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Date: Fri, 11 Aug 2023 15:10:49 +0200
From: Christoph Hellwig <hch@lst.de>
To: Ming Lei <ming.lei@redhat.com>
Subject: Re: [PATCH V3 01/14] blk-mq: add blk_mq_max_nr_hw_queues()
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References: <20230808104239.146085-1-ming.lei@redhat.com> <20230808104239.146085-2-ming.lei@redhat.com> <20230809134401.GA31852@lst.de> <ZNQqt1C0pXspGl3d@fedora>
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Cc: Jens Axboe <axboe@kernel.dk>, "Martin K . Petersen" <martin.petersen@oracle.com>, linux-scsi@vger.kernel.org, Pingfan Liu <piliu@redhat.com>, Dave Young <dyoung@redhat.com>, kexec@lists.infradead.org, linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, Wen Xiong <wenxiong@linux.ibm.com>, Baoquan He <bhe@redhat.com>, Keith Busch <kbusch@kernel.org>, linuxppc-dev@lists.ozlabs.org, Christoph Hellwig <hch@lst.de>
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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.