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

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Date:   Fri, 6 Sep 2019 16:08:20 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     longli@...uxonhyperv.com
Cc:     "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-hyperv@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org, Long Li <longli@...rosoft.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@...uxonhyperv.com wrote:
>From: Long Li <longli@...rosoft.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@...rosoft.com>
>---
>
>Changes:
>v2: rely on default upper layer function to map queues. (suggested by Ming Lei
><tom.leiming@...il.com>)
>v3: use num_present_cpus() instead of num_online_cpus(). Hyper-v doesn't
>support hot-add CPUs. (suggested by Michael Kelley <mikelley@...rosoft.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?

--
Thanks,
Sasha