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

[Long Li <longli@microsoft.com>]

>Subject: Re: [Patch v4] storvsc: setup 1:1 mapping between hardware queue
>and CPU queue
>
>On Fri, Sep 06, 2019 at 10:24:20AM -0700, longli@linuxonhyperv.com wrote:
>>From: Long Li <longli@microsoft.com>
>>
>>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@microsoft.com>
>>---
>>
>>Changes:
>>v2: rely on default upper layer function to map queues. (suggested by
>>Ming Lei
>><tom.leiming@gmail.com>)
>>v3: use num_present_cpus() instead of num_online_cpus(). Hyper-v
>>doesn't support hot-add CPUs. (suggested by Michael Kelley
>><mikelley@microsoft.com>)
>>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?

The isolated CPU can be made online at run time. For example, even maxcpus=x is put on the boot line, individual CPUs can still be made online/offline.

>
>--
>Thanks,
>Sasha