[RFC PATCH] blkcg: prevent priority inversion problem during sync()

[Andrea Righi <righi.andrea@gmail.com>]

This is an attempt to mitigate the priority inversion problem of a
high-priority blkcg issuing a sync() and being forced to wait the
completion of all the writeback I/O generated by any other low-priority
blkcg, causing massive latencies to processes that shouldn't be
I/O-throttled at all.

The idea is to save a list of blkcg's that are waiting for writeback:
every time a sync() is executed the current blkcg is added to the list.

Then, when I/O is throttled, if there's a blkcg waiting for writeback
different than the current blkcg, no throttling is applied (we can
probably refine this logic later, i.e., a better policy could be to
adjust the throttling rate using the blkcg with the highest speed from
the list of waiters - priority inheritance, kinda).

This topic has been discussed here:
https://lwn.net/ml/cgroups/20190118103127.325-1-righi.andrea@gmail.com/

But we didn't come up with any definitive solution.

This patch is not a definitive solution either, but it's an attempt to
continue addressing the issue and, hopefully, handle the priority
inversion problem with sync() in a better way.

Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
---
 block/blk-cgroup.c               | 69 ++++++++++++++++++++++++++++++++
 block/blk-throttle.c             | 10 +++--
 fs/fs-writeback.c                |  4 ++
 include/linux/backing-dev-defs.h |  2 +
 include/linux/blk-cgroup.h       | 15 +++++++
 mm/backing-dev.c                 |  2 +
 6 files changed, 99 insertions(+), 3 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 2bed5725aa03..d71e3cb0688d 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1635,6 +1635,75 @@ static void blkcg_scale_delay(struct blkcg_gq *blkg, u64 now)
 	}
 }
 
+/**
+ * blkcg_wb_waiters_on_bdi - check for writeback waiters on a block device
+ * @bdi: block device to check
+ *
+ * Return true if any other blkcg is waiting for writeback on the target block
+ * device, false otherwise.
+ */
+bool blkcg_wb_waiters_on_bdi(struct backing_dev_info *bdi)
+{
+	struct blkcg *blkcg, *curr_blkcg;
+	bool ret = false;
+
+	if (unlikely(!bdi))
+		return false;
+
+	rcu_read_lock();
+	curr_blkcg = css_to_blkcg(task_css(current, io_cgrp_id));
+	list_for_each_entry_rcu(blkcg, &bdi->cgwb_waiters, cgwb_wait_node)
+		if (blkcg != curr_blkcg) {
+			ret = true;
+			break;
+		}
+	rcu_read_unlock();
+
+	return ret;
+}
+
+/**
+ * blkcg_start_wb_wait_on_bdi - add current blkcg to writeback waiters list
+ * @bdi: target block device
+ *
+ * Add current blkcg to the list of writeback waiters on target block device.
+ */
+void blkcg_start_wb_wait_on_bdi(struct backing_dev_info *bdi)
+{
+	struct blkcg *blkcg;
+
+	rcu_read_lock();
+	blkcg = css_to_blkcg(task_css(current, io_cgrp_id));
+	if (blkcg) {
+		spin_lock(&bdi->cgwb_waiters_lock);
+		list_add_rcu(&blkcg->cgwb_wait_node, &bdi->cgwb_waiters);
+		spin_unlock(&bdi->cgwb_waiters_lock);
+	}
+	rcu_read_unlock();
+}
+
+/**
+ * blkcg_stop_wb_wait_on_bdi - remove current blkcg from writeback waiters list
+ * @bdi: target block device
+ *
+ * Remove current blkcg from the list of writeback waiters on target block
+ * device.
+ */
+void blkcg_stop_wb_wait_on_bdi(struct backing_dev_info *bdi)
+{
+	struct blkcg *blkcg;
+
+	rcu_read_lock();
+	blkcg = css_to_blkcg(task_css(current, io_cgrp_id));
+	if (blkcg) {
+		spin_lock(&bdi->cgwb_waiters_lock);
+		list_del_rcu(&blkcg->cgwb_wait_node);
+		spin_unlock(&bdi->cgwb_waiters_lock);
+	}
+	rcu_read_unlock();
+	synchronize_rcu();
+}
+
 /*
  * This is called when we want to actually walk up the hierarchy and check to
  * see if we need to throttle, and then actually throttle if there is some
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 1b97a73d2fb1..14d9cd6e702d 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -970,9 +970,12 @@ static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio,
 {
 	bool rw = bio_data_dir(bio);
 	unsigned long bps_wait = 0, iops_wait = 0, max_wait = 0;
+	struct throtl_data *td = tg->td;
+	struct request_queue *q = td->queue;
+	struct backing_dev_info *bdi = q->backing_dev_info;
 
 	/*
- 	 * Currently whole state machine of group depends on first bio
+	 * Currently whole state machine of group depends on first bio
 	 * queued in the group bio list. So one should not be calling
 	 * this function with a different bio if there are other bios
 	 * queued.
@@ -981,8 +984,9 @@ static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio,
 	       bio != throtl_peek_queued(&tg->service_queue.queued[rw]));
 
 	/* If tg->bps = -1, then BW is unlimited */
-	if (tg_bps_limit(tg, rw) == U64_MAX &&
-	    tg_iops_limit(tg, rw) == UINT_MAX) {
+	if (blkcg_wb_waiters_on_bdi(bdi) ||
+	    (tg_bps_limit(tg, rw) == U64_MAX &&
+	    tg_iops_limit(tg, rw) == UINT_MAX)) {
 		if (wait)
 			*wait = 0;
 		return true;
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 36855c1f8daf..13880774af3c 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -2446,6 +2446,8 @@ void sync_inodes_sb(struct super_block *sb)
 		return;
 	WARN_ON(!rwsem_is_locked(&sb->s_umount));
 
+	blkcg_start_wb_wait_on_bdi(bdi);
+
 	/* protect against inode wb switch, see inode_switch_wbs_work_fn() */
 	bdi_down_write_wb_switch_rwsem(bdi);
 	bdi_split_work_to_wbs(bdi, &work, false);
@@ -2453,6 +2455,8 @@ void sync_inodes_sb(struct super_block *sb)
 	bdi_up_write_wb_switch_rwsem(bdi);
 
 	wait_sb_inodes(sb);
+
+	blkcg_stop_wb_wait_on_bdi(bdi);
 }
 EXPORT_SYMBOL(sync_inodes_sb);
 
diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 07e02d6df5ad..095e4dd0427b 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -191,6 +191,8 @@ struct backing_dev_info {
 	struct rb_root cgwb_congested_tree; /* their congested states */
 	struct mutex cgwb_release_mutex;  /* protect shutdown of wb structs */
 	struct rw_semaphore wb_switch_rwsem; /* no cgwb switch while syncing */
+	struct list_head cgwb_waiters; /* list of all waiters for writeback */
+	spinlock_t cgwb_waiters_lock; /* protect cgwb_waiters list */
 #else
 	struct bdi_writeback_congested *wb_congested;
 #endif
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 76c61318fda5..b8fe0c603ef8 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -56,6 +56,7 @@ struct blkcg {
 
 	struct list_head		all_blkcgs_node;
 #ifdef CONFIG_CGROUP_WRITEBACK
+	struct list_head		cgwb_wait_node;
 	struct list_head		cgwb_list;
 	refcount_t			cgwb_refcnt;
 #endif
@@ -454,6 +455,10 @@ static inline void blkcg_cgwb_put(struct blkcg *blkcg)
 		blkcg_destroy_blkgs(blkcg);
 }
 
+bool blkcg_wb_waiters_on_bdi(struct backing_dev_info *bdi);
+void blkcg_start_wb_wait_on_bdi(struct backing_dev_info *bdi);
+void blkcg_stop_wb_wait_on_bdi(struct backing_dev_info *bdi);
+
 #else
 
 static inline void blkcg_cgwb_get(struct blkcg *blkcg) { }
@@ -464,6 +469,13 @@ static inline void blkcg_cgwb_put(struct blkcg *blkcg)
 	blkcg_destroy_blkgs(blkcg);
 }
 
+static inline bool blkcg_wb_waiters_on_bdi(struct backing_dev_info *bdi)
+{
+	return false;
+}
+static inline void blkcg_start_wb_wait_on_bdi(struct backing_dev_info *bdi) { }
+static inline void blkcg_stop_wb_wait_on_bdi(struct backing_dev_info *bdi) { }
+
 #endif
 
 /**
@@ -772,6 +784,7 @@ static inline void blkcg_bio_issue_init(struct bio *bio)
 static inline bool blkcg_bio_issue_check(struct request_queue *q,
 					 struct bio *bio)
 {
+	struct backing_dev_info *bdi = q->backing_dev_info;
 	struct blkcg_gq *blkg;
 	bool throtl = false;
 
@@ -785,6 +798,8 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 			  bio_devname(bio, b));
 		bio_associate_blkg(bio);
 	}
+	if (blkcg_wb_waiters_on_bdi(bdi))
+		bio_set_flag(bio, BIO_THROTTLED);
 
 	blkg = bio->bi_blkg;
 
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 72e6d0c55cfa..8848d26e8bf6 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -686,10 +686,12 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
 {
 	int ret;
 
+	INIT_LIST_HEAD(&bdi->cgwb_waiters);
 	INIT_RADIX_TREE(&bdi->cgwb_tree, GFP_ATOMIC);
 	bdi->cgwb_congested_tree = RB_ROOT;
 	mutex_init(&bdi->cgwb_release_mutex);
 	init_rwsem(&bdi->wb_switch_rwsem);
+	spin_lock_init(&bdi->cgwb_waiters_lock);
 
 	ret = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
 	if (!ret) {
-- 
2.17.1