scsi: storvsc: Fix calculation of sub-channel count

kelleymh · martinkpetersen · commit 382e06d11e07 · 2019-04-03T23:30:08.000-04:00
When the number of sub-channels offered by Hyper-V is &gt;= the number of CPUs
in the VM, calculate the correct number of sub-channels.  The current code
produces one too many.

This scenario arises only when the number of CPUs is artificially
restricted (for example, with maxcpus=&lt;n&gt; on the kernel boot line), because
Hyper-V normally offers a sub-channel count &lt; number of CPUs.  While the
current code doesn't break, the extra sub-channel is unbalanced across the
CPUs (for example, a total of 5 channels on a VM with 4 CPUs).

Signed-off-by: Michael Kelley &lt;mikelley@microsoft.com&gt;
Reviewed-by: Vitaly Kuznetsov &lt;vkuznets@redhat.com&gt;
Reviewed-by: Long Li &lt;longli@microsoft.com&gt;
Signed-off-by: Martin K. Petersen &lt;martin.petersen@oracle.com&gt;
diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
@@ -668,13 +668,22 @@ static void  handle_multichannel_storage(struct hv_device *device, int max_chns)
 {
 	struct device *dev = &device->device;
 	struct storvsc_device *stor_device;
-	int num_cpus = num_online_cpus();
 	int num_sc;
 	struct storvsc_cmd_request *request;
 	struct vstor_packet *vstor_packet;
 	int ret, t;
 
-	num_sc = ((max_chns > num_cpus) ? num_cpus : max_chns);
+	/*
+	 * If the number of CPUs is artificially restricted, such as
+	 * with maxcpus=1 on the kernel boot line, Hyper-V could offer
+	 * sub-channels >= the number of CPUs. These sub-channels
+	 * should not be created. The primary channel is already created
+	 * and assigned to one CPU, so check against # CPUs - 1.
+	 */
+	num_sc = min((int)(num_online_cpus() - 1), max_chns);
+	if (!num_sc)
+		return;
+
 	stor_device = get_out_stor_device(device);
 	if (!stor_device)
 		return;
