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

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Date:	Wed, 02 Oct 2013 13:33:11 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Chen Gang F T <chen.gang.flying.transformer@...il.com>
Cc:	Chen Gang <gang.chen@...anux.com>,
	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 00:41 +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.

> 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.

Thanks,
-Toshi

> And I am analyzing (just constructing environments: KVM, kgdb ...),
> before let kernel start successfully, only according to my current
> proofs, we can not say "let laptop cpu0 offline" must be an issue.



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