On 12.05.21 16:51, Baoquan He wrote:
On 05/11/21 at 07:07pm, David Hildenbrand wrote:
If the way adding default value into kernel config is disliked,
this a) option looks good. We can get value with x% of system RAM, but
clamp it with CRASH_KERNEL_MIN/MAX. The CRASH_KERNEL_MIN/MAX may need be
defined with a default value for different ARCHes. It's very close to
our current implementation, and handling 'auto' in kernel.
And kernel config provided so that people can tune the MIN/MAX value,
but no need to post patch to do the tuning each time if have to?
Maybe I'm missing something, but the whole point is to avoid kernel
configuration option at all. If the crashkernel=auto works good for 99% of
the cases, there is no need to provide build time configuration along with
it. There are plenty of ways users can control crashkernel reservations
with the existing 2-4 (depending on architecture) command line options.
Simply hard coding a reasonable defaults (e.g.
"1G-64G:128M,64G-1T:256M,1T-:512M"), and using these defaults when
crashkernel=auto is set would cover the same 99% of users you referred to.
Right, and we can easily allocate a bit more as a safety net temporarily
when we can actually shrink the area later.
If we can resize the reservation later during boot this will also address
David's concern about the wasted memory.
Yes.
You mentioned that amount of memory that is required for crash kernel
reservation depends on the devices present on the system. Is is possible to
detect how much memory is required at late stages of boot?
Here is my thinking:
There seems to be some kind of formula we can roughly use to come up with
the final crashkernel size. Baoquan for sure knows all the dirty details, I
assume it's roughly "core kernel + drivers + user space".
In the kernel, we can only come up with "core kernel + drivers" expecting
that we will run
a) roughly the same kernel
b) with roughly the same drivers
As replied to Mike, kernel size is undecided for different kernel with
different configs. We can define a default minimal size to cover kernel
and driver on systems with not many devices, but hardcoding the size
into upstream is not helpful. If the size is big, users will be asked to
check and shrink always. If the size is too small, a new value need be
got and added to cmdline and reboot.
Hi Baoquan, Kairui, Dave,
so IIUC now, our "old" kernel cannot actually tell us any reliable
"crashkernel area size" because
a) it has no idea with which cmdline parameters the crashkernel will be
started with, and these can have a big impact.
b) it has no idea which driver will be loaded in the crashkernel.
c) It has no idea what will be running in the crashkernel user space.
AFAIKS, best we can do without further information is, therefore, use
some heuristic to a) allocate some memory early during boot in the
kernel and b) later refine our allocation, triggered by user space (->
shrink the crashkernel area).
I dislike calling a) "auto". It provides a default based on some
heuristic (boot memory size), and that default might be very unfortunate
in some scenarios (-> waste memory).
While we could discuss calling the current approach ( a)
)"crashkernel=default", whereby the default is encoded at compile time
as determined by a distributor, I still still quite don't like it
because it feels like this is not necessary. We have a way to pass
something like that via the cmdline, so it's just a matter of properly
using that feature from user space.
AFAIKS, all you want is most probably a more dynamic way to construct a
kernel cmdline, with some properties specific to a kernel.
Let's assume the following:
a) When a distributor ships a kernel, he also ships some kind of
defaults file. Let's assume for simplicity
/lib/modules/5.11.19-200.fc33.x86_64/defaults.conf
The file might contain
CRASHKERNEL_DEFAULT=WHATEVER
b) When generating the cmdline for e.g.,
/boot/loader/entries/XXX-5.11.19-200.fc33.x86_64.conf we run some script
that consult that file in addition to /etc/default/grub. For example, if
the kdump service was installed and /etc/default/grub does not contain
"crashkernel=" (except when we encounter "crashkernel=auto" for compat
handling), we add "crashkernel=WHATEVER". Of course, we might do more
involved stuff based on the current setup, user config, etc.
c) When we install the kdump service, all we have to do is re-generate
the boot entries AFAIKS. Just like we would when adding
"crashkernel=auto" right now.
The end result would also allow for having per-kernel defaults and
change them on kernel updates. Would require some thought on how to make
it fly in user space, how to "ship" the defaults etc.
--
Thanks,
David / dhildenb
