Re: [PATCH RFC v3 04/41] csiostor: use reserved command for LUN reset

[Ming Lei <ming.lei@redhat.com>]

On Fri, May 01, 2020 at 03:01:14PM +0200, Hannes Reinecke wrote:
> On 4/30/20 5:15 PM, Ming Lei wrote:
> > On Thu, Apr 30, 2020 at 03:18:27PM +0200, Hannes Reinecke wrote:
> > > When issuing a LUN reset we should be using a reserved command
> > > to avoid overwriting the original command.
> > > 
> > > Signed-off-by: Hannes Reinecke <hare@suse.com>
> > > ---
> > >   drivers/scsi/csiostor/csio_init.c |  1 +
> > >   drivers/scsi/csiostor/csio_scsi.c | 48 +++++++++++++++++++++++----------------
> > >   2 files changed, 30 insertions(+), 19 deletions(-)
> > > 
> > > diff --git a/drivers/scsi/csiostor/csio_init.c b/drivers/scsi/csiostor/csio_init.c
> > > index 8dea7d53788a..5e1b0a24caf6 100644
> > > --- a/drivers/scsi/csiostor/csio_init.c
> > > +++ b/drivers/scsi/csiostor/csio_init.c
> > > @@ -622,6 +622,7 @@ csio_shost_init(struct csio_hw *hw, struct device *dev,
> > >   	ln->dev_num = (shost->host_no << 16);
> > >   	shost->can_queue = CSIO_MAX_QUEUE;
> > > +	shost->nr_reserved_cmds = 1;
> > 
> > ->can_queue isn't increased by 1 given CSIO_MAX_QUEUE isn't changed, so
> > setting shost->nr_reserved_cmds as 1 will cause io queue depth reduced by 1,
> > that is supposed to not happen.
> > 
> We cannot increase MAX_QUEUE arbitrarily as this is a compile time variable,
> which seems to relate to a hardware setting.
> 
> But I can see to update the reserved command functionality for allowing to
> fetch commands from the normal I/O tag pool; in the case of LUN reset it
> shouldn't make much of a difference as the all I/O is quiesced anyway.

It isn't related with reset.

This patch reduces active IO queue depth by 1 anytime no matter there is reset
or not, and this way may cause performance regression.

Thanks, 
Ming