Re: [PATCH V3 01/14] blk-mq: add blk_mq_max_nr_hw_queues()

[Hari Bathini]

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