On 10.05.21 13:44, Dave Young wrote:
Hi David,
Hi Dave,
On 05/10/21 at 01:01pm, David Hildenbrand wrote:
[snip]
It also bugged me for quite a bit that we don't have a sane way to achieve
what we're doing here upstream. It somewhat feels like "this doesn't belong
in the kernel and is user policy" but then, the existing kernel support is
suboptimal.
Maybe reserving some "maybe too big but okayish to boot the system in a sane
environment -- e.g., X% of system RAM and at least Y" size first and
shrinking it later as triggered by user space early (where we do seem to
have a way to pre-calculate things now) might actually be a good direction
to look into.
Hmm, that is also an option we considered before. Even for your
suggestion we still need a kernel option to set the default ratio/value.
and the ratio/value should be another patch which expands crashkernel
syntax.
Right.
Actually the kconfig help text in this patch is indeed misleading, it is
not introducing crashkernel=a:b... and no need to explain about the
crashkernel syntax, the config option is actually just some interface we
can add any valid crashkernel settings to be used by default. So current
patch help text describes the default value of crash auto str, instead
of describes what crash auto str is.
Right. And I would much rather prefer either
a) handling "auto" completely in the kernel, not just setting some
questionable default at compile time
b) passing it explicitly in via the cmdline
And crashkernel=auto makes this more flexibly. We can tune the values
easily when upgrading. But if we pass a fixed value in userspace we
can not know if the value is set by distribution automatically or by user
manually thus we can not blindly update it.
I think there are two different cases:
1. kernel space updates the value later during boot. "crashkernel=auto"
really does the right thing, meaning
a) allocate something reasonable and safe during early boot
b) update the allocation during late boot when we know what kind of
system we're running on
Then, we indeed care about "crashkernel=auto" in the kernel and I think
it would be a nice thing to have. The only question is on how to make
that a little configurable, depending on different thingies we might
want to run in the crashkernel (assuming someone doesn't want kdump).
2. user space updates the value later during boot
IMHO we don't really car who decided on the value as we do the update
from user space. If an admin messes with crashkernel=, the admin can
also mess with kdump not doing any overwrites (e.g., make that
configurable, or detect the overwrite in kdump somehow).
--
Thanks,
David / dhildenb
