On Fri, Jul 19, 2019 at 4:43 PM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> wrote: > > 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. > > True, though people with systems having hundreds of CPUs might not > thank you for forcing an NMI backtrace on each of them. Is it possible > to NMI only the ones that are holding up the CPU stopper? What a good idea! I think it's possible! But we need to think about the case NMI doesn't work when the holding-up was caused by IRQ disabled. Though it's just around the corner of weekend, I will keep thinking on it during weekend! Thanks, Byungchul > Thanx, Paul > > > 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. > > > > May be once the test case is ready (no rush!) , then it will be more > > clear what can help. > > > > J. > > -- Thanks, Byungchul
