linux-kernel - Re: [v4.6-rc1 Regression] x86/topology: Create logical package id

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Message-ID: <alpine.DEB.2.11.1605062046270.3540@nanos>
Date:	Fri, 6 May 2016 20:48:16 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Joseph Salisbury <joseph.salisbury@...onical.com>
cc:	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Andi Kleen <andi.kleen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Harish Chegondi <harish.chegondi@...el.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Jiri Olsa <jolsa@...hat.com>, Kan Liang <kan.liang@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>,
	Toshi Kani <toshi.kani@...com>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	LKML <linux-kernel@...r.kernel.org>, yingying.zhao@...el.com,
	"Zhang, Xiong Y" <xiong.y.zhang@...el.com>,
	1573231@...s.launchpad.net, 1397880@...s.launchpad.net,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Chen Yucong <slaoub@...il.com>,
	Laura Abbott <labbott@...oraproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [v4.6-rc1 Regression] x86/topology: Create logical package id

On Fri, 6 May 2016, Joseph Salisbury wrote:
> A kernel bug report was opened against Ubuntu [0].  After a kernel
> bisect, it was found that reverting the following commit resolved this bug:
> 
> commit 1f12e32f4cd5243ae46d8b933181be0d022c6793
> Author: Thomas Gleixner <tglx@...utronix.de>
> Date:   Mon Feb 22 22:19:15 2016 +0000
> 
>     x86/topology: Create logical package id
> 
> To build successfully with this commit reverted, I also had to revert
> commits: e7ee3e8,2d4de83,87f01cc and 33c3cc7.
> 
> The regression was introduced as of v4.6-rc1.
>     
> I was hoping to get your feedback, since you are the patch author.  Do
> you think gathering any additional data will help diagnose this issue,
> or would it be best to submit a revert request?

Yuck. That dies with a divide error. And that looks like XEN is supplying crap
data in the CPUID.

Does the patch below cure the issue?

Thanks,

        tglx

8<---------------

--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -332,6 +332,11 @@ static void __init smp_init_package_map(
 	 * primary cores.
 	 */
 	ncpus = boot_cpu_data.x86_max_cores;
+	if (!ncpus) {
+		pr_warn("x86_max_cores == zero !?!?");
+		ncpus = 1;
+	}
+
 	__max_logical_packages = DIV_ROUND_UP(total_cpus, ncpus);
 
 	/*