Re: [Patch v4] storvsc: setup 1:1 mapping between hardware queue and CPU queue

[Sasha Levin]

On Fri, Sep 06, 2019 at 10:24:20AM -0700, longli@xxxxxxxxxxxxxxxxx wrote:
> From: Long Li <longli@xxxxxxxxxxxxx>
>
> storvsc doesn't use a dedicated hardware queue for a given CPU queue. When
> issuing I/O, it selects returning CPU (hardware queue) dynamically based on
> vmbus channel usage across all channels.
>
> This patch advertises num_present_cpus() as number of hardware queues. This
> will have upper layer setup 1:1 mapping between hardware queue and CPU queue
> and avoid unnecessary locking when issuing I/O.
>
> Signed-off-by: Long Li <longli@xxxxxxxxxxxxx>
> ---
>
> Changes:
> v2: rely on default upper layer function to map queues. (suggested by Ming Lei
> <tom.leiming@xxxxxxxxx>)
> v3: use num_present_cpus() instead of num_online_cpus(). Hyper-v doesn't
> support hot-add CPUs. (suggested by Michael Kelley <mikelley@xxxxxxxxxxxxx>)
> v4: move change logs to after Signed-of-by
>
> drivers/scsi/storvsc_drv.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> index b89269120a2d..cf987712041a 100644
> --- a/drivers/scsi/storvsc_drv.c
> +++ b/drivers/scsi/storvsc_drv.c
> @@ -1836,8 +1836,7 @@ static int storvsc_probe(struct hv_device *device,
> 	/*
> 	 * Set the number of HW queues we are supporting.
> 	 */
> -	if (stor_device->num_sc != 0)
> -		host->nr_hw_queues = stor_device->num_sc + 1;
> +	host->nr_hw_queues = num_present_cpus();

Just looking at the change notes for v3: why isn't this
num_active_cpus() then? One can still isolate CPUs on hyper-v, no?

--
Thanks,
Sasha