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

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Message-ID: <CY4PR21MB0741A256EEF9D8DBAF50ED03CEBA0@CY4PR21MB0741.namprd21.prod.outlook.com>
Date:   Fri, 6 Sep 2019 21:37:09 +0000
From:   Long Li <longli@...rosoft.com>
To:     Sasha Levin <sashal@...nel.org>,
        "longli@...uxonhyperv.com" <longli@...uxonhyperv.com>
CC:     KY 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-hyperv@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [Patch v4] storvsc: setup 1:1 mapping between hardware queue and
 CPU queue

>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?

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