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

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@mail.gmail.com/ (local)
    https://lore.kernel.org/linux-block/ZKz912KyFQ7q9qwL@MiWiFi-R3L-srv/ (local)

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.

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