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    Subject[PATCH 4.4 128/266] scsi: storvsc: Fix calculation of sub-channel count
    Date
    [ Upstream commit 382e06d11e075a40b4094b6ef809f8d4bcc7ab2a ]

    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>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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 44b7a69d022a..45cd4cf93af3 100644
    --- a/drivers/scsi/storvsc_drv.c
    +++ b/drivers/scsi/storvsc_drv.c
    @@ -613,13 +613,22 @@ static void handle_sc_creation(struct vmbus_channel *new_sc)
    static void handle_multichannel_storage(struct hv_device *device, int max_chns)
    {
    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;
    --
    2.20.1


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