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

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Message-ID: <DM5PR21MB06352133A72ED856E43ED396CE550@DM5PR21MB0635.namprd21.prod.outlook.com>
Date:   Mon, 1 Apr 2019 21:38:43 +0000
From:   Long Li <longli@...rosoft.com>
To:     Michael Kelley <mikelley@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.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>
Subject: RE: [PATCH v2 1/1] scsi: storvsc: Fix calculation of sub-channel
 count

>>>-----Original Message-----
>>>From: Michael Kelley <mikelley@...rosoft.com>
>>>Sent: Monday, April 1, 2019 9:11 AM
>>>To: KY Srinivasan <kys@...rosoft.com>; martin.petersen@...cle.com; Long
>>>Li <longli@...rosoft.com>; James.Bottomley@...senpartnership.com;
>>>emilne@...hat.com; linux-hyperv@...r.kernel.org; linux-
>>>kernel@...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]
>>>

Reviewed-by: Long Li <longli@...rosoft.com>

>>>---
>>> 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