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SubjectRe: [PATCH 1/1] scsi: storvsc: Fix calculation of sub-channel count
Date
Michael Kelley <mikelley@microsoft.com> writes:

> 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>
> ---
> drivers/scsi/storvsc_drv.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> index 84380ba..fbaa11a 100644
> --- a/drivers/scsi/storvsc_drv.c
> +++ b/drivers/scsi/storvsc_drv.c
> @@ -674,7 +674,17 @@ static void handle_multichannel_storage(struct hv_device *device, int max_chns)
> 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.
> + */

Indeed.

> + num_sc = min((num_cpus - 1), max_chns);

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

(nit: you could've written

num_sc = min((num_online_cpus() - 1), max_chns);

and got rid of the now-unneeded num_cpus variable).

> + if (!num_sc)
> + return;
> +
> stor_device = get_out_stor_device(device);
> if (!stor_device)
> return;

--
Vitaly

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