kdump info request

[Vivek Goyal]

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