Linux Virtualization - Jun 2016 - [PATCH] drivers: virtio_blk: notify blk-core when hw-queue number changes

On 13/06/2016 11:58, Bob Liu wrote:
> A guest might be migrated to other hosts with different num_queues, the
> blk-core should aware of that else the reference of &vblk->vqs[qid]
may be wrong.
> 
> Signed-off-by: Bob Liu <bob.liu at oracle.com>
> ---
>  drivers/block/virtio_blk.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 42758b5..c169238 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -819,6 +819,9 @@ static int virtblk_restore(struct virtio_device *vdev)
>  	if (ret)
>  		return ret;
>  
> +	if (vblk->num_vqs != vblk->tag_set.nr_hw_queues)
> +		blk_mq_update_nr_hw_queues(&vblk->tag_set, vblk->num_vqs);
> +
>  	virtio_device_ready(vdev);
>  
>  	blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
> 
This should never happen; it'd be a configuration problem.

Paolo

On 06/19/2016 06:10 AM, Paolo Bonzini wrote:
> 
> 
> On 13/06/2016 11:58, Bob Liu wrote:
>> A guest might be migrated to other hosts with different num_queues, the
>> blk-core should aware of that else the reference of
&vblk->vqs[qid] may be wrong.
>>
>> Signed-off-by: Bob Liu <bob.liu at oracle.com>
>> ---
>>  drivers/block/virtio_blk.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>> index 42758b5..c169238 100644
>> --- a/drivers/block/virtio_blk.c
>> +++ b/drivers/block/virtio_blk.c
>> @@ -819,6 +819,9 @@ static int virtblk_restore(struct virtio_device
*vdev)
>>  	if (ret)
>>  		return ret;
>>  
>> +	if (vblk->num_vqs != vblk->tag_set.nr_hw_queues)
>> +		blk_mq_update_nr_hw_queues(&vblk->tag_set, vblk->num_vqs);
>> +
>>  	virtio_device_ready(vdev);
>>  
>>  	blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
>>
> 
> This should never happen; it'd be a configuration problem.
> 
Do you mean all hosts have to be configured with the same number of
->num_vqs?
What about cases like migrating a guest from HostA to HostB while HostB is much
more powerful
and would like to run more hardware queues to get better performance.

Thanks,
Bob Liu

On 28/07/2016 10:18, Bob Liu wrote:
>>> >> A guest might be migrated to other hosts with different
num_queues, the
>>> >> blk-core should aware of that else the reference of
&vblk->vqs[qid] may be wrong.
>>> >>
>>> >> Signed-off-by: Bob Liu <bob.liu at oracle.com>
>>> >> ---
>>> >>  drivers/block/virtio_blk.c | 3 +++
>>> >>  1 file changed, 3 insertions(+)
>>> >>
>>> >> diff --git a/drivers/block/virtio_blk.c
b/drivers/block/virtio_blk.c
>>> >> index 42758b5..c169238 100644
>>> >> --- a/drivers/block/virtio_blk.c
>>> >> +++ b/drivers/block/virtio_blk.c
>>> >> @@ -819,6 +819,9 @@ static int virtblk_restore(struct
virtio_device *vdev)
>>> >>  	if (ret)
>>> >>  		return ret;
>>> >>  
>>> >> +	if (vblk->num_vqs != vblk->tag_set.nr_hw_queues)
>>> >> +		blk_mq_update_nr_hw_queues(&vblk->tag_set,
vblk->num_vqs);
>>> >> +
>>> >>  	virtio_device_ready(vdev);
>>> >>  
>>> >>  	blk_mq_start_stopped_hw_queues(vblk->disk->queue,
true);
>>> >>
>> > 
>> > This should never happen; it'd be a configuration problem.
>> > 
> Do you mean all hosts have to be configured with the same number of
->num_vqs?
> What about cases like migrating a guest from HostA to HostB while HostB is
much more powerful
> and would like to run more hardware queues to get better performance.
The number of queues should be equal to the number of CPUs for the
guest, not to the number of CPUs or queues on the host.  The idea is to
have one queue per guest vCPU.

Paolo