linux-kernel - Re: [PATCH V3 01/14] blk-mq: add blk_mq_max_nr_hw

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Date:   Thu, 10 Aug 2023 08:09:27 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-nvme@...ts.infradead.org,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-block@...r.kernel.org,
        Wen Xiong <wenxiong@...ux.ibm.com>,
        Keith Busch <kbusch@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
        Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Baoquan He <bhe@...hat.com>,
        Pingfan Liu <piliu@...hat.com>
Subject: 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/
    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.

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