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

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Message-ID: <20230811131049.GA20991@lst.de>
Date:   Fri, 11 Aug 2023 15:10:49 +0200
From:   Christoph Hellwig <hch@....de>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, 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 Thu, Aug 10, 2023 at 08:09:27AM +0800, Ming Lei wrote:
> 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.

And we need to fix case 2.  We need to drop the is_kdump support, and
if they want to force less cpus they need to make nr_cpus=1 work.