Re: [PATCH v1 5/6] blk-mq: fix for trace_block_plug()

[Jeff Moyer <jmoyer@redhat.com>]

Ming Lei <ming.lei@canonical.com> writes:

> On Mon, Oct 19, 2015 at 11:38 PM, Jeff Moyer <jmoyer@redhat.com> wrote:
>> Ming Lei <ming.lei@canonical.com> writes:
>>
>>> The trace point is for tracing plug event of each request
>>> queue instead of each task, so we should check the request
>>> count in the plug list from current queue instead of
>>> current task.
>>
>> If blk_queue_nomerges returns true, request_count won't be updated, so
>> you've essentially broken plugging for those queues.
>
> Yes, you are right, and blk_queue_bio() has the same problem too,
> and looks automatic flush plug can't work for nomerges queue at all.

Hi Ming,

Something like this would fix it.  Feel free to add it into your patch
set if you're going to do another spin.  Otherwise I'll just send it off
separately.

-Jeff

From: Jeff Moyer <jmoyer@redhat.com>
Subject: block: fix plug list flushing for nomerge queues

Request queues with merging disabled will not flush the plug list after
BLK_MAX_REQUEST_COUNT requests have been queued, since the code relies
on blk_attempt_plug_merge to compute the request_count.  Fix this by
computing the number of queued requests even for nomerge queues.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>

diff --git a/block/blk-core.c b/block/blk-core.c
index 2eb722d..f0ae087 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1594,6 +1594,30 @@ out:
 	return ret;
 }
 
+unsigned int blk_plug_queued_count(struct request_queue *q)
+{
+	struct blk_plug *plug;
+	struct request *rq;
+	struct list_head *plug_list;
+	unsigned int ret = 0;
+
+	plug = current->plug;
+	if (!plug)
+		goto out;
+
+	if (q->mq_ops)
+		plug_list = &plug->mq_list;
+	else
+		plug_list = &plug->list;
+
+	list_for_each_entry(rq, plug_list, queuelist) {
+		if (rq->q == q)
+			ret++;
+	}
+out:
+	return ret;
+}
+
 void init_request_from_bio(struct request *req, struct bio *bio)
 {
 	req->cmd_type = REQ_TYPE_FS;
@@ -1641,9 +1665,11 @@ static void blk_queue_bio(struct request_queue *q, struct bio *bio)
 	 * Check if we can merge with the plugged list before grabbing
 	 * any locks.
 	 */
-	if (!blk_queue_nomerges(q) &&
-	    blk_attempt_plug_merge(q, bio, &request_count, NULL))
-		return;
+	if (!blk_queue_nomerges(q)) {
+		if (blk_attempt_plug_merge(q, bio, &request_count, NULL))
+			return;
+	} else
+		request_count = blk_plug_queued_count(q);
 
 	spin_lock_irq(q->queue_lock);
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 7785ae9..604a1f3 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1267,9 +1267,12 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
 
 	blk_queue_split(q, &bio, q->bio_split);
 
-	if (!is_flush_fua && !blk_queue_nomerges(q) &&
-	    blk_attempt_plug_merge(q, bio, &request_count, &same_queue_rq))
-		return;
+	if (!is_flush_fua && !blk_queue_nomerges(q)) {
+		if (blk_attempt_plug_merge(q, bio, &request_count,
+					   &same_queue_rq))
+			return;
+	} else
+		request_count = blk_plug_queued_count(q);
 
 	rq = blk_mq_map_request(q, bio, &data);
 	if (unlikely(!rq))
diff --git a/block/blk.h b/block/blk.h
index 98614ad..aa27d02 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -86,6 +86,7 @@ bool bio_attempt_back_merge(struct request_queue *q, struct request *req,
 bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
 			    unsigned int *request_count,
 			    struct request **same_queue_rq);
+unsigned int blk_plug_queued_count(struct request_queue *q);
 
 void blk_account_io_start(struct request *req, bool new_io);
 void blk_account_io_completion(struct request *req, unsigned int bytes);