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

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

[Ming Lei]

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@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 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

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Re: [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@redhat.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: virtualization@lists.linux-foundation.org
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

superficially:

Acked-by: Michael S. Tsirkin <mst@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

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