linux-kernel - Re: 4.5.0+ panic when setup loop device

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Message-ID: <alpine.DEB.2.11.1603181254330.3978@nanos>
Date:	Fri, 18 Mar 2016 12:55:56 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Xiong Zhou <jencce.kernel@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Andreas Herrmann <aherrmann@...e.com>
Subject: Re: 4.5.0+ panic when setup loop device

On Fri, 18 Mar 2016, 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.
> 
> Below is my 8 socket DL980 G7, which has SMT turned off for RT testing,
> booting NOPREEMPT master tuned for maximum bloat ala distro and getting
> confused by me telling it (as always) nr_cpus=64.  Bad juju ensues.

:)

Does the patch below fix the wreckage?

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 643dbdccf4bc..c5ac71276076 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -319,7 +319,7 @@ static void __init smp_init_package_map(void)
 	 * might change in the future....
 	 */
 	ncpus = boot_cpu_data.x86_max_cores * smp_num_siblings;
-	__max_logical_packages = DIV_ROUND_UP(nr_cpu_ids, ncpus);
+	__max_logical_packages = DIV_ROUND_UP(total_cpus, ncpus);
 
 	/*
 	 * Possibly larger than what we need as the number of apic ids per