. 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. Thanks.
