Re: [PATCH 4/5] blk-mq: do limited block plug for multiple queue case

[Jeff Moyer <jmoyer@redhat.com>]

Shaohua Li <shli@fb.com> writes:

> plug is still helpful for workload with IO merge, but it can be harmful
> otherwise especially with multiple hardware queues, as there is
> (supposed) no lock contention in this case and plug can introduce
> latency. For multiple queues, we do limited plug, eg plug only if there
> is request merge. If a request doesn't have merge with following
> request, the requet will be dispatched immediately.
>
> This also fixes a bug. If we directly issue a request and it fails, we
> use blk_mq_merge_queue_io(). But we already assigned bio to a request in
> blk_mq_bio_to_request. blk_mq_merge_queue_io shouldn't run
> blk_mq_bio_to_request again.

Good catch.  Might've been better to split that out first for easy
backport to stable kernels, but I won't hold you to that.

> @@ -1243,6 +1277,10 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
>  		return;
>  	}
>  
> +	if (likely(!is_flush_fua) && !blk_queue_nomerges(q) &&
> +	    blk_attempt_plug_merge(q, bio, &request_count))
> +		return;
> +
>  	rq = blk_mq_map_request(q, bio, &data);
>  	if (unlikely(!rq))
>  		return;

After this patch, everything up to this point in blk_mq_make_request and
blk_sq_make_request is the same.  This can be factored out (in another
patch) to a common function.

> @@ -1253,38 +1291,38 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
>  		goto run_queue;
>  	}
>  
> +	plug = current->plug;
>  	/*
>  	 * If the driver supports defer issued based on 'last', then
>  	 * queue it up like normal since we can potentially save some
>  	 * CPU this way.
>  	 */
> -	if (is_sync && !(data.hctx->flags & BLK_MQ_F_DEFER_ISSUE)) {
> -		struct blk_mq_queue_data bd = {
> -			.rq = rq,
> -			.list = NULL,
> -			.last = 1
> -		};
> -		int ret;
> +	if ((plug || is_sync) && !(data.hctx->flags & BLK_MQ_F_DEFER_ISSUE)) {
> +		struct request *old_rq = NULL;

I would add a !blk_queue_nomerges(q) to that conditional.  There's no
point holding back an I/O when we won't merge it anyway.

That brings up another quirk of the current implementation (not your
patches) that bugs me.

BLK_MQ_F_SHOULD_MERGE
QUEUE_FLAG_NOMERGES

Those two flags are set independently, one via the driver and the other
via a sysfs file.  So the user could set the nomerges flag to 1 or 2,
and still potentially get merges (see blk_mq_merge_queue_io).  That's
something that should be fixed, albeit that can wait.

>  		blk_mq_bio_to_request(rq, bio);
>  
>  		/*
> -		 * For OK queue, we are done. For error, kill it. Any other
> -		 * error (busy), just add it to our list as we previously
> -		 * would have done
> +		 * we do limited pluging. If bio can be merged, do merge.
> +		 * Otherwise the existing request in the plug list will be
> +		 * issued. So the plug list will have one request at most
>  		 */
> -		ret = q->mq_ops->queue_rq(data.hctx, &bd);
> -		if (ret == BLK_MQ_RQ_QUEUE_OK)
> -			goto done;
> -		else {
> -			__blk_mq_requeue_request(rq);
> -
> -			if (ret == BLK_MQ_RQ_QUEUE_ERROR) {
> -				rq->errors = -EIO;
> -				blk_mq_end_request(rq, rq->errors);
> -				goto done;
> +		if (plug) {
> +			if (!list_empty(&plug->mq_list)) {
> +				old_rq = list_first_entry(&plug->mq_list,
> +					struct request, queuelist);
> +				list_del_init(&old_rq->queuelist);
>  			}
> -		}
> +			list_add_tail(&rq->queuelist, &plug->mq_list);
> +		} else /* is_sync */
> +			old_rq = rq;
> +		blk_mq_put_ctx(data.ctx);
> +		if (!old_rq)
> +			return;
> +		if (!blk_mq_direct_issue_request(old_rq))
> +			return;
> +		blk_mq_insert_request(old_rq, false, true, true);
> +		return;
>  	}

Now there is no way to exit that if block, we always return.  It may be
worth cosidering moving that block to its own function, if you can think
of a good name for it.

Other than those minor issues, this looks good to me.

Cheers,
Jeff