RE: rcu_preempt self-detected stall on CPU from 4.5-rc3, since 3.17

[Chatre, Reinette]

Hi Paul,

On 2016-03-22, Paul E. McKenney wrote:
> On Tue, Mar 22, 2016 at 04:35:32PM +0000, Chatre, Reinette wrote:
>> On 2016-03-21, Paul E. McKenney wrote:
>>> On Mon, Mar 21, 2016 at 09:22:30AM -0700, Jacob Pan wrote:
>>>> On Fri, 18 Mar 2016 16:56:41 -0700
>>>> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>>>>> On Fri, Mar 18, 2016 at 02:00:11PM -0700, Josh Triplett wrote:
>>>>>> On Thu, Feb 25, 2016 at 04:56:38PM -0800, Paul E. McKenney wrote:
>>> 
>>> [ . . . ]
>>> 
>>>>>> We're seeing a similar stall (~60 seconds) on an x86 development
>>>>>> system here.  Any luck tracking down the cause of this?  If not, any
>>>>>> suggestions for traces that might be helpful?
>>>>> 
>>>>> The dmesg containing the stall, the kernel version, and the .config
>>>>> would be helpful!  Working on a torture test specific to this bug...
> 
> And thank you for the .config.  Your kenrle version looks to be 4.5.0.
> 
>>>> +Reinette, she has the system that can reproduce the issue. I
>>>> believe she is having some other problems with it at the moment. But
>>>> the .config should be available. Version is v4.5.
>>> 
>>> A couple of additional questions:
>>> 
>>> 1.	Is the test running on bare metal or virtualized?  If the
>>> 	latter, what is the host?
>> 
>> Bare metal.
> 
> OK, you are ahead of me.  Mine is virtualized.
> 
>>> 2.	Does the workload involve CPU hotplug?
>> 
>> No.
> 
> Again, you are ahead of me.  Mine makes extremely heavy use of CPU hotplug.
> 
>>> 3.	Are you seeing things like this in dmesg?
>>> 
>>> 	"rcu_preempt kthread starved for 21033 jiffies"
>>> 	"rcu_sched kthread starved for 32103 jiffies"
>>> 	"rcu_bh kthread starved for 84031 jiffies"
>>> 
>>> 	If not, you are probably facing some other bug, and should
>>> 	proceed debugging as described in Documentation/RCU/stallwarn.txt.
>> 
>> Below is a sample of what I see as captured with v4.5. The kernel
>> configuration is attached.
>> 
>> [  135.456197] INFO: rcu_preempt detected stalls on CPUs/tasks: [ 
>> 135.457729]  3-...: (0 ticks this GP) idle=722/0/0 softirq=5532/5532
>> fqs=0 [  135.459604]  (detected by 2, t=60004 jiffies, g=2105, c=2104,
>> q=165) [  135.461318] Task dump for CPU 3: [  135.461321] swapper/3    
>>   R  running task        0     0      1 0x00200000 [  135.461325] 
>> 00000078560040e5 ffff88017846fed0 ffffffff818af2cc ffff880100000000 [ 
>> 135.461330]  0000000600000003 ffff880178470000 ffff880072f32200
>> ffffffff822dcec0 [  135.461334]  ffff88017846c000 ffff88017846c000
>> ffff88017846fee0 ffffffff818af517 [  135.461338] Call Trace: [ 
>> 135.461345]  [<ffffffff818af2cc>] ? cpuidle_enter_state+0xfc/0x310 [ 
>> 135.461349]  [<ffffffff818af517>] ? cpuidle_enter+0x17/0x20 [ 
>> 135.461353]  [<ffffffff811515aa>] ? call_cpuidle+0x2a/0x40 [ 
>> 135.461355]  [<ffffffff8115197d>] ? cpu_startup_entry+0x28d/0x360 [ 
>> 135.461360]  [<ffffffff8108c874>] ? start_secondary+0x114/0x140 [ 
>> 135.461365] rcu_preempt kthread starved for 60004 jiffies! g2105 c2104
> f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
> 
> And yes, it looks like you are seeing the same bug that I am tracing.
> 
> The kthread is blocked on a schedule_timeout_interruptible().  Given
> default configuration, this would have a three-jiffy timeout.
> 
> You set CONFIG_RCU_CPU_STALL_TIMEOUT=60, which matches the 60004 jiffies
> above.  Is that value due to a distro setting or something?  Mainline
> uses CONFIG_RCU_CPU_STALL_TIMEOUT=21.

Indeed ... this value originated from a Fedora configuration. 

>> [  135.463965] rcu_preempt     S ffff88017844fd68     0     7      2
>> 0x00000000 [  135.463969]  ffff88017844fd68 ffff88017dd8cc80
>> ffff880177ff0000 ffff880178443b80 [  135.463973]  ffff880178450000
>> ffff88017844fda0 ffff88017dd8cc80 ffff88017dd8cc80 [  135.463977] 
>> 0000000000000003 ffff88017844fd80 ffffffff81ab031f 0000000100031504 [ 
>> 135.463981] Call Trace: [  135.463986]  [<ffffffff81ab031f>]
>> schedule+0x3f/0xa0 [  135.463989]  [<ffffffff81ab42d7>]
>> schedule_timeout+0x127/0x270 [  135.463993]  [<ffffffff81171a50>] ?
>> detach_if_pending+0x120/0x120 [  135.463997]  [<ffffffff8116da5d>]
>> rcu_gp_kthread+0x6bd/0xa30 [  135.464000]  [<ffffffff81151390>] ?
>> wake_atomic_t_function+0x70/0x70 [  135.464003]  [<ffffffff8116d3a0>] ?
>> force_qs_rnp+0x1b0/0x1b0 [  135.464006]  [<ffffffff8112f846>]
>> kthread+0xe6/0x100 [  135.464009]  [<ffffffff8112f760>] ?
>> kthread_worker_fn+0x190/0x190 [  135.464012]  [<ffffffff81ab5c0f>]
>> ret_from_fork+0x3f/0x70 [  135.464015]  [<ffffffff8112f760>] ?
>> kthread_worker_fn+0x190/0x190
> 
> How long does it take to reproduce this?  If it reproduces in minutes
> or hours, could you please boot with the following on the kernel command
> line and dump the trace buffer shortly after the stall?
> 
> ftrace trace_event=sched_waking,sched_wakeup,sched_wake_idle_without_ipi

The trace I provided above appeared after a few minutes and not again. On previous occasions I had to wait a few hours. I tried running with the above added to the kernel command line but I have not seen the trace yet. I will leave the system overnight but then may risk not capturing the data you need so ...

> If dumping manually shortly after the stall is at all non-trivial
> (for example, if your reproduction time is many minute or hours),
> I can supply some patches that automate this.  Or you can pick
> them up from -rcu:

... could you please point me to the patches you refer to? Or would you like me to try with the entire kernel from rcu/dev?

> 
> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
> 
> Branch rcu/dev has these patches (and much else besides).
> 
> 							Thanx, Paul
> 
> PS:  In case you are curious, when I enable those tracepoints, it
>      shows me that the timer is firing every three jiffies, as it
>      should, but that something happens between the sched_waking
>      and the IPI handler that should actually do the wakeup.
>      However, adding the traces significantly slows reproduction,
>      so I am writing a stress test specific to this bug to try to
>      speed things up, hopefully allowing more tracing to be added
>      while still retaining non-geologic reproduction times.

Thank you very much for these details. 

Reinette