On 12/11/2014 03:02 PM, Jeff Moyer wrote:
> 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).
Good catch! Yes, single queue used to plug on just !is_flush_fua.
Shaohua has a patch in this area too, CC'ing him.
But this looks correct for the single queue case, there _should_ be a
nice win for retaining plugging there. You now added merging for flush
fua though, intentional or an oversight? I'd retain the checks there,
basically just make use_plug !is_flush_fua, perhaps.
Leaving patch below.
> 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)) {
>
--
Jens Axboe
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