On Fri, Jun 28, 2019 at 04:15:22PM +0200, Peter Zijlstra wrote: > On Thu, Jun 27, 2019 at 01:36:12PM -0700, Paul E. McKenney wrote: > > On Thu, Jun 27, 2019 at 03:17:27PM -0500, Scott Wood wrote: > > > On Thu, 2019-06-27 at 11:41 -0700, Paul E. McKenney wrote: > > > > On Thu, Jun 27, 2019 at 02:16:38PM -0400, Joel Fernandes wrote: > > > > > > > > > > I think the fix should be to prevent the wake-up not based on whether we > > > > > are > > > > > in hard/soft-interrupt mode but that we are doing the rcu_read_unlock() > > > > > from > > > > > a scheduler path (if we can detect that) > > > > > > > > Or just don't do the wakeup at all, if it comes to that. I don't know > > > > of any way to determine whether rcu_read_unlock() is being called from > > > > the scheduler, but it has been some time since I asked Peter Zijlstra > > > > about that. > > > > > > > > Of course, unconditionally refusing to do the wakeup might not be happy > > > > thing for NO_HZ_FULL kernels that don't implement IRQ work. > > > > > > Couldn't smp_send_reschedule() be used instead? > > > > Good point. If current -rcu doesn't fix things for Sebastian's case, > > that would be well worth looking at. But there must be some reason > > why Peter Zijlstra didn't suggest it when he instead suggested using > > the IRQ work approach. > > > > Peter, thoughts? > > I've not exactly kept up with the thread; but irq_work allows you to run > some actual code on the remote CPU which is often useful and it is only > a little more expensive than smp_send_reschedule(). > > Also, just smp_send_reschedule() doesn't really do anything without > first poking TIF_NEED_RESCHED (or other scheduler state) and if you want > to do both, there's other helpers you should use, like resched_cpu(). Thank you! Plus it looks like scheduler_ipi() takes an early exit if ->wake_list is empty, regardless of TIF_NEED_RESCHED, right? Thanx, Paul
