Re: [PATCH] blk-mq: Fix blk_mq_tagset_busy_iter() for shared tags

[John Garry <john.garry@xxxxxxxxxx>]

On 13/10/2021 10:22, Ming Lei wrote:
> On Wed, Oct 13, 2021 at 04:40:59PM +0800, John Garry wrote:
> > Since it is now possible for a tagset to share a single set of tags, the
> > iter function should not re-iter the tags for the count of #hw queues in
> > that case. Rather it should just iter once.
> >
> > Fixes: e0fdf846c7bb ("blk-mq: Use shared tags for shared sbitmap support")
> > Reported-by: Kashyap Desai<kashyap.desai@xxxxxxxxxxxx>
> > Signed-off-by: John Garry<john.garry@xxxxxxxxxx>
> >
> > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> > index 72a2724a4eee..c943b6529619 100644
> > --- a/block/blk-mq-tag.c
> > +++ b/block/blk-mq-tag.c
> > @@ -378,9 +378,12 @@ void blk_mq_all_tag_iter(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
> >   void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
> >   		busy_tag_iter_fn *fn, void *priv)
> >   {
> > -	int i;
> > +	unsigned int flags = tagset->flags;
> > +	int i, nr_tags;
> > +
> > +	nr_tags = blk_mq_is_shared_tags(flags) ? 1 : tagset->nr_hw_queues;
> >   
> > -	for (i = 0; i < tagset->nr_hw_queues; i++) {
> > +	for (i = 0; i < nr_tags; i++) {
> >   		if (tagset->tags && tagset->tags[i])
> >   			__blk_mq_all_tag_iter(tagset->tags[i], fn, priv,
> >   					      BT_TAG_ITER_STARTED);
> blk_mq_queue_tag_busy_iter() needn't such change?

I didn't think so.

blk_mq_queue_tag_busy_iter() will indeed re-iter the tags per hctx. 
However in bt_iter(), we check rq->mq_hctx == hctx for calling the iter 
callback:

static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
{
	...

	if (rq->q == hctx->queue && rq->mq_hctx == hctx)
		ret = iter_data->fn(hctx, rq, iter_data->data, reserved);

And this would only pass for the correct hctx which we're iter'ing for. 
Indeed, it would be nice not to iter excessive times, but I didn't see a 
straightforward way to change that.

There is also blk_mq_all_tag_iter():

void blk_mq_all_tag_iter(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
		void *priv)
{
	__blk_mq_all_tag_iter(tags, fn, priv, BT_TAG_ITER_STATIC_RQS);
}

But then the only user is blk_mq_hctx_has_requests():

static bool blk_mq_hctx_has_requests(struct blk_mq_hw_ctx *hctx)
{
	struct blk_mq_tags *tags = hctx->sched_tags ?
			hctx->sched_tags : hctx->tags;
	struct rq_iter_data data = {
		.hctx	= hctx,
	};

	blk_mq_all_tag_iter(tags, blk_mq_has_request, &data);
	return data.has_rq;
}

But, again like bt_iter(), blk_mq_has_request() will check the hctx matches:

static bool blk_mq_has_request(struct request *rq, void *data, bool 
reserved)
{
	struct rq_iter_data *iter_data = data;

	if (rq->mq_hctx != iter_data->hctx)
		return true;
	iter_data->has_rq = true;
	return false;
}

Thanks,
John