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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@microsoft.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
This commit is contained in:
Michael Kelley 2019-04-01 16:10:52 +00:00 committed by Martin K. Petersen
parent 1cb1d2c64e
commit 382e06d11e
1 changed files with 11 additions and 2 deletions
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13
drivers/scsi/storvsc_drv.c
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@ -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;