linux-kernel - Re: Random panic in load

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Date:	Wed, 23 Jul 2014 17:05:24 +0900
From:	Michel Dänzer <michel@...nzer.net>
To:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Random panic in load_balance() with 3.16-rc

On 23.07.2014 15:49, Peter Zijlstra wrote:
> On Tue, Jul 22, 2014 at 09:21:40PM -0700, Linus Torvalds wrote:
>> On Tue, Jul 22, 2014 at 8:53 PM, Michel Dänzer <michel@...nzer.net> wrote:
>>>
>>> Just happened again with the same change on top of 3.16-rc6.
>>
>> The (maybe) related bugzilla entry is just odd. Bruno Wolff reports
>> that the BUG_ON() in his added patch triggers:
>>
>> +               cpumask_clear(sched_group_cpus(sg));
>> +               sg->sgc->capacity = 0;
>> +               BUG_ON(!cpumask_empty(sched_group_cpus(sg)));
>>
>> where it *just* did a cpumask_clear(), and now the BUG_ON() triggers
>> that it's no longer empty?
>>
>> That would imply an allocation error, but all the sched groups seem to
>> be properly allocated with the proper addition of cpumask_size().
>>
>> And his config file even has NR_CPUS being 32, so it should be a
>> single word of bitmap, which triggers all the simple code.
>>
>> Completely insane, in other words.
> 
> So we've had this other thread where the same happened:
> 
>  lkml.kernel.org/r/20140716145546.GA6922@...ff.to
> 
> (pointed Michel to that earlier)
> 
> And that seems to be sorted now (just found positive feedback in my
> Inbox this morning), it was a question of the arch code supplying
> completely 'broken' topology information, and the scheduler trusting it
> too much.
> 
> The real fix in that thread is:
> 
>  lkml.kernel.org/r/20140722133514.GM12054@...top.lan
> 
> And I'll also add this to make the scheduler less trusting:
> 
>  lkml.kernel.org/r/20140722094740.GJ12054@...top.lan
> 
> Michael, that's not going to tell us what's wrong with your machine, as
> you've not got the ancient dual P4 Xeon Bruno's got. Seeing how your
> cpuinfo says:
> 
> model name      : AMD A10-7850K Radeon R7, 12 Compute Cores 4C+8G
> 
> but we can start the same debugging session I suppose.
> 
> Could you run with this patch on top:
> 
>  lkml.kernel.org/r/20140718101633.GP9918@...ns.programming.kicks-ass.net
> 
> And provide us with the dmesg after boot?

Attached. No FAIL messages yet.


-- 
Earthling Michel Dänzer            |                  http://www.amd.com
Libre software enthusiast          |                Mesa and X developer

View attachment "dmesg.txt" of type "text/plain" (83536 bytes)