On Fri, Jun 28, 2019 at 03:01:36PM -0500, Scott Wood wrote: > On Fri, 2019-06-28 at 16:15 +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: > > > > > 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(). > > resched_cpu() will not send an IPI to the current CPU[1]. Correct, smp_send_reschedule() might not work for self, not all hardware can self-IPI. > Plus, the RCU > code needs to set need_resched even in cases where it doesn't need to send > the IPI. And worst of all, resched_cpu() takes the rq lock which is the > deadlock scenario we're trying to avoid. > > -Scott > > [1] Which makes me nervous about latency if there are any wakeups with irqs > disabled, without a preempt_enable() after irqs are enabled again, and not > inside an interrupt. All good points; and those are all solved with irq_work. That provides a 'clean' IRQ context.
