On Mon, Aug 04, 2025 at 09:13:08AM +0200, Hannes Reinecke wrote:
> On 8/1/25 13:44, Ming Lei wrote:
> > Replace the spinlock in blk_mq_find_and_get_req() with an SRCU read lock
> > around the tag iterators.
> >
> > This is done by:
> >
> > - Holding the SRCU read lock in blk_mq_queue_tag_busy_iter(),
> > blk_mq_tagset_busy_iter(), and blk_mq_hctx_has_requests().
> >
> > - Removing the now-redundant tags->lock from blk_mq_find_and_get_req().
> >
> > This change improves performance by replacing a spinlock with a more
> > scalable SRCU lock, and fixes lockup issue in scsi_host_busy() in case of
> > shost->host_blocked.
> >
> > Meantime it becomes possible to use blk_mq_in_driver_rw() for io
> > accounting.
> >
> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > ---
> > block/blk-mq-tag.c | 12 ++++++++----
> > block/blk-mq.c | 24 ++++--------------------
> > 2 files changed, 12 insertions(+), 24 deletions(-)
> >
> > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> > index 6c2f5881e0de..7ae431077a32 100644
> > --- a/block/blk-mq-tag.c
> > +++ b/block/blk-mq-tag.c
> > @@ -256,13 +256,10 @@ static struct request *blk_mq_find_and_get_req(struct blk_mq_tags *tags,
> > unsigned int bitnr)
> > {
> > struct request *rq;
> > - unsigned long flags;
> > - spin_lock_irqsave(&tags->lock, flags);
> > rq = tags->rqs[bitnr];
> > if (!rq || rq->tag != bitnr || !req_ref_inc_not_zero(rq))
> > rq = NULL;
> > - spin_unlock_irqrestore(&tags->lock, flags);
> > return rq;
> > }
> > @@ -440,7 +437,9 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
> > busy_tag_iter_fn *fn, void *priv)
> > {
> > unsigned int flags = tagset->flags;
> > - int i, nr_tags;
> > + int i, nr_tags, srcu_idx;
> > +
> > + srcu_idx = srcu_read_lock(&tagset->tags_srcu);
> > nr_tags = blk_mq_is_shared_tags(flags) ? 1 : tagset->nr_hw_queues;
> > @@ -449,6 +448,7 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
> > __blk_mq_all_tag_iter(tagset->tags[i], fn, priv,
> > BT_TAG_ITER_STARTED);
> > }
> > + srcu_read_unlock(&tagset->tags_srcu, srcu_idx);
> > }
> > EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
> > @@ -499,6 +499,8 @@ EXPORT_SYMBOL(blk_mq_tagset_wait_completed_request);
> > void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_tag_iter_fn *fn,
> > void *priv)
> > {
> > + int srcu_idx;
> > +
> > /*
> > * __blk_mq_update_nr_hw_queues() updates nr_hw_queues and hctx_table
> > * while the queue is frozen. So we can use q_usage_counter to avoid
> > @@ -507,6 +509,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_tag_iter_fn *fn,
> > if (!percpu_ref_tryget(&q->q_usage_counter))
> > return;
> > + srcu_idx = srcu_read_lock(&q->tag_set->tags_srcu);
> > if (blk_mq_is_shared_tags(q->tag_set->flags)) {
> > struct blk_mq_tags *tags = q->tag_set->shared_tags;
> > struct sbitmap_queue *bresv = &tags->breserved_tags;
> > @@ -536,6 +539,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_tag_iter_fn *fn,
> > bt_for_each(hctx, q, btags, fn, priv, false);
> > }
> > }
> > + srcu_read_unlock(&q->tag_set->tags_srcu, srcu_idx);
> > blk_queue_exit(q);
> > }
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 7b4ab8e398b6..43b15e58ffe1 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -3415,7 +3415,6 @@ static void blk_mq_clear_rq_mapping(struct blk_mq_tags *drv_tags,
> > struct blk_mq_tags *tags)
> > {
> > struct page *page;
> > - unsigned long flags;
> > /*
> > * There is no need to clear mapping if driver tags is not initialized
> > @@ -3439,15 +3438,6 @@ static void blk_mq_clear_rq_mapping(struct blk_mq_tags *drv_tags,
> > }
> > }
> > }
> > -
> > - /*
> > - * Wait until all pending iteration is done.
> > - *
> > - * Request reference is cleared and it is guaranteed to be observed
> > - * after the ->lock is released.
> > - */
> > - spin_lock_irqsave(&drv_tags->lock, flags);
> > - spin_unlock_irqrestore(&drv_tags->lock, flags);
> > }
> > void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
> > @@ -3670,8 +3660,12 @@ static bool blk_mq_hctx_has_requests(struct blk_mq_hw_ctx *hctx)
> > struct rq_iter_data data = {
> > .hctx = hctx,
> > };
> > + int srcu_idx;
> > + srcu_idx = srcu_read_lock(&hctx->queue->tag_set->tags_srcu);
> > blk_mq_all_tag_iter(tags, blk_mq_has_request, &data);
> > + srcu_read_unlock(&hctx->queue->tag_set->tags_srcu, srcu_idx);
> > +
> > return data.has_rq;
> > }
> > @@ -3891,7 +3885,6 @@ static void blk_mq_clear_flush_rq_mapping(struct blk_mq_tags *tags,
> > unsigned int queue_depth, struct request *flush_rq)
> > {
> > int i;
> > - unsigned long flags;
> > /* The hw queue may not be mapped yet */
> > if (!tags)
> > @@ -3901,15 +3894,6 @@ static void blk_mq_clear_flush_rq_mapping(struct blk_mq_tags *tags,
> > for (i = 0; i < queue_depth; i++)
> > cmpxchg(&tags->rqs[i], flush_rq, NULL);
> > -
> > - /*
> > - * Wait until all pending iteration is done.
> > - *
> > - * Request reference is cleared and it is guaranteed to be observed
> > - * after the ->lock is released.
> > - */
> > - spin_lock_irqsave(&tags->lock, flags);
> > - spin_unlock_irqrestore(&tags->lock, flags);
> > }
> > static void blk_free_flush_queue_callback(struct rcu_head *head)
>
> While this looks good, I do wonder what happened to the 'fq' srcu.
> Don't we need to insert an srcu_read_lock() when we're trying to
> access it?
That is exactly the srcu read lock added in this patch.
Thanks,
Ming
