linux-kernel - Re: [RESEND PATCH v10] x86, apic, kexec, Documentation: Add disable_cpu

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Message-ID: <52D6CB57.8030804@zytor.com>
Date:	Wed, 15 Jan 2014 09:54:31 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>, hpa@...ux.intel.com,
	jingbai.ma@...com, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, bp@...en8.de, ebiederm@...ssion.com,
	akpm@...ux-foundation.org, fengguang.wu@...el.com
Subject: Re: [RESEND PATCH v10] x86, apic, kexec, Documentation: Add disable_cpu_apicid
 kernel parameter

On 01/15/2014 09:47 AM, Vivek Goyal wrote:
> On Wed, Jan 15, 2014 at 09:26:14AM -0800, H. Peter Anvin wrote:
>> On 01/15/2014 09:05 AM, Vivek Goyal wrote:
>>>
>>> I think this is a reasonable approach to solve the issue. Use a command
>>> line to not bring up specific cpu in second kernel which can create
>>> problems.
>>>
>>> Acked-by: Vivek Goyal <vgoyal@...hat.com>
>>>
>>> hpa, I know you are not excited about this approach. If you made up your
>>> mind that this appoarch is not worth pursuing, please do suggest what
>>> would you like to see and we can give that a try.
>>>
>>> We want to solve this problem as on large memory machines saving dump can
>>> take lot of time and we want to bring up multiple cpus and speed up
>>> compression and save on dump time.
>>>
>>
>> I'm not excited about kdump's reliance on the command line, since it
>> seems to be a neverending source of trouble, simply because the command
>> line is fundamentally intended as a human interface.
> 
> So in general, what are the alternatives? Either we figure out that kernel
> is booting as kdump kernel and do things differently. That seems even
> worse as what do we want in kdump kernel will change over a period of
> time.
> 
> Other thing is that pass more information in bootparams. But that does
> not seem much different than command line to me.
> 

It is the commingling of semantics that is the problem.  Command line
options are generally imperative, "do this".  What you want in the kdump
situation, as you yourself state above, is get a description of the
current situation and let the kdump side choose the action to take.

As a transport mechanism the command line suffers from limited size and
that you have to share it with an arbitrary amount of user-provided
options that may or may not be essential.

>>  However, this
>> seems relatively harmless in comparison with everything else and I am
>> much happier with saving the ID in the first kernel rather than trying
>> to guess if a currently-downed CPU is the BSP.
> 
> So looks like you are alright with this patch. Can you please queue it
> up in your tree.
> 

In process.

	-hpa


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