On Tue, Jan 16 2018 at 10:01P -0500,
Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> From: Ming Lei <ming.lei@xxxxxxxxxx>
>
> blk_insert_cloned_request() is called in fast path of dm-rq driver, and
> in this function we append request to hctx->dispatch_list of the underlying
> queue directly.
>
> 1) This way isn't efficient enough because hctx lock is always required
>
> 2) With blk_insert_cloned_request(), we bypass underlying queue's IO
> scheduler totally, and depend on DM rq driver to do IO schedule
> completely. But DM rq driver can't get underlying queue's dispatch
> feedback at all, and this information is extreamly useful for IO merge.
> Without that IO merge can't be done basically by blk-mq, which causes
> very bad sequential IO performance.
>
> Fix this by having blk_insert_cloned_request() make use of
> blk_mq_try_issue_directly() via blk_mq_request_direct_issue().
> blk_mq_request_direct_issue() allows a request to be dispatched to be
> issue directly to the underlying queue and provides dispatch result to
> dm-rq and blk-mq.
>
> With this, the DM's blk-mq sequential IO performance is vastly
> improved (as much as 3X in mpath/virtio-scsi testing).
>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
> ---
> block/blk-core.c | 3 +--
> block/blk-mq.c | 42 ++++++++++++++++++++++++++++++++++++------
> block/blk-mq.h | 3 +++
> drivers/md/dm-rq.c | 19 ++++++++++++++++---
> 4 files changed, 56 insertions(+), 11 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 7ba607527487..55f338020254 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -2500,8 +2500,7 @@ blk_status_t blk_insert_cloned_request(struct request_queue *q, struct request *
> * bypass a potential scheduler on the bottom device for
> * insert.
> */
> - blk_mq_request_bypass_insert(rq, true);
> - return BLK_STS_OK;
> + return blk_mq_request_direct_issue(rq);
> }
>
> spin_lock_irqsave(q->queue_lock, flags);
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 55f3a27fb2e6..3168a13cb012 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1706,6 +1706,12 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> blk_qc_t new_cookie;
> blk_status_t ret = BLK_STS_OK;
> bool run_queue = true;
> + /*
> + * If @cookie is NULL do not insert the request, this mode is used
> + * by blk_insert_cloned_request() via blk_mq_request_direct_issue()
> + */
> + bool dispatch_only = !cookie;
> + bool need_insert = false;
>
> /* RCU or SRCU read lock is needed before checking quiesced flag */
> if (blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)) {
> @@ -1713,25 +1719,38 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> goto insert;
> }
>
> - if (q->elevator)
> + if (q->elevator && !dispatch_only)
> goto insert;
>
> if (!blk_mq_get_driver_tag(rq, NULL, false))
> - goto insert;
> + need_insert = true;
>
> - if (!blk_mq_get_dispatch_budget(hctx)) {
> + if (!need_insert && !blk_mq_get_dispatch_budget(hctx)) {
> blk_mq_put_driver_tag(rq);
> + need_insert = true;
> + }
> +
> + if (need_insert) {
> + if (dispatch_only)
> + return BLK_STS_RESOURCE;
> goto insert;
> }
>
> new_cookie = request_to_qc_t(hctx, rq);
>
> + ret = q->mq_ops->queue_rq(hctx, &bd);
> +
> + if (dispatch_only) {
> + if (ret == BLK_STS_RESOURCE)
> + __blk_mq_requeue_request(rq);
> + return ret;
> + }
> +
> /*
> * For OK queue, we are done. For error, kill it. Any other
> * error (busy), just add it to our list as we previously
> * would have done
> */
> - ret = q->mq_ops->queue_rq(hctx, &bd);
> switch (ret) {
> case BLK_STS_OK:
> *cookie = new_cookie;
FYI, because blk_mq_put_driver_tag() is idempotent, the above can be
simplified by eliminating 'need_insert' like so (Jens feel free to fold
this in?):
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 3168a13cb012..c2f66a5c5242 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1711,7 +1711,6 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
* by blk_insert_cloned_request() via blk_mq_request_direct_issue()
*/
bool dispatch_only = !cookie;
- bool need_insert = false;
/* RCU or SRCU read lock is needed before checking quiesced flag */
if (blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)) {
@@ -1722,15 +1721,9 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
if (q->elevator && !dispatch_only)
goto insert;
- if (!blk_mq_get_driver_tag(rq, NULL, false))
- need_insert = true;
-
- if (!need_insert && !blk_mq_get_dispatch_budget(hctx)) {
+ if (!blk_mq_get_driver_tag(rq, NULL, false) ||
+ !blk_mq_get_dispatch_budget(hctx)) {
blk_mq_put_driver_tag(rq);
- need_insert = true;
- }
-
- if (need_insert) {
if (dispatch_only)
return BLK_STS_RESOURCE;
goto insert;
--
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