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

[Ming Lei]

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.

thanks,
Ming