[PATCH V3 04/14] virtio-blk: limit max allowed submit queues

[Michael S. Tsirkin]

On Tue, Aug 08, 2023 at 06:42:29PM +0800, Ming Lei wrote:
> Take blk-mq's knowledge into account for calculating io queues.
> 
> Fix wrong queue mapping in case of kdump kernel.
> 
> On arm and ppc64, 'maxcpus=1' is passed to kdump command line, see
> `Documentation/admin-guide/kdump/kdump.rst`, so num_possible_cpus()
> still returns all CPUs because 'maxcpus=1' just bring up one single
> cpu core during booting.
> 
> blk-mq sees single queue in kdump kernel, and in driver's viewpoint
> there are still multiple queues, this inconsistency causes driver to apply
> wrong queue mapping for handling IO, and IO timeout is triggered.
> 
> Meantime, single queue makes much less resource utilization, and reduce
> risk of kernel failure.
> 
> Cc: Michael S. Tsirkin <mst at redhat.com>
> Cc: Jason Wang <jasowang at redhat.com>
> Cc: virtualization at lists.linux-foundation.org
> Signed-off-by: Ming Lei <ming.lei at redhat.com>

superficially:

Acked-by: Michael S. Tsirkin <mst at redhat.com>

but this patch only makes sense if the rest of patchset is merged.
feel free to merge directly.

> ---
>  drivers/block/virtio_blk.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 1fe011676d07..4ba79fe2a1b4 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -1047,7 +1047,8 @@ static int init_vq(struct virtio_blk *vblk)
>  
>  	num_poll_vqs = min_t(unsigned int, poll_queues, num_vqs - 1);
>  
> -	vblk->io_queues[HCTX_TYPE_DEFAULT] = num_vqs - num_poll_vqs;
> +	vblk->io_queues[HCTX_TYPE_DEFAULT] = min_t(unsigned,
> +			num_vqs - num_poll_vqs, blk_mq_max_nr_hw_queues());
>  	vblk->io_queues[HCTX_TYPE_READ] = 0;
>  	vblk->io_queues[HCTX_TYPE_POLL] = num_poll_vqs;
>  
> -- 
> 2.40.1