Re: [PATCH 3/3] blk-mq: centralise related handling into blk_mq_get_driver_tag

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

On Tue, Jun 30, 2020 at 08:26:11AM +0200, Hannes Reinecke wrote:
> On 6/30/20 4:23 AM, Ming Lei wrote:
> > Move blk_mq_tag_busy(), .nr_active update and request assignment into
> > blk_mq_get_driver_tag(), all are good to do during getting driver tag.
> > 
> > Meantime blk-flush related code is simplified and flush request needn't
> > to update the request table manually any more.
> > 
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >   block/blk-flush.c  | 13 ++++---------
> >   block/blk-mq-tag.h | 12 ------------
> >   block/blk-mq.c     | 33 +++++++++++++--------------------
> >   block/blk.h        |  5 -----
> >   4 files changed, 17 insertions(+), 46 deletions(-)
> > 
> > diff --git a/block/blk-flush.c b/block/blk-flush.c
> > index 21108a550fbf..3b0c5cfe922a 100644
> > --- a/block/blk-flush.c
> > +++ b/block/blk-flush.c
> > @@ -236,12 +236,10 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
> >   		error = fq->rq_status;
> >   	hctx = flush_rq->mq_hctx;
> > -	if (!q->elevator) {
> > -		blk_mq_tag_set_rq(hctx, flush_rq->tag, fq->orig_rq);
> > +	if (!q->elevator)
> >   		flush_rq->tag = -1;
> > -	} else {
> > +	else
> >   		flush_rq->internal_tag = -1;
> > -	}
> >   	running = &fq->flush_queue[fq->flush_running_idx];
> >   	BUG_ON(fq->flush_pending_idx == fq->flush_running_idx);
> > @@ -315,13 +313,10 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
> >   	flush_rq->mq_ctx = first_rq->mq_ctx;
> >   	flush_rq->mq_hctx = first_rq->mq_hctx;
> > -	if (!q->elevator) {
> > -		fq->orig_rq = first_rq;
> > +	if (!q->elevator)
> >   		flush_rq->tag = first_rq->tag;
> > -		blk_mq_tag_set_rq(flush_rq->mq_hctx, first_rq->tag, flush_rq);
> > -	} else {
> > +	else
> >   		flush_rq->internal_tag = first_rq->internal_tag;
> > -	}
> >   	flush_rq->cmd_flags = REQ_OP_FLUSH | REQ_PREFLUSH;
> >   	flush_rq->cmd_flags |= (flags & REQ_DRV) | (flags & REQ_FAILFAST_MASK);
> > diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
> > index 3945c7f5b944..b1acac518c4e 100644
> > --- a/block/blk-mq-tag.h
> > +++ b/block/blk-mq-tag.h
> > @@ -101,18 +101,6 @@ static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
> >   	return atomic_read(&hctx->nr_active) < depth;
> >   }
> > -/*
> > - * This helper should only be used for flush request to share tag
> > - * with the request cloned from, and both the two requests can't be
> > - * in flight at the same time. The caller has to make sure the tag
> > - * can't be freed.
> > - */
> > -static inline void blk_mq_tag_set_rq(struct blk_mq_hw_ctx *hctx,
> > -		unsigned int tag, struct request *rq)
> > -{
> > -	hctx->tags->rqs[tag] = rq;
> > -}
> > -
> >   static inline bool blk_mq_tag_is_reserved(struct blk_mq_tags *tags,
> >   					  unsigned int tag)
> >   {
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index cabeeeb3d56c..44b101757d33 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -277,26 +277,20 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
> >   {
> >   	struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
> >   	struct request *rq = tags->static_rqs[tag];
> > -	req_flags_t rq_flags = 0;
> >   	if (data->flags & BLK_MQ_REQ_INTERNAL) {
> >   		rq->tag = BLK_MQ_NO_TAG;
> >   		rq->internal_tag = tag;
> >   	} else {
> > -		if (data->hctx->flags & BLK_MQ_F_TAG_SHARED) {
> > -			rq_flags = RQF_MQ_INFLIGHT;
> > -			atomic_inc(&data->hctx->nr_active);
> > -		}
> >   		rq->tag = tag;
> >   		rq->internal_tag = BLK_MQ_NO_TAG;
> > -		data->hctx->tags->rqs[rq->tag] = rq;
> >   	}
> >   	/* csd/requeue_work/fifo_time is initialized before use */
> >   	rq->q = data->q;
> >   	rq->mq_ctx = data->ctx;
> >   	rq->mq_hctx = data->hctx;
> > -	rq->rq_flags = rq_flags;
> > +	rq->rq_flags = 0;
> >   	rq->cmd_flags = data->cmd_flags;
> >   	if (data->flags & BLK_MQ_REQ_PREEMPT)
> >   		rq->rq_flags |= RQF_PREEMPT;
> > @@ -380,8 +374,6 @@ static struct request *__blk_mq_alloc_request(struct blk_mq_alloc_data *data)
> >   retry:
> >   	data->ctx = blk_mq_get_ctx(q);
> >   	data->hctx = blk_mq_map_queue(q, data->cmd_flags, data->ctx);
> > -	if (!(data->flags & BLK_MQ_REQ_INTERNAL))
> > -		blk_mq_tag_busy(data->hctx);
> >   	/*
> >   	 * Waiting allocations only fail because of an inactive hctx.  In that
> > @@ -478,8 +470,6 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
> >   	if (q->elevator)
> >   		data.flags |= BLK_MQ_REQ_INTERNAL;
> > -	else
> > -		blk_mq_tag_busy(data.hctx);
> >   	ret = -EWOULDBLOCK;
> >   	tag = blk_mq_get_tag(&data);
> > @@ -1131,7 +1121,6 @@ static bool __blk_mq_get_driver_tag(struct request *rq)
> >   {
> >   	struct sbitmap_queue *bt = &rq->mq_hctx->tags->bitmap_tags;
> >   	unsigned int tag_offset = rq->mq_hctx->tags->nr_reserved_tags;
> > -	bool shared = blk_mq_tag_busy(rq->mq_hctx);
> >   	int tag;
> >   	if (blk_mq_tag_is_reserved(rq->mq_hctx->sched_tags, rq->internal_tag)) {
> > @@ -1146,19 +1135,23 @@ static bool __blk_mq_get_driver_tag(struct request *rq)
> >   		return false;
> >   	rq->tag = tag + tag_offset;
> > -	if (shared) {
> > -		rq->rq_flags |= RQF_MQ_INFLIGHT;
> > -		atomic_inc(&rq->mq_hctx->nr_active);
> > -	}
> > -	rq->mq_hctx->tags->rqs[rq->tag] = rq;
> >   	return true;
> >   }
> >   static bool blk_mq_get_driver_tag(struct request *rq)
> >   {
> > -	if (rq->tag != BLK_MQ_NO_TAG)
> > -		return true;
> > -	return __blk_mq_get_driver_tag(rq);
> > +	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
> > +	bool shared = blk_mq_tag_busy(rq->mq_hctx);
> > +
> > +	if (rq->tag == BLK_MQ_NO_TAG && !__blk_mq_get_driver_tag(rq))
> > +		return false;
> > +
> > +	if (shared) {
> > +		rq->rq_flags |= RQF_MQ_INFLIGHT;
> > +		atomic_inc(&hctx->nr_active);
> > +	}
> > +	hctx->tags->rqs[rq->tag] = rq;
> > +	return true;
> >   }
> >   static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode,
> > diff --git a/block/blk.h b/block/blk.h
> > index 3a120a070dac..459790f9783d 100644
> > --- a/block/blk.h
> > +++ b/block/blk.h
> > @@ -25,11 +25,6 @@ struct blk_flush_queue {
> >   	struct list_head	flush_data_in_flight;
> >   	struct request		*flush_rq;
> > -	/*
> > -	 * flush_rq shares tag with this rq, both can't be active
> > -	 * at the same time
> > -	 */
> > -	struct request		*orig_rq;
> >   	struct lock_class_key	key;
> >   	spinlock_t		mq_flush_lock;
> >   };
> > 
> Can you give some more explanation why it's safe to move blk_mq_tag_busy()
> into blk_mq_get_driver_tag(), seeing that it was called before
> blk_mq_get_tag() initially?

In theory, it should be done before blk_mq_get_tag() in case of none
because hctx_may_queue() will use this info for deciding if one driver
tag can be allocated for this lun/ns for the sake of fairness.

However, blk_mq_tag_busy() is just one shot thing, and very short time
of unfairness shouldn't be a big deal.

Looks you guys care this change, I will avoid this kind of change in V2.

Thanks,
Ming