Re: [PATCH 3/4] blk-mq: use hw tag for scheduling if hw tag space is big enough

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

On Wed, May 03, 2017 at 09:29:36AM -0700, Omar Sandoval wrote:
> On Fri, Apr 28, 2017 at 11:15:38PM +0800, Ming Lei wrote:
> > When tag space of one device is big enough, we use hw tag
> > directly for I/O scheduling.
> > 
> > Now the decision is made if hw queue depth is not less than
> > q->nr_requests and the tag set isn't shared.
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  block/blk-mq-sched.c |  8 ++++++++
> >  block/blk-mq-sched.h | 15 +++++++++++++++
> >  block/blk-mq.c       | 18 +++++++++++++++++-
> >  3 files changed, 40 insertions(+), 1 deletion(-)
> > 
> > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > index 45a675f07b8b..4681e27c127e 100644
> > --- a/block/blk-mq-sched.c
> > +++ b/block/blk-mq-sched.c
> > @@ -507,6 +507,7 @@ int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
> >  	struct elevator_queue *eq;
> >  	unsigned int i;
> >  	int ret;
> > +	bool auto_hw_tag;
> >  
> >  	if (!e) {
> >  		q->elevator = NULL;
> > @@ -519,7 +520,14 @@ int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
> >  	 */
> >  	q->nr_requests = 2 * BLKDEV_MAX_RQ;
> >  
> > +	auto_hw_tag = blk_mq_sched_may_use_hw_tag(q);
> > +
> >  	queue_for_each_hw_ctx(q, hctx, i) {
> > +		if (auto_hw_tag)
> > +			hctx->flags |= BLK_MQ_F_SCHED_USE_HW_TAG;
> > +		else
> > +			hctx->flags &= ~BLK_MQ_F_SCHED_USE_HW_TAG;
> > +
> >  		ret = blk_mq_sched_alloc_tags(q, hctx, i);
> >  		if (ret)
> >  			goto err;
> 
> I think you should also clear the BLK_MQ_F_SCHED_USE_HW_TAG flag in
> blk_mq_exit_sched()?

Looks not necessary since the flag is always evaluated in
blk_mq_init_sched().

Thanks,
Ming