[PATCH v2 5/6] block: prevent merging of requests with different priorities

[Damien Le Moal]

Growing in size a high priority request by merging it with a lower
priority BIO or request will increase the request execution time. This
is the opposite result of the desired effect of high I/O priorities,
namely getting low I/O latencies. Prevent merging of requests and BIOs
that have different I/O priorities to fix this.

Signed-off-by: Damien Le Moal <damien.lem...@wdc.com>
---
 block/blk-core.c  | 3 ---
 block/blk-merge.c | 8 ++++++--
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 4450d3c08f25..dde30b08aa14 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -662,7 +662,6 @@ bool bio_attempt_back_merge(struct request_queue *q, struct 
request *req,
        req->biotail->bi_next = bio;
        req->biotail = bio;
        req->__data_len += bio->bi_iter.bi_size;
-       req->ioprio = ioprio_best(req->ioprio, bio_prio(bio));
 
        blk_account_io_start(req, false);
        return true;
@@ -686,7 +685,6 @@ bool bio_attempt_front_merge(struct request_queue *q, 
struct request *req,
 
        req->__sector = bio->bi_iter.bi_sector;
        req->__data_len += bio->bi_iter.bi_size;
-       req->ioprio = ioprio_best(req->ioprio, bio_prio(bio));
 
        blk_account_io_start(req, false);
        return true;
@@ -706,7 +704,6 @@ bool bio_attempt_discard_merge(struct request_queue *q, 
struct request *req,
        req->biotail->bi_next = bio;
        req->biotail = bio;
        req->__data_len += bio->bi_iter.bi_size;
-       req->ioprio = ioprio_best(req->ioprio, bio_prio(bio));
        req->nr_phys_segments = segments + 1;
 
        blk_account_io_start(req, false);
diff --git a/block/blk-merge.c b/block/blk-merge.c
index b1df622cbd85..6be04ef8da5b 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -752,6 +752,9 @@ static struct request *attempt_merge(struct request_queue 
*q,
        if (req->write_hint != next->write_hint)
                return NULL;
 
+       if (req->ioprio != next->ioprio)
+               return NULL;
+
        /*
         * If we are allowed to merge, then append bio list
         * from next to rq and release next. merge_requests_fn
@@ -807,8 +810,6 @@ static struct request *attempt_merge(struct request_queue 
*q,
         */
        blk_account_io_merge(next);
 
-       req->ioprio = ioprio_best(req->ioprio, next->ioprio);
-
        /*
         * ownership of bio passed from next to req, return 'next' for
         * the caller to free
@@ -883,6 +884,9 @@ bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
        if (rq->write_hint != bio->bi_write_hint)
                return false;
 
+       if (rq->ioprio != bio_prio(bio))
+               return false;
+
        return true;
 }
 
-- 
2.19.1