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From:   Michael Kelley <mikelley@microsoft.com>
To:     "longli@linuxonhyperv.com" <longli@linuxonhyperv.com>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
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        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Long Li <longli@microsoft.com>
Subject: RE: [PATCH] scsi: storvsc: Correctly set number of hardware queues
 for IDE disk
Thread-Topic: [PATCH] scsi: storvsc: Correctly set number of hardware queues
 for IDE disk
Thread-Index: AQHVyFePib13rsBp3ki2r0uiVO4ISafnOM/Q
Date:   Sun, 12 Jan 2020 16:27:55 +0000
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From: Long Li <longli@microsoft.com>  Sent: Saturday, January 11, 2020 12:1=
7 AM
>=20
> Commit 0ed881027690 ("scsi: storvsc: setup 1:1 mapping between hardware q=
ueue and
> CPU queue")
> introduced a regression for disks attached to IDE. For these disks the ho=
st 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 numbe=
r of CPUs.
>=20
> Fix it by leaving the number of hardware queues to 1 (default value) for =
IDE
> disks.
>=20
> Fixes: 0ed881027690 ("scsi: storvsc: setup 1:1 mapping between hardware q=
ueue and CPU
> queue")
> Signed-off-by: Long Li <longli@microsoft.com>
> ---
>  drivers/scsi/storvsc_drv.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>=20
> 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 =3D (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 =3D num_present_cpus();
> +	if (dev_id->driver_data !=3D 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 =3D num_present_cpus();
>=20
>  	/*
>  	 * Set the error handler work queue.
> --
> 2.20.1