linux-kernel - [PATCH v2 1/1] scsi: storvsc: Fix calculation of sub-channel count

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Message-ID: <1554134985-8671-1-git-send-email-mikelley@microsoft.com>
Date:   Mon, 1 Apr 2019 16:10:52 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     KY Srinivasan <kys@...rosoft.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        Long Li <longli@...rosoft.com>,
        "James.Bottomley@...senpartnership.com" 
        <James.Bottomley@...senpartnership.com>,
        "emilne@...hat.com" <emilne@...hat.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>
CC:     Michael Kelley <mikelley@...rosoft.com>
Subject: [PATCH v2 1/1] scsi: storvsc: Fix calculation of sub-channel count

When the number of sub-channels offered by Hyper-V is >= 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=<n> on the kernel boot line),
because Hyper-V normally offers a sub-channel count < 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 <mikelley@...rosoft.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>
---
Changes in v2:
* Put num_online_cpus() inline and eliminate num_cpus local
  variable [Vitaly Kuznetsov]

---
 drivers/scsi/storvsc_drv.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index 84380ba..e186743 100644
--- 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;
-- 
1.8.3.1