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From:   longli@linuxonhyperv.com
To:     "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Sasha Levin <sashal@kernel.org>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        linux-hyperv@vger.kernel.org, linux-scsi@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Long Li <longli@microsoft.com>
Subject: [PATCH] scsi: storvsc: Correctly set number of hardware queues for IDE disk
Date:   Sat, 11 Jan 2020 00:17:14 -0800
Message-Id: <1578730634-109961-1-git-send-email-longli@linuxonhyperv.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Long Li <longli@microsoft.com>

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