linux-kernel - Re: [RFC PATCH 0/5] crash dump bitmap: scan memory pages in kernel to speedup kernel dump process

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Message-ID: <513D9338.20402@hp.com>
Date:	Mon, 11 Mar 2013 16:18:00 +0800
From:	Jingbai Ma <jingbai.ma@...com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jingbai Ma <jingbai.ma@...com>, mingo@...hat.com,
	kumagai-atsushi@....nes.nec.co.jp, hpa@...or.com,
	yinghai@...nel.org, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/5] crash dump bitmap: scan memory pages in kernel
 to speedup kernel dump process

On 03/08/2013 11:52 PM, Vivek Goyal wrote:
> On Thu, Mar 07, 2013 at 01:54:45PM -0800, Eric W. Biederman wrote:
>> Vivek Goyal<vgoyal@...hat.com>  writes:
>>
>>> On Thu, Mar 07, 2013 at 10:58:18PM +0800, Jingbai Ma wrote:
>>>> This patch intend to speedup the memory pages scanning process in
>>>> selective dump mode.
>>>>
>>>> Test result (On HP ProLiant DL980 G7 with 1TB RAM, makedumpfile
>>>> v1.5.3):
>>>>
>>>> 						Total scan Time
>>>> Original kernel
>>>> 	+ makedumpfile v1.5.3 cyclic mode	1958.05 seconds
>>>> Original kernel
>>>> 	+ makedumpfile v1.5.3 non-cyclic mode	1151.50 seconds
>>>> Patched kernel
>>>> 	+ patched makedumpfile v1.5.3		17.50 seconds
>>>>
>>>> Traditionally, to reduce the size of dump file, dumper scans all memory
>>>> pages to exclude the unnecessary memory pages after capture kernel
>>>> booted, and scan it in userspace code (makedumpfile).
>>>
>>> I think this is not a good idea. It has several issues.
>>
>> Actually it does not appear to be doing any work in the first kernel.
>
> Looks like patch3 in series is doing that.
>
>                          machine_crash_shutdown(&fixed_regs);
> +                       generate_crash_dump_bitmap();
>                          machine_kexec(kexec_crash_image);
>
> So this bitmap seems to be being set just before transitioning into
> second kernel.
>
> I am sure you would not like this extra code in this path. :-)

I was thought this function code is pretty simple, could be called here 
safely.
If it's not proper for here, how about before the function 
machine_crash_shutdown(&fixed_regs)?
Furthermore, could you explain the real risks to execute more codes here?

Thanks!

>
> Thanks
> Vivek


-- 
Jingbai Ma (jingbai.ma@...com)
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