kdump info request

[Randy Dunlap]

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 to
> 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 of
> > capture kernel, we can dynamically lower driver memory requirement (at
> > 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 this
> 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 functionality
> 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 as
> > fast
> > as possible to reduce the down time is also an important
> consideration.
> > 
> > [AM] We believe our normal runtime memory requirement is significant.
> > Also, even with the lower memory, there would not be a noticeable dump
> > time difference since lot of memory is for supporting multiple
> > outstanding commands in driver's raid core and other raid operations,
> > which will not be running under capture kernel. Thanks again for your
> > feedback.
> > 
> 
> Makes sense.
> 
> Thanks
> Vivek
> 
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---
~Randy
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