On Thu, Jul 18, 2019 at 08:52:52PM -0400, Joel Fernandes wrote: > On Thu, Jul 18, 2019 at 8:40 PM Byungchul Park <byungchul.park@xxxxxxx> wrote: > [snip] > > > - There is a bug in the CPU stopper machinery itself preventing it > > > from scheduling the stopper on Y. Even though Y is not holding up the > > > grace period. > > > > Or any thread on Y is busy with preemption/irq disabled preventing the > > stopper from being scheduled on Y. > > > > Or something is stuck in ttwu() to wake up the stopper on Y due to any > > scheduler locks such as pi_lock or rq->lock or something. > > > > I think what you mentioned can happen easily. > > > > Basically we would need information about preemption/irq disabled > > sections on Y and scheduler's current activity on every cpu at that time. > > I think all that's needed is an NMI backtrace on all CPUs. An ARM we > don't have NMI solutions and only IPI or interrupt based backtrace > works which should at least catch and the preempt disable and softirq > disable cases. > > But yeah I don't see why just the stacks of those CPUs that are > blocking the CPU X would not suffice for the trivial cases where a > piece of misbehaving code disable interrupts / preemption and > prevented the stopper thread from executing. Right. So it makes more interesting tho! :-) > May be once the test case is ready (no rush!) , then it will be more > clear what can help. Yes. I'm really happy to help things about RCU that I love, fixed or improved. And with the the test case or a real issue, I believe I can do more helpful work. Looking forward to it, too (no rush!). Thanks, Byungchul
