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

From: Baoquan He <[email-masked]>
Date: Thu, 10 Aug 2023 09:18:27 +0800

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/[email-masked]/
>     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?

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.

> 
> 2) some archs do not support 'nr_cpus=1', and have to rely on
> 'max_cpus=1', so num_possible_cpus isn't changed, and kernel just boots
> with single online cpu. That causes trouble because blk-mq limits single
> queue.
> 
> Documentation/admin-guide/kdump/kdump.rst
> 
> Thanks, 
> Ming
>