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From:   Long Li <longli@microsoft.com>
To:     Michael Kelley <mikelley@microsoft.com>,
        "longli@linuxonhyperv.com" <longli@linuxonhyperv.com>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
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        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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
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Date:   Tue, 14 Jan 2020 00:09:13 +0000
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>Subject: RE: [PATCH] scsi: storvsc: Correctly set number of hardware queue=
s for
>IDE disk
>
>From: Long Li <longli@microsoft.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@microsoft.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 =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

I will send v2 to address this.

Long

>
>> +		host->nr_hw_queues =3D num_present_cpus();
>>
>>  	/*
>>  	 * Set the error handler work queue.
>> --
>> 2.20.1