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blk-mq: fix plugging in blk_sq_make_request 

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>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Jeff Moyer 2015-05-08 10:51:30 -07:00 committed by Jens Axboe
parent 5596d0d591
commit e6c4438ba7
1 changed files with 14 additions and 22 deletions
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36
block/blk-mq.c
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@ -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)) {