Re: [PATCH 3/3] scsi: avoid to hold host-wide counter of host_busy for scsi_mq

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

On Fri, Apr 27, 2018 at 04:16:48PM +0000, Bart Van Assche wrote:
> On Fri, 2018-04-20 at 14:57 +0800, Ming Lei wrote:
> > +struct scsi_host_mq_in_flight {
> > +	int cnt;
> > +};
> > +
> > +static void scsi_host_check_in_flight(struct request *rq, void *data,
> > +		bool reserved)
> > +{
> > +	struct scsi_host_mq_in_flight *in_flight = data;
> > +
> > +	if (blk_mq_request_started(rq))
> > +		in_flight->cnt++;
> > +}
> > +
> >  /**
> >   * scsi_host_busy - Return the host busy counter
> >   * @shost:	Pointer to Scsi_Host to inc.
> >   **/
> >  int scsi_host_busy(struct Scsi_Host *shost)
> >  {
> > -	return atomic_read(&shost->host_busy);
> > +	struct scsi_host_mq_in_flight in_flight = {
> > +		.cnt = 0,
> > +	};
> > +
> > +	if (!shost->use_blk_mq)
> > +		return atomic_read(&shost->host_busy);
> > +
> > +	blk_mq_tagset_busy_iter(&shost->tag_set, scsi_host_check_in_flight,
> > +			&in_flight);
> > +	return in_flight.cnt;
> >  }
> >  EXPORT_SYMBOL(scsi_host_busy);
> 
> This patch introduces a subtle behavior change that has not been explained
> in the commit message. If a SCSI request gets requeued that results in a
> decrease of the .host_busy counter by scsi_device_unbusy() before the request
> is requeued and an increase of the host_busy counter when scsi_queue_rq() is
> called again. During that time such requests have the state MQ_RQ_COMPLETE and
> hence blk_mq_request_started() will return true and scsi_host_check_in_flight()

No, __blk_mq_requeue_request() will change the rq state into MQ_RQ_IDLE,
so such issue you worried about, please look at scsi_mq_requeue_cmd(),
which calls blk_mq_requeue_request(), which puts driver tag and updates
rq's state to IDLE.

> will include these requests. In other words, this patch introduces a subtle
> behavior change that has not been explained in the commit message. Hence I'm
> doubt that this change is correct.

As I explained above, no such issue.


Thanks,
Ming