Messages in this thread Patch in this message | | | From | Jeff Moyer <> | Subject | [patch] blk-mq: fix plugging in blk_sq_make_request | Date | Thu, 11 Dec 2014 17:02:49 -0500 |
| |
Hi,
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).
I haven't done any performance testing of this yet. I'd really just like to see if I've got the intent right first. Jens?
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
diff --git a/block/blk-mq.c b/block/blk-mq.c index 1d016fc..1cd90c0 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -1208,16 +1208,11 @@ 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)) { @@ -1225,7 +1220,7 @@ static void blk_sq_make_request(struct request_queue *q, struct bio *bio) return; } - if (use_plug && !blk_queue_nomerges(q) && + if (!blk_queue_nomerges(q) && blk_attempt_plug_merge(q, bio, &request_count)) return; @@ -1244,21 +1239,18 @@ 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)) {
| |