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Subject[PATCH 3/6 v2] blk-mq: fix plugging in blk_sq_make_request
Date
From: Jeff Moyer <jmoyer@redhat.com>

The following appears in blk_sq_make_request:

/*
* If we have multiple hardware queues, just go directly to
* one of those for sync IO.
*/

We clearly don't have multiple hardware queues, here! This comment was
introduced with this commit 07068d5b8e (blk-mq: split make request
handler for multi and single queue):

We want slightly different behavior from them:

- On single queue devices, we currently use the per-process plug
for deferred IO and for merging.

- On multi queue devices, we don't use the per-process plug, but
we want to go straight to hardware for SYNC IO.

The old code had this:

use_plug = !is_flush_fua && ((q->nr_hw_queues == 1) || !is_sync);

and that was converted to:

use_plug = !is_flush_fua && !is_sync;

which is not equivalent. For the single queue case, that second half of
the && expression is always true. So, what I think was actually inteded
follows (and this more closely matches what is done in blk_queue_bio).

V2: delete the 'likely', which should not be a big deal

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Shaohua Li <shli@fb.com>
---
block/blk-mq.c | 36 ++++++++++++++----------------------
1 file changed, 14 insertions(+), 22 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index ade8a2d..a65acff 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1309,16 +1309,11 @@ static void blk_sq_make_request(struct request_queue *q, struct bio *bio)
{
const int is_sync = rw_is_sync(bio->bi_rw);
const int is_flush_fua = bio->bi_rw & (REQ_FLUSH | REQ_FUA);
- unsigned int use_plug, request_count = 0;
+ struct blk_plug *plug;
+ unsigned int request_count = 0;
struct blk_map_ctx data;
struct request *rq;

- /*
- * If we have multiple hardware queues, just go directly to
- * one of those for sync IO.
- */
- use_plug = !is_flush_fua && !is_sync;
-
blk_queue_bounce(q, &bio);

if (bio_integrity_enabled(bio) && bio_integrity_prep(bio)) {
@@ -1326,7 +1321,7 @@ static void blk_sq_make_request(struct request_queue *q, struct bio *bio)
return;
}

- if (use_plug && !blk_queue_nomerges(q) &&
+ if (!is_flush_fua && !blk_queue_nomerges(q) &&
blk_attempt_plug_merge(q, bio, &request_count))
return;

@@ -1345,21 +1340,18 @@ static void blk_sq_make_request(struct request_queue *q, struct bio *bio)
* utilize that to temporarily store requests until the task is
* either done or scheduled away.
*/
- if (use_plug) {
- struct blk_plug *plug = current->plug;
-
- if (plug) {
- blk_mq_bio_to_request(rq, bio);
- if (list_empty(&plug->mq_list))
- trace_block_plug(q);
- else if (request_count >= BLK_MAX_REQUEST_COUNT) {
- blk_flush_plug_list(plug, false);
- trace_block_plug(q);
- }
- list_add_tail(&rq->queuelist, &plug->mq_list);
- blk_mq_put_ctx(data.ctx);
- return;
+ plug = current->plug;
+ if (plug) {
+ blk_mq_bio_to_request(rq, bio);
+ if (list_empty(&plug->mq_list))
+ trace_block_plug(q);
+ else if (request_count >= BLK_MAX_REQUEST_COUNT) {
+ blk_flush_plug_list(plug, false);
+ trace_block_plug(q);
}
+ list_add_tail(&rq->queuelist, &plug->mq_list);
+ blk_mq_put_ctx(data.ctx);
+ return;
}

if (!blk_mq_merge_queue_io(data.hctx, data.ctx, rq, bio)) {
--
1.8.1


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