Re: [RFD] efi assisted kdump

[Dave Young <dyoung@redhat.com>]

Hi, Petr

On 01/24/15 at 05:03pm, Petr Tesarik wrote:
> On Sat, 24 Jan 2015 21:26:37 +0800
> Dave Young <dyoung@redhat.com> wrote:
> 
> > Hi,
> > 
> > Kdump has several limitations currently such as kdump kernel reboot will bypass
> > device shutdown path so device drivers should reset during initialization.
> > 
> >[...]
> > 
> > ppc64 has a feature "firmware assisted kdump", see below documentation:
> > Documentation/powerpc/firmware-assisted-dump.txt
> 
> Hi Dave,
> 
> while I'm no expert on either UEFI or IBM POWER, I'd like to warn you
> that fadump (firmware-assisted dump) on PPC is not quite optimal in its
> current form. One of the things that have always irritated me are
> excessive RAM requirements.
> 
> The problem is that there is only one reboot in fadump - after saving
> the dump, the secondary kernel discards the saved area and continues
> booting as usual. However, many kernel structuers must be already
> allocated at that point, e.g. the memmap array(s), but they are sized
> by the total RAM, not the limited amount available to the 2nd kernel.

Thanks for the comment.

I'm not sure I understand the RAM requirements you mentioned. I think you
are worrying about freeing oldmem to be used by 2nd kernel.

But we do not need to do same as power we can just reboot another time
as long as vmcore is saved. The main advantage is the capture kernel can
boot with all devices being reset. 

> 
> This issue is not so bad on a typical IBM POWER system, because these
> machines tend to be partitioned into many small LPARs, where each has
> relatively little RAM. But on a machine with 6TB RAM, you'll have to
> reserve approx. 84G merely for the memmap array on the secondary
> kernel's boot, which looks just a bit wasteful...
> 
> Maybe the two approaches can be combined somehow, so that the
> kdump system uses kexec() to transition into the production system, but
> that only works if a normal kexec does not have any of the issues
> you're trying to address.
> 
> Just my two cents,
> Petr Tesarik
> 
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