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Date:	Fri, 25 Jul 2014 11:05:01 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Daniel J Blueman <daniel@...ascale.com>
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 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.

Thanks,

	tglx


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