On 10/08/23 8:31 am, Baoquan He wrote:
On 08/10/23 at 10:06am, Ming Lei wrote:
On Thu, Aug 10, 2023 at 09:18:27AM +0800, Baoquan He wrote:
On 08/10/23 at 08:09am, Ming Lei wrote:
On Wed, Aug 09, 2023 at 03:44:01PM +0200, Christoph Hellwig wrote:
I'm starting to sound like a broken record, but we can't just do random
is_kdump checks, and it's not going to get better by resending it again and
again. If kdump kernels limit the number of possible CPUs, it needs to
reflected in cpu_possible_map and we need to use that information.
Can you look at previous kdump/arch guys' comment about kdump usage &
num_possible_cpus?
https://lore.kernel.org/linux-block/CAF+s44RuqswbosY9kMDx35crviQnxOeuvgNsuE75Bb0Y2Jg2uw@xxxxxxxxxxxxxx/
https://lore.kernel.org/linux-block/ZKz912KyFQ7q9qwL@MiWiFi-R3L-srv/
The point is that kdump kernels does not limit the number of possible CPUs.
1) some archs support 'nr_cpus=1' for kdump kernel, which is fine, since
num_possible_cpus becomes 1.
Yes, "nr_cpus=" is strongly suggested in kdump kernel because "nr_cpus="
limits the possible cpu numbers, while "maxcpuss=" only limits the cpu
number which can be brought up during bootup. We noticed this diference
because a large number of possible cpus will cost more memory in kdump
kernel. e.g percpu initialization, even though kdump kernel have set
"maxcpus=1".
Currently x86 and arm64 all support "nr_cpus=". Pingfan ever spent much
effort to make patches to add "nr_cpus=" support to ppc64, seems ppc64
dev and maintainers do not care about it. Finally the patches are not
accepted, and the work is not continued.
Now, I am wondering what is the barrier to add "nr_cpus=" to power ach.
Can we reconsider adding 'nr_cpus=' to power arch since real issue
occurred in kdump kernel?
If 'nr_cpus=' can be supported on ppc64, this patchset isn't needed.
As for this patchset, it can be accpeted so that no failure in kdump
kernel is seen on ARCHes w/o "nr_cpus=" support? My personal opinion.
IMO 'nr_cpus=' support should be preferred, given it is annoying to
maintain two kinds of implementation for kdump kernel from driver
viewpoint. I guess kdump things can be simplified too with supporting
'nr_cpus=' only.
Yes, 'nr_cpus=' is ideal. Not sure if there's some underlying concerns so
that power people decided to not support it.
Though "nr_cpus=1" is an ideal solution, maintainer was not happy with
the patch as the code changes have impact for regular boot path and
it is likely to cause breakages. So, even if "nr_cpus=1" support for
ppc64 is revived, the change is going to take time to be accepted
upstream.
Also, I see is_kdump_kernel() being used irrespective of "nr_cpus=1"
support for other optimizations in the driver for the special dump
capture environment kdump is.
If there is no other downside for driver code, to use is_kdump_kernel(),
other than the maintainability aspect, I think the above changes are
worth considering.
Thanks
Hari
