linux-kernel - RE: [Patch v2] storvsc: setup 1:1 mapping between hardware queue and CPU queue

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Date:   Thu, 22 Aug 2019 21:01:32 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     "longli@...uxonhyperv.com" <longli@...uxonhyperv.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Long Li <longli@...rosoft.com>
Subject: RE: [Patch v2] storvsc: setup 1:1 mapping between hardware queue and
 CPU queue

From: Long Li <longli@...uxonhyperv.com> Sent: Thursday, August 22, 2019 1:42 PM
> 
> 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_possible_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.
> 
> Changes:
> v2: rely on default upper layer function to map queues. (suggested by Ming Lei
> <tom.leiming@...il.com>)
> 
> Signed-off-by: Long Li <longli@...rosoft.com>
> ---
>  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..dfd3b76a4f89 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_possible_cpus();

For a lot of the VM sizes in Azure, num_possible_cpus() is 128, even if
the VM has only 4 or 8 or some other smaller number of vCPUs.
So I'm wondering if you really want num_present_cpus() here instead,
which would include only the vCPUs that actually exist in the VM.

Michael

> 
>  	/*
>  	 * Set the error handler work queue.
> --
> 2.17.1