Re: Firmware assisted dump support in dracut

[Mahesh Jagannath Salgaonkar <mahesh-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

On 06/04/2013 07:51 PM, Vivek Goyal wrote:
> On Tue, Jun 04, 2013 at 10:14:03AM -0400, Vivek Goyal wrote:
>> On Tue, Jun 04, 2013 at 01:50:19PM +0530, Mahesh Jagannath Salgaonkar wrote:
>>> On 05/30/2013 07:53 PM, Vivek Goyal wrote:
>>>> On Thu, May 30, 2013 at 08:49:07AM +0200, Harald Hoyer wrote:
>>>>
>>>> [..]
>>>>>> Sorry for restarting this discussion very late. I would like to know how
>>>>>> safe is to rebuild kernel's default (boot) initramfs for an already
>>>>>> installed kernel ?
>>>>>>
>>>>>> Also, Does dracut provides any of following mechanism?
>>>>>> a) Mechanism where dracut can detect what options were used during first
>>>>>> build for a given (exsisting) initramfs. (This mechanism may help one to
>>>>>> regenerate similar initramfs with additional dracut modules.)
>>>>>
>>>>> currently dracut only stores which modules were used to generate the image in
>>>>> usr/lib/dracut/modules.txt
>>>>>
>>>>> But yes, you are right. Would be nice to save all the options and have a
>>>>> mechanism to regenerate it with those.
>>>>>
>>>>
>>>> I am not sure how well it will work in the context of kdump.  kdump
>>>> options and original options might conflict. kdump might drop some of
>>>> its own cmdline options in initramfs which might not make any sense
>>>> in regular boot.
>>>>
>>>> Kdump will specify additional mount points. IIUC, then we will try
>>>> to bring up those targets in regular boot too and mount at user
>>>> specified mount point. And later init services might get confused
>>>> when respective daemon tries to bring up same targets again.
>>>
>>> I think we should be able differentiate between regular boot and boot
>>> after crash by checking existence of '/proc/vmcore' file, and do the
>>> kdump specific configs/mount if this file exists. In regular boot we can
>>> mute the kdump code path.
>>
>> Also what about kernel command line? Kernel command line can be very
>> specific to kdump environment. It is atleast in x86. I am hoping that's
>> not the case with firmware assisted dump.
> 
> If first kernel and kdump kernel command line are same in fadump and
> boot environment is same, I think it is worth exploring the option
> of saving kdump from kdump service when system boots back. That will
> do away with any need to tweak initramfs.

This needs larger memory to be reserved. We may need to make sure that
kdump service is invoked at very early stage before other services kicks
in which may start requesting for memory and run into OOM issue. If we
can make sure that no other services are invoked when kdump service is
saving dump then it will help to fix the size of reserved memory. The
systemd infrastructure provides aggressive parallelization capabilities.
I am not expert on systemd but will need to explore whether with systemd
is this possible.

> 
> It just requires more memory to be reserved so that system can boot
> , mount root, run services and then save dump. I am hoping that firmware
> can set aside enough memory to (1G?), to copy the memory contents and
> boot the kernel in that 1G.

It's the OS who reserves the memory from the available system RAM and
provides it to firmware to use. This means production kernel will have
set aside this memory for firmware to use.

> 
> Also does fadump work well with new mmap() interface /proc/vmcore is
> going to support. Patches are in -mm now. s390 zfcpdump is going to
> face some issues and michael is working on resolving these. I suspect
> that fadump will run into similar issues if firmware is copying memory
> contents somewhere else.

Will need to check this.

> 
> Thanks
> Vivek
>