Message-ID: <52D6CB57.8030804@zytor.com>
Date: Wed, 15 Jan 2014 09:54:31 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Vivek Goyal <vgoyal@redhat.com>
CC: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>, hpa@linux.intel.com,
        jingbai.ma@hp.com, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org, bp@alien8.de, ebiederm@xmission.com,
        akpm@linux-foundation.org, fengguang.wu@intel.com
Subject: Re: [RESEND PATCH v10] x86, apic, kexec, Documentation: Add disable_cpu_apicid
 kernel parameter
References: <20140115064458.1545.38775.stgit@localhost6.localdomain6> <20140115170525.GF3180@redhat.com> <52D6C4B6.8010804@zytor.com> <20140115174714.GG3180@redhat.com>
In-Reply-To: <20140115174714.GG3180@redhat.com>
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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@redhat.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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