kdump info request

Mukker, Atul Atul.Mukker at lsi.com
Mon Oct 1 10:35:28 EDT 2007

Hello Randy,

I tried this today but did not succeed :-( Can you please see the logs
appended below and point out what I might be doing wrong? This is on a
Red Hat 5 system.


Sample code
#include <linux/init.h>
#include <linux/version.h>
#include <linux/module.h>

MODULE_AUTHOR("LSI Corporation");
MODULE_DESCRIPTION("Kernel Command Line");

static int __init cline_init(void)
	printk("enter module:[%s]\n", saved_command_line);
	return 0;

static void __exit cline_exit(void)
	printk("exit module.\n");


obj-m	+= cline.o

Build log

make: Entering directory `/usr/src/kernels/2.6.18-8.el5-i686'
  CC [M]  /usr/src/kernels/2.6.18-8.el5-i686/drivers/scsi/cline/cline.o
  Building modules, stage 2.
WARNING: "saved_command_line"
  LD [M]  /usr/src/kernels/2.6.18-8.el5-i686/drivers/scsi/cline/cline.ko
make: Leaving directory `/usr/src/kernels/2.6.18-8.el5-i686'

-----Original Message-----
From: Randy Dunlap [mailto:rdunlap at xenotime.net] 
Sent: Saturday, September 22, 2007 2:36 PM
To: Mukker, Atul
Cc: vgoyal at in.ibm.com; Moore, Eric; Kexec Mailing List
Subject: Re: kdump info request

On Fri, 21 Sep 2007 06:48:01 -0600 Mukker, Atul wrote:

> One more question, and hopefully I will not bug you further on this
> How exactly do you parse the kernel parameters? I thought it would be
> plain simple, but seems like the command line string is not available
> SCSI driver like ours, or I am missing something?

#include <linux/init.h>  // for extern of 'saved_command_line' pointer.

and use calls to any functions in lib/cmdline.c or write your
own functions.  Should be fairly simple to scan for a fixed string
of "elfcorehdr=".
You don't care about the variable parameters.. or do you?

> Thanks
> -Atul 
> -----Original Message-----
> From: Vivek Goyal [mailto:vgoyal at in.ibm.com] 
> Sent: Friday, September 21, 2007 12:15 AM
> To: Mukker, Atul
> Cc: Kexec Mailing List; Moore, Eric
> Subject: Re: kdump info request
> On Wed, Sep 19, 2007 at 07:26:19AM -0600, Mukker, Atul wrote:
> [..]
> > > 
> > > If we could somehow determine that we are being called in context
> > capture kernel, we can dynamically lower driver memory requirement
> > cost of lower IO throughout of-course, which is ok for this brief
> > context).
> > 
> > You can parse the command line and look for presence of elfcorehdr=
> > option.
> > This is internally appended to command line by kexec tools to tell
> > capture
> > kernel the address of ELF core headers.
> > 
> > 
> > [AM] Is this a standard way? Doesn't look like one. According to
> > kernel-parameters.txt, kexec would "generally" pass this option to
> > kernel command line. Can we look at "struct resource crashk_res" and
> > check if start and end member have different value, which indicates
> > capture kernel?
> > 
> Well, nobody else so far has had such requirements so can't say if
> is standard way. But this is the best way I can think of so far.
> Using crashk_res will not work. Not all users will use kdump and will
> not
> reserve any memory for capture kernel. In that case crashk_res will be

> zero for start and end and you don't want to trim down the
> of your driver.
> > What's the memory allocation requirement of current RAID driver? How
> > much
> > memory you are reserving for capture kernel? Are you already seeing
> the
> > memory allocation failure?
> > 
> > [AM] Our normal runtime memory is about 20MB. For the test beds, we
> use
> > "crashkernel=192M at 16M". We have not yet seen the allocation failure
> but
> > we would like to build the fallback mechanism if it does fail under
> > capture kernel. Only if we are not able to get the normal runtime
> > memory, we plan to switch to a lower memory model.
> > 
> 20MB is huge. I agree that it is a good idea to bring it down for
> capture
> kernel if performance is not significantly impacted.
> > I feel until and unless memory requirements are huge, we should not
> > compromise with IO throughput. Capability to save the dump to disk
> > fast
> > as possible to reduce the down time is also an important
> consideration.
> > 
> > [AM] We believe our normal runtime memory requirement is
> > Also, even with the lower memory, there would not be a noticeable
> > time difference since lot of memory is for supporting multiple
> > outstanding commands in driver's raid core and other raid
> > which will not be running under capture kernel. Thanks again for
> > feedback.
> > 
> Makes sense.
> Thanks
> Vivek
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