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

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Date:	Tue, 14 May 2013 10:20:50 +0200
From:	Bjørn Mork <bjorn@...k.no>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
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

"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.


Thanks,
Bjørn
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