[PATCH RFC] blk-throttle: add feedback to cgroup writeback about throttled writes

[Konstantin Khlebnikov]

Throttler steals bio before allocating requests for them,
thus throttled writeback never reaches congestion.

This adds bit WB_write_throttled into per-cgroup bdi congestion control.
It's set when write bandwidth limit is exceeded and throttler has at least
one bio inside and cleared when last throttled bio is gone.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
---
 block/blk-throttle.c             |    8 ++++++++
 include/linux/backing-dev-defs.h |   19 +++++++++++++++++++
 include/linux/backing-dev.h      |    6 ++++--
 3 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 0fea76aa0f3f..7ec0aaf9efa7 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1145,6 +1145,10 @@ static void tg_dispatch_one_bio(struct throtl_grp *tg, bool rw)
 	bio = throtl_pop_queued(&sq->queued[rw], &tg_to_put);
 	sq->nr_queued[rw]--;
 
+	/* Last throttled @bio is gone, clear congestion bit. */
+	if (rw && !sq->nr_queued[rw])
+		clear_wb_throttled(tg_to_blkg(tg)->wb_congested);
+
 	throtl_charge_bio(tg, bio);
 
 	/*
@@ -2215,6 +2219,10 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 	 * its @tg's disptime is not in the future.
 	 */
 	if (tg->flags & THROTL_TG_WAS_EMPTY) {
+		/* Next write will be throttled, set congestion bit. */
+		if (rw && blkg)
+			set_wb_throttled(blkg->wb_congested);
+
 		tg_update_disptime(tg);
 		throtl_schedule_next_dispatch(tg->service_queue.parent_sq, true);
 	}
diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 866c433e7d32..343107fd4eff 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -29,6 +29,7 @@ enum wb_state {
 enum wb_congested_state {
 	WB_async_congested,	/* The async (write) queue is getting full */
 	WB_sync_congested,	/* The sync queue is getting full */
+	WB_write_throttled,	/* Associated blkcg is throttled */
 };
 
 typedef int (congested_fn)(void *, int);
@@ -200,6 +201,16 @@ static inline void set_bdi_congested(struct backing_dev_info *bdi, int sync)
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 
+static inline void set_wb_throttled(struct bdi_writeback_congested *congested)
+{
+	set_bit(WB_write_throttled, &congested->state);
+}
+
+static inline void clear_wb_throttled(struct bdi_writeback_congested *congested)
+{
+	clear_bit(WB_write_throttled, &congested->state);
+}
+
 /**
  * wb_tryget - try to increment a wb's refcount
  * @wb: bdi_writeback to get
@@ -244,6 +255,14 @@ static inline bool wb_dying(struct bdi_writeback *wb)
 
 #else	/* CONFIG_CGROUP_WRITEBACK */
 
+static inline void set_wb_throttled(struct bdi_writeback_congested *congested)
+{
+}
+
+static inline void clear_wb_throttled(struct bdi_writeback_congested *congested)
+{
+}
+
 static inline bool wb_tryget(struct bdi_writeback *wb)
 {
 	return true;
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 854e1bdd0b2a..3619641b5ec5 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -460,13 +460,15 @@ static inline int inode_read_congested(struct inode *inode)
 
 static inline int inode_write_congested(struct inode *inode)
 {
-	return inode_congested(inode, 1 << WB_async_congested);
+	return inode_congested(inode, (1 << WB_async_congested) |
+				      (1 << WB_write_throttled));
 }
 
 static inline int inode_rw_congested(struct inode *inode)
 {
 	return inode_congested(inode, (1 << WB_sync_congested) |
-				      (1 << WB_async_congested));
+				      (1 << WB_async_congested) |
+				      (1 << WB_write_throttled));
 }
 
 static inline int bdi_congested(struct backing_dev_info *bdi, int cong_bits)