Re: [PATCH V4 2/6] blk-mq: pass hctx to blk_mq_dispatch_rq_list

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

On Tue, Jun 02, 2020 at 09:25:01AM +0000, Damien Le Moal wrote:
> On 2020/06/02 18:15, Ming Lei wrote:
> > All requests in the 'list' of blk_mq_dispatch_rq_list belong to same
> > hctx, so it is better to pass hctx instead of request queue, because
> > blk-mq's dispatch target is hctx instead of request queue.
> > 
> > Cc: Sagi Grimberg <sagi@grimberg.me>
> > Cc: Baolin Wang <baolin.wang7@gmail.com>
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
> > Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> > Tested-by: Baolin Wang <baolin.wang7@gmail.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  block/blk-mq-sched.c | 14 ++++++--------
> >  block/blk-mq.c       |  6 +++---
> >  block/blk-mq.h       |  2 +-
> >  3 files changed, 10 insertions(+), 12 deletions(-)
> > 
> > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > index a31e281e9d31..632c6f8b63f7 100644
> > --- a/block/blk-mq-sched.c
> > +++ b/block/blk-mq-sched.c
> > @@ -96,10 +96,9 @@ static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> >  	struct elevator_queue *e = q->elevator;
> >  	LIST_HEAD(rq_list);
> >  	int ret = 0;
> > +	struct request *rq;
> >  
> >  	do {
> > -		struct request *rq;
> > -
> >  		if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
> >  			break;
> >  
> > @@ -131,7 +130,7 @@ static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> >  		 * in blk_mq_dispatch_rq_list().
> >  		 */
> >  		list_add(&rq->queuelist, &rq_list);
> > -	} while (blk_mq_dispatch_rq_list(q, &rq_list, true));
> > +	} while (blk_mq_dispatch_rq_list(rq->mq_hctx, &rq_list, true));
> 
> Why not use the hctx argument passed to the function instead of rq->mq_hctx ?

e->type->ops.dispatch_request(hctx) may return one request which's
.mq_hctx isn't same with the 'hctx' argument, so far bfq and deadline
may do that.


Thanks,
Ming