Re: [PATCH 1/8] blk-mq: add blk_mq_max_nr_hw_queues()

From: Ming Lei <ming.lei@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-nvme@lists.infradead.org,
	"Martin K . Petersen" <martin.petersen@oracle.com>,
	linux-scsi@vger.kernel.org, linux-block@vger.kernel.org,
	Wen Xiong <wenxiong@linux.ibm.com>,
	Keith Busch <kbusch@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/8] blk-mq: add blk_mq_max_nr_hw_queues()
Date: Wed, 12 Jul 2023 21:16:11 +0800	[thread overview]
Message-ID: <ZK6nm2koR+TfeMcs@ovpn-8-25.pek2.redhat.com> (raw)
In-Reply-To: <20230712130017.GA12417@lst.de>

On Wed, Jul 12, 2023 at 03:00:17PM +0200, Christoph Hellwig wrote:
> On Wed, Jul 12, 2023 at 08:54:48PM +0800, Ming Lei wrote:
> > +/* Max nr_hw_queues for each hw queue type */
> > +unsigned int blk_mq_max_nr_hw_queues(void)
> > +{
> > +	if (is_kdump_kernel())
> > +		return 1;
> > +	return nr_cpu_ids;
> 
> Again, these is_kdump_kernel hacks don't make any sense.   The amount
> of maximum available CPU needs to come through a proper API, and we
> need to use it, not add hacks like this.
> 
> The only thing that makes sense here is to find the last CPU
> in cpu_possible_mask, and for kdump kernels to ensure that number
> is 1 or whatever low value they want.

It doesn't matter how many cpus are available, given at least one
cpu is online.

The problem is that blk_mq_alloc_tag_set() forces to set nr_hw_queues
as 1 for kdump kernel, that is why blk_mq_max_nr_hw_queues() has to
return 1 for kdump kernel.

We have to tell driver that blk-mq only supports 1 queue for kdump
kernel.

Or:

Thomas, can we disable managed irq for kdump kernel and switch to
non-managed irq? Then we can avoid driver's change. I'd suggest
this way if it is possible.

Thanks,
Ming