linux-kernel - RE: [PATCH] scsi: storvsc: Correctly set number of hardware queues for IDE disk

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Date:   Sun, 12 Jan 2020 16:27:55 +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] scsi: storvsc: Correctly set number of hardware queues
 for IDE disk

From: Long Li <longli@...rosoft.com>  Sent: Saturday, January 11, 2020 12:17 AM
> 
> Commit 0ed881027690 ("scsi: storvsc: setup 1:1 mapping between hardware queue and
> CPU queue")
> introduced a regression for disks attached to IDE. For these disks the host VSP only offers
> one VMBUS channel. Setting multiple queues can overload the VMBUS channel and result
> in
> performance drop for high queue depth workload on system with large number of CPUs.
> 
> Fix it by leaving the number of hardware queues to 1 (default value) for IDE
> disks.
> 
> Fixes: 0ed881027690 ("scsi: storvsc: setup 1:1 mapping between hardware queue and CPU
> queue")
> Signed-off-by: Long Li <longli@...rosoft.com>
> ---
>  drivers/scsi/storvsc_drv.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> index f8faf8b3d965..992b28e40374 100644
> --- a/drivers/scsi/storvsc_drv.c
> +++ b/drivers/scsi/storvsc_drv.c
> @@ -1842,9 +1842,11 @@ static int storvsc_probe(struct hv_device *device,
>  	 */
>  	host->sg_tablesize = (stor_device->max_transfer_bytes >> PAGE_SHIFT);
>  	/*
> +	 * For non-IDE disks, the host supports multiple channels.
>  	 * Set the number of HW queues we are supporting.
>  	 */
> -	host->nr_hw_queues = num_present_cpus();
> +	if (dev_id->driver_data != IDE_GUID)

This function already has a pre-computed value of this test in
the local variable "dev_is_ide".   It would be more consistent
to just use it.

Michael

> +		host->nr_hw_queues = num_present_cpus();
> 
>  	/*
>  	 * Set the error handler work queue.
> --
> 2.20.1