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Date: Fri, 18 Mar 2016 08:51:15 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Xiong Zhou <jencce.kernel@gmail.com>, Thomas Gleixner <tglx@linutronix.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Andreas Herrmann <aherrmann@suse.com>
Subject: Re: 4.5.0+ panic when setup loop device
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On Fri, Mar 18, 2016 at 05:11:54AM +0100, Mike Galbraith wrote:
> On Thu, 2016-03-17 at 10:52 +0100, Peter Zijlstra wrote:
> 
> > Andreas; Borislav said to Cc you since you wrote all this.
> > The issue is that Linux assumes:
> > 
> > 	> nr_logical_cpus = nr_cores * nr_siblings
> 
> It also seems to now assume that if SMT is possible, it's enabled.

Urgh..

What I think, with my pre wakeup brain, happens is that the CPUID
topology muck still reports 2 siblings (it tends to do that).

But the BIOS only reports APIC-IDs for all your cores, so our
nr_cpu_ids is reduced, while the nr_siblings count it not.

And them *boom*.

This is the same old problem that is nearly impossible to tell if HT is
enabled or not -- complete and utter trainwreck :/

I need to go make wake-up-juice and ponder wth to do about this.