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Date:	Fri, 25 Jul 2014 17:36:51 +0800
From:	Daniel J Blueman <daniel@...ascale.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Hillf Danton <dhillf@...il.com>,
	Borislav Petkov <bp@...64.org>, Ingo Molnar <mingo@...hat.com>,
	Igor Mammedov <imammedo@...hat.com>,
	Steffen Persvold <sp@...ascale.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [3.14] core onlining/hotplug regression

On 07/25/2014 05:05 PM, Thomas Gleixner wrote:
> On Fri, 25 Jul 2014, Daniel J Blueman wrote:
>> On a larger x86 system with 1728 cores, 3.15(.6) asserts on
>> smpboot_thread_fn's td->cpu != smp_processor_id() consistently after ~1500
>> cores are online.
>>
>> Reverting the only directly related changes I could find [1,2] doesn't help.
>> Debugging indicates there is a race where the created thread is quickly
>> migrated to core 0 when this occurs, since smp_processor_id returns 0 in these
>> cases. Thomas introduced a thread parked state to fix related issues a year
>> back. Linux 3.14(.13) boots just nice.
>
> Weird. Commits [1,2] are definitely not the culprits.
>
>> Full boot output is at:
>> https://resources.numascale.com/linux-315-thread-mig.txt
>
> Not really helpful, as we don't see what causes it. We just see the
> wreckage.
>
>> Any theories so far? I'll start bisecting when I have full access to the
>> system again in a week and I'll do some more debugging with intermittent
>> access before then.
>
> One thing you could try is enabling tracing.
>
>      "ftrace=function ftrace_dump_on_oops"
>
> It'll take a looooong time to spill out the traces, but that should
> give us the root cause precisely.

Good trick. I'll get this early next week and we'll see what's up.

Thanks,
   Daniel
-- 
Daniel J Blueman
Principal Software Engineer, Numascale
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