Re: [PATCH v4 02/10] crash hp: Introduce CRASH_HOTPLUG configuration options

David Hildenbrand
2022-03-02T09:20:55Z

On 01.03.22 21:04, Eric DeVolder wrote:
>
>
> On 2/22/22 21:25, Baoquan He wrote:
>> On 02/09/22 at 02:56pm, Eric DeVolder wrote:
>>> Support for CPU and memory hotplug for crash is controlled by the
>>> CRASH_HOTPLUG configuration option, introduced by this patch.
>>>
>>> The CRASH_HOTPLUG_ELFCOREHDR_SZ related configuration option is
>>> also introduced with this patch.
>>>
>>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
>>> ---
>>> arch/x86/Kconfig | 26 ++++++++++++++++++++++++++
>>> 1 file changed, 26 insertions(+)
>>>
>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>>> index ebe8fc76949a..4e3374edab02 100644
>>> --- a/arch/x86/Kconfig
>>> +++ b/arch/x86/Kconfig
>>> @@ -2060,6 +2060,32 @@ 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 hotplugging a machine
>>> + with up to 1024 CPUs and up to 1024 memory regions (e.g. 1TiB
>>> + with 1024 1GiB memory DIMMs).
>>
>> This example of memory could be a little misleading. The memory regions
>> may not be related to memory DIMMs. System could split them into many
>> smaller regions during bootup.
>
> I changed "with 1024 1GiB memory DIMMs" to "with 1024 1GiB hotplug memories".
> eric

It's still not quite precise. Essentially it's the individual "System
RAM" entries in /proc/iomem

Boot memory (i.e., a single DIMM) might be represented by multiple
entries due to rearranged holes (by the BIOS).

While hoplugged DIMMs (under virt!) are usually represented using a
single range, it can be different on physical machines. Last but not
least, dax/kmem and virtio-mem behave in a different way.

--
Thanks,

David / dhildenb