From: Long Li <longli@microsoft.com>
Date: Mon, 13 Jan 2020 16:08:36 -0800
Patch-mainline: v5.5-rc7
Subject: scsi: storvsc: Correctly set number of hardware queues for IDE disk
Git-commit: 7b571c19d4c0b78d27dd3bf1f3c42e4032390af6
References: git-fixes

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")
Link: https://lore.kernel.org/r/1578960516-108228-1-git-send-email-longli@linuxonhyperv.com
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Acked-by: Olaf Hering <ohering@suse.de>
---
 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
--- 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_is_ide)
+		host->nr_hw_queues = num_present_cpus();
 
 	/*
 	 * Set the error handler work queue.