Re: [patch 0/9] kdump: Patch series for s390 support

[Vivek Goyal]

On Fri, Jul 08, 2011 at 03:04:03PM +0200, Michael Holzheu wrote:
> Hello Vivek,
> 
> On Thu, 2011-07-07 at 15:33 -0400, Vivek Goyal wrote:
> > > Another advantage is
> > > that since it is different code, it is much less likely that the dump
> > > tool will run into the same problem than the previously crashed kernel.
> > 
> > I think in practice this is not really a problem. If your kernel
> > is not stable enough to even boot and copy a file, then most likely
> > it has not even been deployed. The very fact that a kernel has been
> > up and running verifies that it is a stable kernel for that machine
> > and is capable of capturing the dump.
> 
> I don't want to argue, about probabilities. Even if we gain only a
> little more reliability this is important for us. Don't forget that we
> write software for mainframes. We accept that the last 0.1 percent of
> reliability can be very expensive compared to the first 99.9 percent.
> 
> [snip]
> 
> > > And last but not least, with the stand-alone dump tools you can
> > > dump early kernel problems which is not possible using kdump, because
> > > you can't dump before the kdump kernel has been loaded with kexec.
> > > 
> > 
> > That is one limitation but again if your kernel can't even boot,
> > it is not ready to ship and it is more of a development issue and
> > there are other ways to debug problems. So I would not worry too
> > much about it.
> 
> We worry about that. See the comment above regarding the 100 percent.
> 
> > On a side note, few months back there were folks who were trying
> > to enhance bootloaders to be able to prepare basic environment so
> > that a kdump kernel can boot even in the event of early first
> > kernel boot.
> 
> This is one more argument to create the ELF header in the 2nd kernel.
> With our approach loading the kdump kernel at boot time is almost
> trivial.

I think ELF header is just the way of passing some required information
from first kernel to second kernel. In second kernel, we anyway prepare
fresh headers for /proc/vmcore.

So in your mechanism if you don't need any info from second kernel it
is fine to not use ELF. But if you do need, then it makes sense to
use existing mechanism instead of creating a new one (seems to be
meminfo in your case).

I think at the end of the day it would not matter much whether kexec-tools
created those headers or boot loader did. But there are advantages to
doing things in kexec-tools.

- A user space is fully booted and it provides scope for enhancements and
  intellingent things.

	- Depending on dump target a user can filter out some of the
	  modules from kdump ramdisk and reduce the size of memory
	  required. With a pure bootloader approach, I guess one will
	  do the change, generate a new initrd and then reboot the
	  system.

	  With kexec-tools it is just a matter of regnerating initrd
	  and reloading the kernel using kexec system call.

	  So we avoid extra reboot.

This is just one of the arguments. I think key thing here seems to be
that whatever kexec-tools is doing, should we do that in bootloader 
to serve the case of early crash. 

IMHO, I am not too concerned about early crash at this point of time for
the simple reason that you can't even deploy the kernel which can't boot.
This is a developer environment issue and and not a customer deployment
scenario. But other people ofcourse might have different requirement.

So cater to those requirements, I think it is fine that bootloader
does what kexec-tools is doing. Load kdump kernel, tell first kernel
about it, load purgatory (which enables transition between two kernels,
does checksums, sets up right page tables etc). Looks like s390
wants to take this path, I guess it is fine as long as it is clear
from the patches.

> 
> Example (e.g. crashkernel=xxxM@256M):
> 
> 1. The boot loader loads standard kernel and kdump kernel into memory.
> The kdump kernel is loaded into crashkernel memory to 256M. No more
> setup (e.g. creating ELF headers) is necessary.
> 2. We could add a kernel parameter "kexec_load=<segm addr>,<segm
> size>, ..." that does an internal kexec_load(). After this kernel
> parameter is processed, kdump is armed.

I think I am not worried about kexec_load() as such. I am just trying
to understand the theme of the patchset and a mixed approach of using
kexec-tools as well as using boot loader is confusing me. 

I am still trying to figure out what is short term plan and what is
long term and whether you are going for kexec-tools as bootloader
for loading kdump kernel approach or s390 boot loader loading second
kernel approach.

> 
> What do you think?
> 
> > > That were more or less the arguments, why we did not support kdump in
> > > the past.
> > > 
> > > In order to increase dump reliability with kdump, we now implemented a
> > > two stage approach. The stand-alone dump tools first check via meminfo,
> > > if kdump is valid using checksums. If kdump is loaded and healthy it is
> > > started. Otherwise the stand-alone dump tools create a full-blown
> > > stand-alone dump.
> > 
> > kexec-tools purgatory code also checks the checksum of loaded kernel
> > and other information and next kernel boot starts only if nothing
> > has been corrupted in first kernel. 
> 
> Can you point me to the code where this is done and from where in the
> kernel that code is called? Currently with our implementation we do not
> use any purgatory code from kexec tools.

kexec-tools/purgatory/purgatory.c (verify_sha256_digest()).

> > and need of checksums sounds unnecessary. I think what you do need is
> > that somehow invoking second hook (s390 specific stand alone kernel)
> > in case primary kernel is corrupted.
> > > 
> > > With this approach we still keep our s390 dump reliability and gain the
> > > great kdump features, e.g. distributor installer support, dump filtering
> > > with makedumpfile, etc.
> > > 
> > > > why the existing
> > > > mechanism of preparing ELF headers to describe all the above info
> > > > and just passing the address of header on kernel commnad line
> > > > (crashkernel=) will not work for s390. Introducing an entirely new
> > > > infrastructure for communicating the same information does not
> > > > sound too exciting.
> > > 
> > > We need the meminfo interface anyway for the two stage approach. The
> > > stand-alone dump tools have to find and verify the kdump kernel in order
> > > to start it.
> > 
> > kexec-tools does this verification already. We verify the checksum of
> > all the loaded information in reserved area. So why introduce this
> > meminfo interface.
> 
> Ok, where is this done and when?

kexec-tools prepares a binary shim (we call purgatory) which is loaded
in kernel using kexec system call. After system crash control is passed
to this pargatory which verifies the checksums of all the loaded
segments and jumps to entry point of second kernel.

verify_sha256_digest() is the function which does all the verification
and loops forever if checksums don't match.

> 
> > > Therefore the interface is there and can be used. Also
> > > creating the ELF header in the 2nd kernel is more flexible and easier
> > > IMHO:
> > > * You do not have to care about memory or CPU hotplug.
> > 
> > Reloading the kernel upon memory or cpu hotplug should be trivial. This
> > does not justify to move away from standard ELF interface and creation
> > of a new one.
> > 
> > > * You do not have to preallocate CPU crash notes etc.
> > 
> > Its a small per cpu area. Looks like otherwise you will create meminfo
> > areas otherwise.
> > 
> > > * It works independently from the tool/mechanism that loads the kdump
> > > kernel into memory. E.g. we have the idea to load the kdump kernel at
> > > boot time into the crashkernel memory (not via the kexec_load system
> > > call). That would solve the main kdump problems: The kdump kernel can't
> > > be overwritten by I/O and also early kernel problems could then be
> > > dumped using kdump.
> > 
> > Can you give more details how exactly it works. I know very little about
> > s390 dump mechanism.
> 
> Maybe I confused you here. What I wanted to describe is the following
> idea:
> 1. The running production kernel starts with "crashkernel=" and reserves
> memory for kdump. No kdump is loaded with kexec.
> 2. The system crashes
> 3. To create the dump, a prepared dump disk is booted. The boot loader
> loads the kdump kernel into crashkernel memory.
> 4. The boot loader starts kdump kernel on s390 with entry point
> <crashkernel base> + 0x10008
> 5. The kdump kernel creates ELF header etc...
> 
> So this is simple for the boot loader code because no preparation steps
> like creating the ELF header are required. This is similar to scenario
> of pre-loading the kdump kernel together with the standard kernel at
> startup that I described above.
> 
> > 
> > When do you load kdump kernel and who does it?
> 
> Currently we load the kdump kernel with kexec like it is done on all
> other architectures. The other options I described above are currently
> just ideas that we have for the future.

So bootloader doing everything is future idea and for the time we still
use kexec_load() for loading kernel? If yes, then we can stop worrying
about early crash kernel case till you implement the future idea?

In fact, if kdump kenrel is not loaded, your existing mechanism of
IPLing stand alone tools should work as it without any modifications,
isn't it? This does not provide you filtering capability in early
crash but does retain ability to capture dumps.

> 
> > Who gets the control first after crash?
> > 
> > To me it looked like that you regularly load kdump kernel and if that
> > is corrupted then somehow you boot standalone kernel. So corruption
> > of kdump kernel should not be a issue for you.
> 
> As Martin already said: It can be the other way round. The stand-alone
> dump tool gets first control. We trust this code because it is freshly
> loaded and has a different code base.

I am not sure having a differnt code base means more reliability or
less reliability. It might also mean a less tested code and less
reliable. But anyway, I will not get into that debate as things have
been working for you.

> This code verifies the kdump setup
> and jumps into the pre-loaded kdump (crashkernel base + 0x10008) if
> everything is ok. Otherwise it creates a traditional s390 dump.

Ok, so the code which does the verification and takes the decision of
either booting kdump kernel or stand alone kernel is part of dump tools?
Is it loaded fresh into memory after crash and who does that?

If you are going for kexec-tools based appraoch, then as I said in
previous mail, looks like you can just create s390 specific purgatory
and just reuse the infrastructure for checksum verification. You
just need to do little enahnacement so that if kdump kernel is
corrupted, you jump to the code which loads s390 stand alone kernel
instead of looping forever.

Thanks
Vivek
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