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

[Long Li <longli@microsoft.com>]

>>>Subject: RE: [Patch v2] storvsc: setup 1:1 mapping between hardware
>>>queue and CPU queue
>>>
>>>From: Long Li <longli@linuxonhyperv.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@gmail.com>)
>>>>
>>>> Signed-off-by: Long Li <longli@microsoft.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.

I think reporting num_possible_cpus() doesn't do more harm or take more resources. Because block layer allocates map for all the possible CPUs.

The actual mapping is done in blk_mq_map_queues(), and it iterates all the possible CPUs. If we report num_present_cpus(), the rest of the CPUs also need to be mapped.

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