Re: RCU CPU stall console spews leads to soft lockup disabled is reasonable ?

[Zhang Zhen <zhenzhang.zhang@huawei.com>]

On 2015/1/22 4:06, Paul E. McKenney wrote:
> On Wed, Jan 21, 2015 at 10:10:51AM -0500, Don Zickus wrote:
>> On Wed, Jan 21, 2015 at 10:26:27AM +0800, Zhang Zhen wrote:
>>>> This may not cause other problems but what happens if you comment out the
>>>> 'touch_softlockup_watchdog' from the touch_nmi_watchdog function like
>>>> below (based on latest upstream cb59670870)?
>>>>
>>>> The idea is that console printing for that cpu won't reset the softlockup
>>>> detector.  Again other bad things might happen and this patch may not be a
>>>> good final solution, but it can help give me a clue about what is going
>>>> on.
>>>
>>> I commented out the 'touch_softlockup_watchdog' from the touch_nmi_watchdog function
>>> (based on latest upstream ec6f34e5b552).
>>> This triggered RCU stall and softlockup, but softlockup just printk only once.
>>
>> Yes, as expected.  Instead of flooding the console with the same message,
>> report only once until the high priority task is re-kicked and can re-arm
>> the softlockup timer again.  So this makes sense.  Based on your double
>> spin lock test, I don't ever expect this to get re-armed, so you should
>> not expect to see another message.
>>
>>
>>> As you mentioned "other bad things" lead to softlockup just printk only once.
>>> What's the bad thing ?
>>
>> Well, one of the assumptions here is that if you are going to have
>> interrupts disabled for a long time and purposely kick the hardlockup
>> detector (with touch_nmi_watchdog), then you can reasonably assume that
>> softlockups will go off too if you don't also kick the softlockup detector
>> (even though there is a window where this isn't true: 2*thresh - thresh).
>>
>>
>> So the patch I gave you, can lead to false softlockup warnings.
>>
>>
>> On the flip side, printing to the console also blocks hardlockups from
>> showing up.  But I believe loooong ago, Paul adjusted the rcu stalls to be
>> longer than a hardlockup/softlockup timeout to prevent such scenarios.
>>
>>
>>
>> I am not sure what to do here.  Printing to the console has traditionally
>> been slow (especially if doing so with interrupts disabled), so a
>> touch_nmi_watchdog has been necessary.  But a side effect of that, is it
>> prevents the lockup detectors from going off if printing repeatedly.
>>
>>
>> Now we can hack up rcu stall to only print once until it is re-armed.
>> This would show rcu stall printing first, followed two minutes later by
>> a softlockup, which sorta achieves what you want.
>>
>>
>>
>> However, at the end of the day, an rcu stall, softlockup or hardlockup is
>> a bad thing, does it matter if you get one or all of them?  One should be
>> enough to start an investigation, no?
> 
> Hear, hear!!!
> 
Ok, thanks for Don's and your patience!
We enriched softlockup printk information, so we want to get softlockup warnning.

> 							Thanx, Paul
> 
>> I don't know.  I am leaning towards 'working as expected' and nothing to
>> really fix here.  Does anyone have any suggestions on what assumptions we
>> could change to handle this better?
>>
>>
>> Cheers,
>> Don
>>
>>>
>>> / #
>>> / # echo 60 > /proc/sys/kernel/watchdog_thresh
>>> / # busybox insmod softlockup_test.ko &
>>> / # [   39.044058] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=21002 jiffies, g=78, c=77, q=4)
>>> [   39.044058] INFO: Stall ended before state dump start
>>> [  102.049045] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=84007 jiffies, g=78, c=77, q=4)
>>> [  102.049045] INFO: Stall ended before state dump start
>>> [  160.482123] BUG: soft lockup - CPU#0 stuck for 134s! [busybox:54]
>>> [  165.054075] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=147012 jiffies, g=78, c=77, q=4)
>>> [  165.054075] INFO: Stall ended before state dump start
>>> [  228.059045] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=210017 jiffies, g=78, c=77, q=4)
>>> [  228.059045] INFO: Stall ended before state dump start
>>> [  291.064099] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=273022 jiffies, g=78, c=77, q=4)
>>> [  291.064099] INFO: Stall ended before state dump start
>>> [  354.069074] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=336027 jiffies, g=78, c=77, q=4)
>>> [  354.069099] INFO: Stall ended before state dump start
>>> [  417.074045] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=399032 jiffies, g=78, c=77, q=4)
>>> [  417.074045] INFO: Stall ended before state dump start
>>> [  480.079099] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=462037 jiffies, g=78, c=77, q=4)
>>> [  480.079099] INFO: Stall ended before state dump start
>>> [  543.084099] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=525042 jiffies, g=78, c=77, q=4)
>>> [  543.084099] INFO: Stall ended before state dump start
>>> [  606.089101] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=588047 jiffies, g=78, c=77, q=4)
>>> [  606.089101] INFO: Stall ended before state dump start
>>> [  669.094099] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=651052 jiffies, g=78, c=77, q=4)
>>> [  669.094099] INFO: Stall ended before state dump start
>>> [  732.099045] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=714057 jiffies, g=78, c=77, q=4)
>>> [  732.099045] INFO: Stall ended before state dump start
>>> [  795.104074] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=777062 jiffies, g=78, c=77, q=4)
>>> [  795.104098] INFO: Stall ended before state dump start
>>> [  858.109046] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=840067 jiffies, g=78, c=77, q=4)
>>> [  858.109046] INFO: Stall ended before state dump start
>>> [  921.114100] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=903072 jiffies, g=78, c=77, q=4)
>>> [  921.114100] INFO: Stall ended before state dump start
>>> [  984.119099] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=966077 jiffies, g=78, c=77, q=4)
>>> [  984.119099] INFO: Stall ended before state dump start
>>> [ 1047.124075] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1029082 jiffies, g=78, c=77, q=4)
>>> [ 1047.124099] INFO: Stall ended before state dump start
>>> [ 1110.129046] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1092087 jiffies, g=78, c=77, q=4)
>>> [ 1110.129046] INFO: Stall ended before state dump start
>>> [ 1173.134045] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1155092 jiffies, g=78, c=77, q=4)
>>> [ 1173.134045] INFO: Stall ended before state dump start
>>> [ 1236.139101] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1218097 jiffies, g=78, c=77, q=4)
>>> [ 1236.139101] INFO: Stall ended before state dump start
>>> [ 1299.144059] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1281102 jiffies, g=78, c=77, q=4)
>>> [ 1299.144059] INFO: Stall ended before state dump start
>>> [ 1362.149099] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1344107 jiffies, g=78, c=77, q=4)
>>> [ 1362.149099] INFO: Stall ended before state dump start
>>> [ 1425.154059] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1407112 jiffies, g=78, c=77, q=4)
>>> [ 1425.154059] INFO: Stall ended before state dump start
>>> [ 1488.159046] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1470117 jiffies, g=78, c=77, q=4)
>>> [ 1488.159046] INFO: Stall ended before state dump start
>>> [ 1551.164045] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1533122 jiffies, g=78, c=77, q=4)
>>> [ 1551.164045] INFO: Stall ended before state dump start
>>> [ 1614.169057] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1596127 jiffies, g=78, c=77, q=4)
>>> [ 1614.169057] INFO: Stall ended before state dump start
>>> [ 1677.174060] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1659132 jiffies, g=78, c=77, q=4)
>>> [ 1677.174060] INFO: Stall ended before state dump start
>>> [ 1740.179045] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1722137 jiffies, g=78, c=77, q=4)
>>> [ 1740.179045] INFO: Stall ended before state dump start
>>> [ 1803.184075] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1785142 jiffies, g=78, c=77, q=4)
>>> [ 1803.184101] INFO: Stall ended before state dump start
>>> [ 1866.189046] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1848147 jiffies, g=78, c=77, q=4)
>>> [ 1866.189046] INFO: Stall ended before state dump start
>>> [ 1929.194045] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1911152 jiffies, g=78, c=77, q=4)
>>> [ 1929.194045] INFO: Stall ended before state dump start
>>> [ 1992.199083] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=1974157 jiffies, g=78, c=77, q=4)
>>> [ 1992.199083] INFO: Stall ended before state dump start
>>> [ 2055.204098] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=2037162 jiffies, g=78, c=77, q=4)
>>> [ 2055.204098] INFO: Stall ended before state dump start
>>> [ 2118.209045] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=2100167 jiffies, g=78, c=77, q=4)
>>> [ 2118.209045] INFO: Stall ended before state dump start
>>> [ 2181.214098] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=2163172 jiffies, g=78, c=77, q=4)
>>> [ 2181.214098] INFO: Stall ended before state dump start
>>>>
>>>> Cheers,
>>>> Don
>>>>
>>>> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
>>>> index 70bf118..833c015 100644
>>>> --- a/kernel/watchdog.c
>>>> +++ b/kernel/watchdog.c
>>>> @@ -209,7 +209,7 @@ void touch_nmi_watchdog(void)
>>>>  	 * going off.
>>>>  	 */
>>>>  	raw_cpu_write(watchdog_nmi_touch, true);
>>>> -	touch_softlockup_watchdog();
>>>> +	//touch_softlockup_watchdog();
>>>>  }
>>>>  EXPORT_SYMBOL(touch_nmi_watchdog);
>>>>  
>>>>
>>>> .
>>>>
>>>
>>>
>>
> 
> 
> .
>