Re: [PATCH v1] scsi: storvsc: Cap cmd_per_lun at can_queue

[Melanie Plageman <melanieplageman@gmail.com>]

On Mon, Mar 08, 2021 at 02:37:40PM +0000, Michael Kelley wrote:
> From: Melanie Plageman (Microsoft) <melanieplageman@gmail.com> Sent: Friday, March 5, 2021 3:22 PM
> > 
> > The scsi_device->queue_depth is set to Scsi_Host->cmd_per_lun during
> > allocation.
> > 
> > Cap cmd_per_lun at can_queue to avoid dispatch errors.
> > 
> > Signed-off-by: Melanie Plageman (Microsoft) <melanieplageman@gmail.com>
> > ---
> >  drivers/scsi/storvsc_drv.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> > index 6bc5453cea8a..d7953a6e00e6 100644
> > --- a/drivers/scsi/storvsc_drv.c
> > +++ b/drivers/scsi/storvsc_drv.c
> > @@ -1946,6 +1946,8 @@ static int storvsc_probe(struct hv_device *device,
> >  				(max_sub_channels + 1) *
> >  				(100 - ring_avail_percent_lowater) / 100;
> > 
> > +	scsi_driver.cmd_per_lun = min_t(u32, scsi_driver.cmd_per_lun, scsi_driver.can_queue);
> > +
> 
> I'm not sure what you mean by "avoid dispatch errors".  Can you elaborate?

The scsi_driver.cmd_per_lun is set to 2048. Which is then used to set
Scsi_Host->cmd_per_lun in storvsc_probe().

In storvsc_probe(), when doing scsi_scan_host(), scsi_alloc_sdev() is
called and sets the scsi_device->queue_depth to the Scsi_Host's
cmd_per_lun with this code:
    
scsi_change_queue_depth(sdev, sdev->host->cmd_per_lun ?    
                                        sdev->host->cmd_per_lun : 1);

During dispatch, the scsi_device->queue_depth is used in
scsi_dev_queue_ready(), called by scsi_mq_get_budget() to determine
whether or not the device can queue another command.    

On some machines, with the 2048 value of cmd_per_lun that was used to
set the initial scsi_device->queue_depth, commands can be queued that
are later not able to be dispatched after running out of space in the
ringbuffer.

On an 8 core Azure VM with 16GB of memory with a single 1 TiB SSD
(running an fio workload that I can provide if needed), storvsc_do_io()
ends up often returning SCSI_MLQUEUE_DEVICE_BUSY.
                                                                      
This is the call stack: 
    
hv_get_bytes_to_write
hv_ringbuffer_write    
vmbus_send_packet      
storvsc_dio_io      
storvsc_queuecommand
scsi_dispatch_cmd    
scsi_queue_rq       
dispatch_rq_list

> Be aware that the calculation of "can_queue" in this driver is somewhat
> flawed -- it should not be based on the size of the ring buffer, but instead on
> the maximum number of requests Hyper-V will queue.  And even then,
> can_queue doesn't provide the cap you might expect because the blk-mq layer
> allocates can_queue tags for each HW queue, not as a total.


The docs for scsi_mid_low_api document Scsi_Host can_queue this way:

  can_queue    
  - must be greater than 0; do not send more than can_queue    
    commands to the adapter. 
        
I did notice that in scsi_host.h, the comment for can_queue does say
can_queue is the "maximum number of simultaneous commands a single hw
queue in HBA will accept." However, I don't see it being used this way
in the code.
                                                                            
During dispatch, In scsi_target_queue_ready(), there is this code:

        if (busy >= starget->can_queue)
                goto starved;

And the scsi_target->can_queue value should be coming from Scsi_host as
mentioned in the scsi_target definition in scsi_device.h
    /*
      * LLDs should set this in the slave_alloc host template callout.
      * If set to zero then there is not limit.
      */
    unsigned int            can_queue;

So, I don't really see how this would be per hardware queue.

> 
> I agree that the cmd_per_lun setting is also too big, but we should fix that in
> the context of getting all of these different settings working together correctly,
> and not piecemeal.
> 

Capping Scsi_Host->cmd_per_lun to scsi_driver.can_queue during probe
will also prevent the LUN queue_depth from being set to a value that is
higher than it can ever be set to again by the user when
storvsc_change_queue_depth() is invoked. 

Also in scsi_sysfs sdev_store_queue_depth() there is this check:

          if (depth < 1 || depth > sdev->host->can_queue)
                return -EINVAL;

I would also note that VirtIO SCSI in virtscsi_probe(), Scsi_Host->cmd_per_lun
is set to the min of the configured cmd_per_lun and
Scsi_Host->can_queue:

    shost->cmd_per_lun = min_t(u32, cmd_per_lun, shost->can_queue);

Best,
Melanie