[PATCH V2 05/20] blk-mq-sched: improve dispatching from sw queue

Ming Lei
2017-08-05T06:57:38Z

SCSI devices use host-wide tagset, and the shared
driver tag space is often quite big. Meantime
there is also queue depth for each lun(.cmd_per_lun),
which is often small.

So lots of requests may stay in sw queue, and we
always flush all belonging to same hw queue and
dispatch them all to driver, unfortunately it is
easy to cause queue busy becasue of the small
per-lun queue depth. Once these requests are flushed
out, they have to stay in hctx->dispatch, and no bio
merge can participate into these requests, and
sequential IO performance is hurted.

This patch improves dispatching from sw queue when
there is per-request-queue queue depth by taking
request one by one from sw queue, just like the way
of IO scheduler.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
block/blk-mq-sched.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 49 insertions(+), 6 deletions(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index f69752961a34..e43c9407d653 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -89,9 +89,9 @@ static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
return false;
}

-static void blk_mq_do_dispatch(struct request_queue *q,
- struct elevator_queue *e,
- struct blk_mq_hw_ctx *hctx)
+static inline void blk_mq_do_dispatch_sched(struct request_queue *q,
+ struct elevator_queue *e,
+ struct blk_mq_hw_ctx *hctx)
{
LIST_HEAD(rq_list);

@@ -105,6 +105,36 @@ static void blk_mq_do_dispatch(struct request_queue *q,
} while (blk_mq_dispatch_rq_list(q, &rq_list));
}

+static inline struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
+ struct blk_mq_ctx *ctx)
+{
+ unsigned idx = ctx->index_hw;
+
+ if (++idx == hctx->nr_ctx)
+ idx = 0;
+
+ return hctx->ctxs[idx];
+}
+
+static inline void blk_mq_do_dispatch_ctx(struct request_queue *q,
+ struct blk_mq_hw_ctx *hctx)
+{
+ LIST_HEAD(rq_list);
+ struct blk_mq_ctx *ctx = NULL;
+
+ do {
+ struct request *rq;
+
+ rq = blk_mq_dispatch_rq_from_ctx(hctx, ctx);
+ if (!rq)
+ break;
+ list_add(&rq->queuelist, &rq_list);
+
+ /* round robin for fair dispatch */
+ ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
+ } while (blk_mq_dispatch_rq_list(q, &rq_list));
+}
+
void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
{
struct request_queue *q = hctx->queue;
@@ -142,18 +172,31 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
if (!list_empty(&rq_list)) {
blk_mq_sched_mark_restart_hctx(hctx);
do_sched_dispatch = blk_mq_dispatch_rq_list(q, &rq_list);
- } else if (!has_sched_dispatch) {
+ } else if (!has_sched_dispatch & !q->queue_depth) {
+ /*
+ * If there is no per-request_queue depth, we
+ * flush all requests in this hw queue, otherwise
+ * pick up request one by one from sw queue for
+ * avoiding to mess up I/O merge when dispatch
+ * is busy, which can be triggeredd easily by
+ * limit of of request_queue's queue depth
+ */
blk_mq_flush_busy_ctxs(hctx, &rq_list);
blk_mq_dispatch_rq_list(q, &rq_list);
}

+ if (!do_sched_dispatch)
+ return;
+
/*
* We want to dispatch from the scheduler if we had no work left
* on the dispatch list, OR if we did have work but weren't able
* to make progress.
*/
- if (do_sched_dispatch && has_sched_dispatch)
- blk_mq_do_dispatch(q, e, hctx);
+ if (has_sched_dispatch)
+ blk_mq_do_dispatch_sched(q, e, hctx);
+ else
+ blk_mq_do_dispatch_ctx(q, hctx);
}

bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
--
2.9.4