On 09/22/2012 05:09 PM, Paul E. McKenney wrote: > On Sat, Sep 22, 2012 at 10:26:09AM +0200, Sasha Levin wrote: >> On 09/21/2012 05:18 PM, Sasha Levin wrote: >>> 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. >> >> Some more info that might help, I'm also occasionally seeing: >> >> [ 42.389345] ------------[ cut here ]------------ >> [ 42.389348] WARNING: at kernel/rcutree.c:375 rcu_eqs_enter+0x5c/0xc0() >> [ 42.389350] Pid: 0, comm: swapper/2 Tainted: G W >> 3.6.0-rc6-next-20120921-sasha-00002-ge9c9495-dirty #378 > > Hmmm... So either RCU is losing count or some CPU that is already > marked as idle from RCU's perspective is trying to re-enter idle. > > This is helpful, thank you! It fits in nicely with the splat that > you got after applying Michael Wang's patch. Could you please try the > diagnostic patch below? > > Thanx, Paul > >> [ 42.389351] Call Trace: >> [ 42.389354] [] ? rcu_eqs_enter+0x5c/0xc0 >> [ 42.389356] [] warn_slowpath_common+0x86/0xb0 >> [ 42.389359] [] warn_slowpath_null+0x15/0x20 >> [ 42.389361] [] rcu_eqs_enter+0x5c/0xc0 >> [ 42.389364] [] rcu_idle_enter+0x43/0xa0 >> [ 42.389366] [] cpu_idle+0x126/0x160 >> [ 42.389369] [] start_secondary+0x26e/0x276 >> [ 42.389370] ---[ end trace 04c11301284d64ee ]--- >> [ 42.389394] ------------[ cut here ]------------ >> [ 42.389396] WARNING: at kernel/rcutree.c:350 rcu_eqs_enter_common+0x709/0x970() >> [ 42.389398] Pid: 0, comm: swapper/2 Tainted: G W >> 3.6.0-rc6-next-20120921-sasha-00002-ge9c9495-dirty #378 >> [ 42.389399] Call Trace: >> [ 42.389402] [] ? rcu_eqs_enter_common+0x709/0x970 >> [ 42.389405] [] warn_slowpath_common+0x86/0xb0 >> [ 42.389407] [] warn_slowpath_null+0x15/0x20 >> [ 42.389410] [] rcu_eqs_enter_common+0x709/0x970 >> [ 42.389412] [] rcu_eqs_enter+0xaf/0xc0 >> [ 42.389414] [] rcu_idle_enter+0x43/0xa0 >> [ 42.389417] [] cpu_idle+0x126/0x160 >> [ 42.389420] [] start_secondary+0x26e/0x276 >> [ 42.389421] ---[ end trace 04c11301284d64ef ]--- >> [ 42.389424] ------------[ cut here ]------------ >> [ 42.389426] WARNING: at kernel/rcutree.c:527 rcu_eqs_exit+0x4f/0xb0() >> [ 42.389427] Pid: 0, comm: swapper/2 Tainted: G W >> 3.6.0-rc6-next-20120921-sasha-00002-ge9c9495-dirty #378 >> [ 42.389428] Call Trace: >> [ 42.389431] [] ? rcu_eqs_exit+0x4f/0xb0 >> [ 42.389433] [] warn_slowpath_common+0x86/0xb0 >> [ 42.389436] [] warn_slowpath_null+0x15/0x20 >> [ 42.389438] [] rcu_eqs_exit+0x4f/0xb0 >> [ 42.389441] [] rcu_idle_exit+0x43/0xa0 >> [ 42.389443] [] cpu_idle+0x13d/0x160 >> [ 42.389445] [] start_secondary+0x26e/0x276 >> [ 42.389447] ---[ end trace 04c11301284d64f0 ]--- >> [ 42.389448] ------------[ cut here ]------------ >> [ 42.389450] WARNING: at kernel/rcutree.c:501 rcu_eqs_exit_common+0x4a/0x3a0() >> [ 42.389451] Pid: 0, comm: swapper/2 Tainted: G W >> 3.6.0-rc6-next-20120921-sasha-00002-ge9c9495-dirty #378 >> [ 42.389452] Call Trace: >> [ 42.389455] [] ? rcu_eqs_exit_common+0x4a/0x3a0 >> [ 42.389458] [] warn_slowpath_common+0x86/0xb0 >> [ 42.389460] [] warn_slowpath_null+0x15/0x20 >> [ 42.389462] [] rcu_eqs_exit_common+0x4a/0x3a0 >> [ 42.389465] [] rcu_eqs_exit+0x9c/0xb0 >> [ 42.389467] [] rcu_idle_exit+0x43/0xa0 >> [ 42.389470] [] cpu_idle+0x13d/0x160 >> [ 42.389472] [] start_secondary+0x26e/0x276 >> [ 42.389474] ---[ end trace 04c11301284d64f1 ]--- > > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c > index da14e5c..39d0aec 100644 > --- a/arch/x86/kernel/process.c > +++ b/arch/x86/kernel/process.c > @@ -418,16 +418,22 @@ void cpu_idle(void) > pm_idle(); > > rcu_idle_exit(); > + WARN_ON(rcu_is_cpu_idle()); > start_critical_timings(); > > /* In many cases the interrupt that ended idle > has already called exit_idle. But some idle > loops can be woken up without interrupt. */ > + WARN_ON(rcu_is_cpu_idle()); > __exit_idle(); > + WARN_ON(rcu_is_cpu_idle()); > } > > + WARN_ON(rcu_is_cpu_idle()); > tick_nohz_idle_exit(); > + WARN_ON(rcu_is_cpu_idle()); > preempt_enable_no_resched(); > + WARN_ON(rcu_is_cpu_idle()); > schedule(); > preempt_disable(); > } > I don't see any new warnings using this patch. I've attached the config. Thanks, Sasha