linux-kernel - Re: [Suggestion] about calling debug_hotplug_cpu() which enabled by 'allmodconfig' for a x86

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Date:	Thu, 03 Oct 2013 08:51:32 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Chen Gang <gang.chen@...anux.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	"rafael.j.wysocki@...el.com" <rafael.j.wysocki@...el.com>,
	"isimatu.yasuaki@...fujitsu.com" <isimatu.yasuaki@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Suggestion] about calling debug_hotplug_cpu() which enabled by
 'allmodconfig' for a x86_64 dual core laptop.

On Thu, 2013-10-03 at 00:49 +0000, Chen Gang wrote:
> On 10/03/2013 03:33 AM, Toshi Kani wrote:
> > On Thu, 2013-10-03 at 00:41 +0800, Chen Gang F T wrote:
> >> On 10/02/2013 11:28 PM, Toshi Kani wrote:
> >>> On Wed, 2013-10-02 at 03:10 +0000, Chen Gang wrote:
> >>>> Hello Maintainers:
> >>>>
> >>>> Under my x86_64 dual core laptop, I build kernel next-20130927 with
> >>>> 'allmodconfig', and install it, the machine can not start. Related
> >>>> information is:
> >>>>
> >>>>   after call debug_hotplug_cpu(), output "cpu 0 is offline" .... and then "Failed to execute /init".
> >>>>
> >>>> After remove "_debug_hotplug_cpu(0, 0);", can pass the issue (but will
> >>>> fail in another place). I guess, the reason is my laptop cpu is not
> >>>> 'hotplug', but have to call debug_hotplug_cpu() with allmodconfig.
> >>>>
> >>>>
> >>>> I will continue analyzing, welcome any additional suggestions or
> >>>> completions.
> >>>
> >>> allmodconfig sets CONFIG_DEBUG_HOTPLUG_CPU0 to y (which is defined as
> >>> "def_bool n"), which puts CPU0 offline during boot for testing.  This
> >>> debug feature is causing the problem on your laptop.  I am not familiar
> >>> with allmodconfig, but it seems that it enables all the config options,
> >>> preferably with 'm'.
> >>>
> >>
> >> Hmm... excuse me, I don't know: for laptop, if cpu0 is offline, whether
> >> it still can work or not. If any members know about it, please tell me,
> >> thanks.
> > 
> > CPU online/offline works on laptops as long as they have more than one
> > CPU.  You can boot with other kernel and test this feature
> > with /sys/devices/system/cpu/cpuX/online on your laptop.  cpu0 is a
> > special case and you may need to specify "cpu0_hotplug" boot option.
> > 
> 
> Thank you for your confirmation, "for dual core (although laptop), after
> system boot up, can let cpu0 offline". :-)

Good. :)

> >> In my opinion, if enable DEBUG_HOTPLUG_CPU0, but "should not let cpu0
> >> offline for laptop", we need let 'offline' operation fail.
> > 
> > Whether it is a laptop or not should not be a matter here.  But I agree
> > with you that enabling CONFIG_DEBUG_HOTPLUG_CPU0 is not useful (and can
> > be harmful) unless a user really intends to test this feature.
> > 
> 
> Hmm... I will continue analyzing. After finish analyzing (let kernel
> boot up OK), I should give a confirmation: "during boot up, can we still
> let cpu0 offline for dual core?".
> 
> In fact, 'allmodconfig' is just a mad user which often does useless
> things to intend to find issues.

I see.  If this option is used for testing, enabling DEBUG_HOTPLUG_CPU0
does make sense.

Thanks,
-Toshi


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