linux-kernel - Re: [v3.10-rc1] WARNING: at kernel/rcutree.c:502

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Message-ID: <5191F3FB.3080004@linux.vnet.ibm.com>
Date:	Tue, 14 May 2013 13:51:15 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Bjørn Mork <bjorn@...k.no>
CC:	paulmck@...ux.vnet.ibm.com, Dipankar Sarma <dipankar@...ibm.com>,
	linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [v3.10-rc1] WARNING: at kernel/rcutree.c:502

On 05/14/2013 01:50 PM, Bjørn Mork wrote:
> "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> writes:
>> On 05/14/2013 01:08 PM, Bjørn Mork wrote:
>>> "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> writes:
>>>
>>>> The problem appears to be in the cpu idle poll implementation. You can trigger
>>>> this problem by passing idle=poll in the kernel cmd-line as well, right?
>>>
>>> That sounded so obvious that it made me think "Doh, why didn't I just
>>> test that before?"  But unfortunately there must be some other factor
>>> involved. No warnings observed during normal use when running with
>>> idle=poll:
>>>
>>
>> I didn't expect warnings with normal use.
>>
>>>  bjorn@...i:~$ dmesg|grep polling
>>>  [    0.000000] process: using polling idle threads
>>>
>>>
>>> I expected a flood of warnings here, but there is none until I start
>>> powertop (to confirm that the original issue is still there).  So it's
>>> more than just entering cpu_idle_poll().
>>>
>>
>> Yeah, of course it is :-) The warning triggers only when you enable the tracepoint
>> in the idle code. And in your case, powertop does that. That's why it only
>> triggers when you run powertop. Alternatively, if you enable the tracepoint
>> yourself manually, I bet you'll see the warnings, even without using powertop.
> 
> Ah, right.  That sounds so obvious I have to go "Doh!" again:)
> 

:-)

>>>> I think I understand what is going on here. Can you please try the fix below?
>>>> (It is only compile-tested since its very late here and I really need to get
>>>> some sleep!).
>>>
>>> Works perfect.  Thanks. 
>>
>> Thanks for your testing!
>>
>>> I assume this is the correct fix even if the
>>> problem isn't completely understood?
>>>
>>
>> Hmm? Why do you say the problem isn't completely understood? I thought I
>> explained the problem in my changelog. Did I miss something?
> 
> No, I did.  I didn't understand the tracepoint part of this.  But it is
> all perfectly clear to me too now, after your excellent explanation
> above.
> 

Great! Thanks :)
 
Regards,
Srivatsa S. Bhat

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