Re: RCU idle CPU detection is broken in linux-next

[Sasha Levin]

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
[   42.389351] Call Trace:
[   42.389354]  [<ffffffff811c653c>] ? rcu_eqs_enter+0x5c/0xc0
[   42.389356]  [<ffffffff81106886>] warn_slowpath_common+0x86/0xb0
[   42.389359]  [<ffffffff81106975>] warn_slowpath_null+0x15/0x20
[   42.389361]  [<ffffffff811c653c>] rcu_eqs_enter+0x5c/0xc0
[   42.389364]  [<ffffffff811c66f3>] rcu_idle_enter+0x43/0xa0
[   42.389366]  [<ffffffff81078956>] cpu_idle+0x126/0x160
[   42.389369]  [<ffffffff83985a64>] 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]  [<ffffffff811c6019>] ? rcu_eqs_enter_common+0x709/0x970
[   42.389405]  [<ffffffff81106886>] warn_slowpath_common+0x86/0xb0
[   42.389407]  [<ffffffff81106975>] warn_slowpath_null+0x15/0x20
[   42.389410]  [<ffffffff811c6019>] rcu_eqs_enter_common+0x709/0x970
[   42.389412]  [<ffffffff811c658f>] rcu_eqs_enter+0xaf/0xc0
[   42.389414]  [<ffffffff811c66f3>] rcu_idle_enter+0x43/0xa0
[   42.389417]  [<ffffffff81078956>] cpu_idle+0x126/0x160
[   42.389420]  [<ffffffff83985a64>] 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]  [<ffffffff811c6d9f>] ? rcu_eqs_exit+0x4f/0xb0
[   42.389433]  [<ffffffff81106886>] warn_slowpath_common+0x86/0xb0
[   42.389436]  [<ffffffff81106975>] warn_slowpath_null+0x15/0x20
[   42.389438]  [<ffffffff811c6d9f>] rcu_eqs_exit+0x4f/0xb0
[   42.389441]  [<ffffffff811c6f13>] rcu_idle_exit+0x43/0xa0
[   42.389443]  [<ffffffff8107896d>] cpu_idle+0x13d/0x160
[   42.389445]  [<ffffffff83985a64>] 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]  [<ffffffff811c679a>] ? rcu_eqs_exit_common+0x4a/0x3a0
[   42.389458]  [<ffffffff81106886>] warn_slowpath_common+0x86/0xb0
[   42.389460]  [<ffffffff81106975>] warn_slowpath_null+0x15/0x20
[   42.389462]  [<ffffffff811c679a>] rcu_eqs_exit_common+0x4a/0x3a0
[   42.389465]  [<ffffffff811c6dec>] rcu_eqs_exit+0x9c/0xb0
[   42.389467]  [<ffffffff811c6f13>] rcu_idle_exit+0x43/0xa0
[   42.389470]  [<ffffffff8107896d>] cpu_idle+0x13d/0x160
[   42.389472]  [<ffffffff83985a64>] start_secondary+0x26e/0x276
[   42.389474] ---[ end trace 04c11301284d64f1 ]---


Thanks,
Sasha
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