On 09/21/2012 05:12 PM, Paul E. McKenney wrote: > On Fri, Sep 21, 2012 at 03:26:27PM +0200, Sasha Levin wrote: >> On 09/21/2012 02:13 PM, Paul E. McKenney wrote: >>>> This might be unrelated, but I got the following dump as well when trinity >>>>> decided it's time to reboot my guest: >>> OK, sounds like we should hold off until you reproduce, then. >> >> I'm not sure what you mean. >> >> There are basically two issues I'm seeing now, which reproduce pretty much >> every >> time: >> >> 1. The "using when idle" warning. >> 2. The rcu related hangs during shutdown. >> >> The first one appears early on when I start fuzzing, the other one happens >> when >> shutting down - so both of them are reproducible in the same session. > > Ah, I misunderstood the "reboot my guest" -- I thought that you were > doing something like repeated modprobe/rmmod cycles on rcutorture while > running the guest for an extended time period. That will teach me not > to reply to email so soon after waking up. ;-) > > That said, #2 is expected behavior given the RCU CPU stall warnings in > your Sept. 20 dmesg. This is because rcutorture does rcu_barrier() on > the way out, which cannot complete if grace periods are not completing. > And the later soft lockup is also likely a consequence of the stall, > because CPU hotplug does a synchronize_sched() while holding the hotplug > lock, which will then cause get_online_cpus() to hang. > > Looking further down, there are hung tasks that are waiting for a > timeout, but this is also a consequence of the hang because they > are waiting for MAX_SCHEDULE_TIMEOUT -- in other words, they are > waiting to be killed at shutdown time. I could suppress this by using > schedule_timeout_interruptible() in a loop in order to reduce the noise > in this case. > > The remaining traces in that email are also consequences of the stall. > > So why the stall? > > Using RCU from a CPU that RCU believes to be idle can cause arbitrary > bad behavior (possibly including stalls), but with very low probability. > The reason that things can go arbitrarily bad is that RCU is ignoring > the CPU, and thus not waiting for any RCU read-side critical sections. > This could of course result in abitrary corruption of memory. The reason > for the low probability is that grace periods tend to be long and RCU > read-side critical sections tend to be short. > > It looks like you are running -next, which has RCU grace periods driven > by a kthread. Is it possible that this kthread is not getting a chance > to run (in fact, the "Stall ended before state dump start" is consistent > with that possibility), but in that case I would expect to see a soft > lockup from it. Furthermore, in that case, it would be expected to > start running again as soon as things started going idle during shutdown. > > Or did the system somehow manage to stay busy despite being in shutdown? > Or, for that matter, are you overcommitting the physical CPUs on your > trinity test setup?
Nope, I originally had 4 vcpus in the guest with the host running 4 physical cpus, but I've also tested it with just 2 vcpus and still see the warnings. Thanks, Sasha -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
