On Tue, 2013-05-21 at 22:14 +0200, Frederic Weisbecker wrote: > 2013/5/21 Steven Rostedt <rost...@goodmis.org>: > > commit a382bf934449 "nohz: Assign timekeeping duty to a CPU outside the > > full dynticks range" added a cpu notifier callback that would prevent > > the time keeping CPU from going offline if the have_nohz_full_mask was > > set. > > > > This also prevents the CPU from going offline on system reboot. > > > > Worse yet, the return code was -EINVAL, but the notifier does not > > recognize error codes, and it must be wrapped by a notifier_from_errno() > > function. This means that even though the CPU would fail to go down, the > > notifier would think it succeeded, and the cpu down process would > > continue. > > > > This caused two different problems. One, the migration thread after > > moving tasks from the CPU would park itself and then a task, namely the > > reboot task, could migrate onto that CPU. Then the reboot task spins > > waiting for the cpu to go idle. But because the reboot task happens to > > be spinning on the cpu its waiting for, the system hangs. > > Can that happen if that CPU is the boot CPU? Note this is the only > possible timekeeper with the upstream code.
Yep it can happen in upstream (that's all I'm using). In tick_broadcast_setup_oneshot(), it sets the tick_do_timer_cpu to the current CPU, which can be something other than the boot CPU. Now that CPU wont be able to be hot plugged. > > > > > The other error that happened was that the sched_domain re-setup would > > get confused, and in get_group() the cpu = cpumask_first() would process > > a mask that had nothing set, and return cpu > nr_cpu_ids. Later it would > > reference the per_cpu sg with this CPU and get a bogus pointer and > > crash. > > Ouch, when are we doing this domain re-setup? I remember we > repartition the domains after cpu down/up but I don't understand how > that can interfere with this issue. I haven't looked hard enough yet, but this problem only appeared when this bug triggered. By telling the system a CPU is offline, but still having tasks schedule to it, causes all sorts of weird side effects. I haven't figured out in detail how this affected the sched domains, but I don't get the sched domain corruption after fixing this bug. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
