Re: [PATCH v7 2/8] x86/crash: Introduce new options to support cpu and memory hotplug

[Eric DeVolder]

On 4/25/22 23:21, Sourabh Jain wrote:
> On 13/04/22 22:12, Eric DeVolder wrote:
> > CRASH_HOTPLUG is to enable cpu and memory hotplug support of crash.
> >
> > CRASH_HOTPLUG_ELFCOREHDR_SZ is used to specify the maximum size of
> > the elfcorehdr buffer/segment.
> >
> > This is a preparation for later usage.
> >
> > Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
> > Acked-by: Baoquan He <bhe@xxxxxxxxxx>
> > ---
> >   arch/x86/Kconfig | 26 ++++++++++++++++++++++++++
> >   1 file changed, 26 insertions(+)
> >
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index b0142e01002e..f7b92ee1bcc7 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -2072,6 +2072,32 @@ config CRASH_DUMP
> >         (CONFIG_RELOCATABLE=y).
> >         For more details see Documentation/admin-guide/kdump/kdump.rst
> > +config CRASH_HOTPLUG
> > +    bool "kernel updates of crash elfcorehdr"
> > +    depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG) && KEXEC_FILE
> > +    help
> > +      Enable the kernel to update the crash elfcorehdr (which contains
> > +      the list of CPUs and memory regions) directly when hot plug/unplug
> > +      of CPUs or memory. Otherwise userspace must monitor these hot
> > +      plug/unplug change notifications via udev in order to
> > +      unload-then-reload the crash kernel so that the list of CPUs and
> > +      memory regions is kept up-to-date. Note that the udev CPU and
> > +      memory change notifications still occur (however, userspace is not
> > +      required to monitor for crash dump purposes).
> > +
> > +config CRASH_HOTPLUG_ELFCOREHDR_SZ
> > +    depends on CRASH_HOTPLUG
> > +    int
> > +    default 131072
> > +    help
> > +      Specify the maximum size of the elfcorehdr buffer/segment.
> > +      The 128KiB default is sized so that it can accommodate 2048
> > +      Elf64_Phdr, where each Phdr represents either a CPU or a
> > +      region of memory.
> > +      For example, this size can accommodate a machine with up to 1024
> > +      CPUs and up to 1024 memory regions, eg. as represented by the
> > +      'System RAM' entries in /proc/iomem.
>
> Is it possible to get rid of CRASH_HOTPLUG_ELFCOREHDR_SZ?
At the moment, I do not think so. The idea behind this value is to represent the largest number of 
CPUs and memory regions possible in the system. Today there is NR_CPUS which could be used for CPUs, 
but there isn't a similar value for memory. I also am not aware of a kernel variable that could be 
utilized to represent the maximum number of memory regions. If there is, please let me know!
> How about finding the additional buffer space needed for future CPU and memory
> add during the kdump load? Not sure about the feasibility of doing this in
> kexec tool (userspace).

I may not understand what you are asking, but the x86 code, for kexec_file_load, does in fact 
allocate all the space needed (currently via CRASH_HOTPLUG_ELFCOREHDR_SZ) upon kdump load.

For kexec_load, I've had no problem asking the kexec tool to allocate a larger piece of memory for 
the elfcorehdr. But it is the same problem as CRASH_HOTPLUG_ELFCOREHDR_SZ; how big? In my workspace 
I tell kexec tool how big. If there are sysfs visible values for NR_CPU and memory, then we could 
have kexec pull those and compute.

I do think the important thing is that this allocation needs to happen once (for either kexec_load 
or kexec_file_load), so that the buffer is always in the same spot and thus the pointer to that 
buffer does not change; else boot_params cmdline would need to change. I once had this coded this 
way, but Baoquan pointed out this simpler way.

Regards,
eric

> Thanks,
> Sourabh Jain