[SRU][PATCH 0/1][B/gke-5.0] blk-wbt: fix performance regression in wbt scale_up/scale_down

[Khalid Elmously]

From: Harshad Shirwadkar <harshadshirwadkar at gmail.com>

BugLink: https://bugs.launchpad.net/bugs/1847641

scale_up wakes up waiters after scaling up. But after scaling max, it
should not wake up more waiters as waiters will not have anything to
do. This patch fixes this by making scale_up (and also scale_down)
return when threshold is reached.

This bug causes increased fdatasync latency when fdatasync and dd
conv=sync are performed in parallel on 4.19 compared to 4.14. This
bug was introduced during refactoring of blk-wbt code.

Fixes: a79050434b45 ("blk-rq-qos: refactor out common elements of blk-wbt")
Cc: stable at vger.kernel.org
Cc: Josef Bacik <jbacik at fb.com>
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar at gmail.com>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
(cherry picked from commit b84477d3ebb96294f87dc3161e53fa8fe22d9bfd linux-block)
Signed-off-by: Khalid Elmously <khalid.elmously at canonical.com>
---
 block/blk-rq-qos.c | 14 +++++++++-----
 block/blk-rq-qos.h |  4 ++--
 block/blk-wbt.c    |  6 ++++--
 3 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index c69a1515a7ee..fc5e1ef5bad9 100644
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -140,24 +140,27 @@ bool rq_depth_calc_max_depth(struct rq_depth *rqd)
 	return ret;
 }
 
-void rq_depth_scale_up(struct rq_depth *rqd)
+/* Returns true on success and false if scaling up wasn't possible */
+bool rq_depth_scale_up(struct rq_depth *rqd)
 {
 	/*
 	 * Hit max in previous round, stop here
 	 */
 	if (rqd->scaled_max)
-		return;
+		return false;
 
 	rqd->scale_step--;
 
 	rqd->scaled_max = rq_depth_calc_max_depth(rqd);
+	return true;
 }
 
 /*
  * Scale rwb down. If 'hard_throttle' is set, do it quicker, since we
- * had a latency violation.
+ * had a latency violation. Returns true on success and returns false if
+ * scaling down wasn't possible.
  */
-void rq_depth_scale_down(struct rq_depth *rqd, bool hard_throttle)
+bool rq_depth_scale_down(struct rq_depth *rqd, bool hard_throttle)
 {
 	/*
 	 * Stop scaling down when we've hit the limit. This also prevents
@@ -165,7 +168,7 @@ void rq_depth_scale_down(struct rq_depth *rqd, bool hard_throttle)
 	 * keep up.
 	 */
 	if (rqd->max_depth == 1)
-		return;
+		return false;
 
 	if (rqd->scale_step < 0 && hard_throttle)
 		rqd->scale_step = 0;
@@ -174,6 +177,7 @@ void rq_depth_scale_down(struct rq_depth *rqd, bool hard_throttle)
 
 	rqd->scaled_max = false;
 	rq_depth_calc_max_depth(rqd);
+	return true;
 }
 
 struct rq_qos_wait_data {
diff --git a/block/blk-rq-qos.h b/block/blk-rq-qos.h
index 564851889550..ddd0c9e6d53c 100644
--- a/block/blk-rq-qos.h
+++ b/block/blk-rq-qos.h
@@ -124,8 +124,8 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data,
 		 acquire_inflight_cb_t *acquire_inflight_cb,
 		 cleanup_cb_t *cleanup_cb);
 bool rq_wait_inc_below(struct rq_wait *rq_wait, unsigned int limit);
-void rq_depth_scale_up(struct rq_depth *rqd);
-void rq_depth_scale_down(struct rq_depth *rqd, bool hard_throttle);
+bool rq_depth_scale_up(struct rq_depth *rqd);
+bool rq_depth_scale_down(struct rq_depth *rqd, bool hard_throttle);
 bool rq_depth_calc_max_depth(struct rq_depth *rqd);
 
 void __rq_qos_cleanup(struct rq_qos *rqos, struct bio *bio);
diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index fd166fbb0f65..205e566b1ac3 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -307,7 +307,8 @@ static void calc_wb_limits(struct rq_wb *rwb)
 
 static void scale_up(struct rq_wb *rwb)
 {
-	rq_depth_scale_up(&rwb->rq_depth);
+	if (!rq_depth_scale_up(&rwb->rq_depth))
+		return;
 	calc_wb_limits(rwb);
 	rwb->unknown_cnt = 0;
 	rwb_wake_all(rwb);
@@ -316,7 +317,8 @@ static void scale_up(struct rq_wb *rwb)
 
 static void scale_down(struct rq_wb *rwb, bool hard_throttle)
 {
-	rq_depth_scale_down(&rwb->rq_depth, hard_throttle);
+	if (!rq_depth_scale_down(&rwb->rq_depth, hard_throttle))
+		return;
 	calc_wb_limits(rwb);
 	rwb->unknown_cnt = 0;
 	rwb_trace_step(rwb, "scale down");
-- 
2.17.1