[PATCH 0/7] bcache patches for Linux v5.7-rc1

[PATCH 0/7] bcache patches for Linux v5.7-rc1

[Coly Li]

Hi Jens,

These are bcache patches for Linux v5.7-rc1.

The major change is to make bcache btree check and dirty secrtors
counting being multithreaded, then the registration time can be
much less. My first four patches are for this purpose.

Davidlohr Bueso contributes a patch to optimize barrier usage for
atomic operations. By his inspiration I also compose a patch for
the rested locations to change.

Takashi Iwai contributes a helpful patch to avoid potential
buffer overflow in bcache sysfs code path.

Please take them, and thank you in advance.

Coly Li
---

Coly Li (5):
  bcache: move macro btree() and btree_root() into btree.h
  bcache: add bcache_ prefix to btree_root() and btree() macros
  bcache: make bch_btree_check() to be multithreaded
  bcache: make bch_sectors_dirty_init() to be multithreaded
  bcache: optimize barrier usage for atomic operations

Davidlohr Bueso (1):
  bcache: optimize barrier usage for Rmw atomic bitops

Takashi Iwai (1):
  bcache: Use scnprintf() for avoiding potential buffer overflow

 drivers/md/bcache/btree.c     | 242 ++++++++++++++++++++++++----------
 drivers/md/bcache/btree.h     |  88 +++++++++++++
 drivers/md/bcache/sysfs.c     |   2 +-
 drivers/md/bcache/writeback.c | 164 ++++++++++++++++++++++-
 drivers/md/bcache/writeback.h |  19 +++
 5 files changed, 440 insertions(+), 75 deletions(-)

-- 
2.25.0

[PATCH 1/7] bcache: move macro btree() and btree_root() into btree.h

[Coly Li]

In order to accelerate bcache registration speed, the macro btree()
and btree_root() will be referenced out of btree.c. This patch moves
them from btree.c into btree.h with other relative function declaration
in btree.h, for the following changes.

Signed-off-by: Coly Li <colyli@suse.de>
---
 drivers/md/bcache/btree.c | 60 +----------------------------------
 drivers/md/bcache/btree.h | 66 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 67 insertions(+), 59 deletions(-)

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index fa872df4e770..99cb201809af 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -101,64 +101,6 @@
 
 #define insert_lock(s, b)	((b)->level <= (s)->lock)
 
-/*
- * These macros are for recursing down the btree - they handle the details of
- * locking and looking up nodes in the cache for you. They're best treated as
- * mere syntax when reading code that uses them.
- *
- * op->lock determines whether we take a read or a write lock at a given depth.
- * If you've got a read lock and find that you need a write lock (i.e. you're
- * going to have to split), set op->lock and return -EINTR; btree_root() will
- * call you again and you'll have the correct lock.
- */
-
-/**
- * btree - recurse down the btree on a specified key
- * @fn:		function to call, which will be passed the child node
- * @key:	key to recurse on
- * @b:		parent btree node
- * @op:		pointer to struct btree_op
- */
-#define btree(fn, key, b, op, ...)					\
-({									\
-	int _r, l = (b)->level - 1;					\
-	bool _w = l <= (op)->lock;					\
-	struct btree *_child = bch_btree_node_get((b)->c, op, key, l,	\
-						  _w, b);		\
-	if (!IS_ERR(_child)) {						\
-		_r = bch_btree_ ## fn(_child, op, ##__VA_ARGS__);	\
-		rw_unlock(_w, _child);					\
-	} else								\
-		_r = PTR_ERR(_child);					\
-	_r;								\
-})
-
-/**
- * btree_root - call a function on the root of the btree
- * @fn:		function to call, which will be passed the child node
- * @c:		cache set
- * @op:		pointer to struct btree_op
- */
-#define btree_root(fn, c, op, ...)					\
-({									\
-	int _r = -EINTR;						\
-	do {								\
-		struct btree *_b = (c)->root;				\
-		bool _w = insert_lock(op, _b);				\
-		rw_lock(_w, _b, _b->level);				\
-		if (_b == (c)->root &&					\
-		    _w == insert_lock(op, _b)) {			\
-			_r = bch_btree_ ## fn(_b, op, ##__VA_ARGS__);	\
-		}							\
-		rw_unlock(_w, _b);					\
-		bch_cannibalize_unlock(c);				\
-		if (_r == -EINTR)					\
-			schedule();					\
-	} while (_r == -EINTR);						\
-									\
-	finish_wait(&(c)->btree_cache_wait, &(op)->wait);		\
-	_r;								\
-})
 
 static inline struct bset *write_block(struct btree *b)
 {
@@ -2422,7 +2364,7 @@ int __bch_btree_map_nodes(struct btree_op *op, struct cache_set *c,
 	return btree_root(map_nodes_recurse, c, op, from, fn, flags);
 }
 
-static int bch_btree_map_keys_recurse(struct btree *b, struct btree_op *op,
+int bch_btree_map_keys_recurse(struct btree *b, struct btree_op *op,
 				      struct bkey *from, btree_map_keys_fn *fn,
 				      int flags)
 {
diff --git a/drivers/md/bcache/btree.h b/drivers/md/bcache/btree.h
index f4dcca449391..f37153db3f6c 100644
--- a/drivers/md/bcache/btree.h
+++ b/drivers/md/bcache/btree.h
@@ -260,6 +260,13 @@ void bch_initial_gc_finish(struct cache_set *c);
 void bch_moving_gc(struct cache_set *c);
 int bch_btree_check(struct cache_set *c);
 void bch_initial_mark_key(struct cache_set *c, int level, struct bkey *k);
+typedef int (btree_map_keys_fn)(struct btree_op *op, struct btree *b,
+				struct bkey *k);
+int bch_btree_map_keys_recurse(struct btree *b, struct btree_op *op,
+			       struct bkey *from, btree_map_keys_fn *fn,
+			       int flags);
+int bch_btree_map_keys(struct btree_op *op, struct cache_set *c,
+		       struct bkey *from, btree_map_keys_fn *fn, int flags);
 
 static inline void wake_up_gc(struct cache_set *c)
 {
@@ -284,6 +291,65 @@ static inline void force_wake_up_gc(struct cache_set *c)
 	wake_up_gc(c);
 }
 
+/*
+ * These macros are for recursing down the btree - they handle the details of
+ * locking and looking up nodes in the cache for you. They're best treated as
+ * mere syntax when reading code that uses them.
+ *
+ * op->lock determines whether we take a read or a write lock at a given depth.
+ * If you've got a read lock and find that you need a write lock (i.e. you're
+ * going to have to split), set op->lock and return -EINTR; btree_root() will
+ * call you again and you'll have the correct lock.
+ */
+
+/**
+ * btree - recurse down the btree on a specified key
+ * @fn:		function to call, which will be passed the child node
+ * @key:	key to recurse on
+ * @b:		parent btree node
+ * @op:		pointer to struct btree_op
+ */
+#define btree(fn, key, b, op, ...)					\
+({									\
+	int _r, l = (b)->level - 1;					\
+	bool _w = l <= (op)->lock;					\
+	struct btree *_child = bch_btree_node_get((b)->c, op, key, l,	\
+						  _w, b);		\
+	if (!IS_ERR(_child)) {						\
+		_r = bch_btree_ ## fn(_child, op, ##__VA_ARGS__);	\
+		rw_unlock(_w, _child);					\
+	} else								\
+		_r = PTR_ERR(_child);					\
+	_r;								\
+})
+
+/**
+ * btree_root - call a function on the root of the btree
+ * @fn:		function to call, which will be passed the child node
+ * @c:		cache set
+ * @op:		pointer to struct btree_op
+ */
+#define btree_root(fn, c, op, ...)					\
+({									\
+	int _r = -EINTR;						\
+	do {								\
+		struct btree *_b = (c)->root;				\
+		bool _w = insert_lock(op, _b);				\
+		rw_lock(_w, _b, _b->level);				\
+		if (_b == (c)->root &&					\
+		    _w == insert_lock(op, _b)) {			\
+			_r = bch_btree_ ## fn(_b, op, ##__VA_ARGS__);	\
+		}							\
+		rw_unlock(_w, _b);					\
+		bch_cannibalize_unlock(c);                              \
+		if (_r == -EINTR)                                       \
+			schedule();                                     \
+	} while (_r == -EINTR);                                         \
+									\
+	finish_wait(&(c)->btree_cache_wait, &(op)->wait);               \
+	_r;                                                             \
+})
+
 #define MAP_DONE	0
 #define MAP_CONTINUE	1
 
-- 
2.25.0

[PATCH 2/7] bcache: add bcache_ prefix to btree_root() and btree() macros

[Coly Li]

This patch changes macro btree_root() and btree() to bcache_btree_root()
and bcache_btree(), to avoid potential generic name clash in future.

NOTE: for porduct kernel maintainers, this patch can be skipped if
you feel the rename stuffs introduce inconvenince to patch backport.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Coly Li <colyli@suse.de>
---
 drivers/md/bcache/btree.c | 15 ++++++++-------
 drivers/md/bcache/btree.h |  4 ++--
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 99cb201809af..faf152524a16 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1790,7 +1790,7 @@ static void bch_btree_gc(struct cache_set *c)
 
 	/* if CACHE_SET_IO_DISABLE set, gc thread should stop too */
 	do {
-		ret = btree_root(gc_root, c, &op, &writes, &stats);
+		ret = bcache_btree_root(gc_root, c, &op, &writes, &stats);
 		closure_sync(&writes);
 		cond_resched();
 
@@ -1888,7 +1888,7 @@ static int bch_btree_check_recurse(struct btree *b, struct btree_op *op)
 			}
 
 			if (p)
-				ret = btree(check_recurse, p, b, op);
+				ret = bcache_btree(check_recurse, p, b, op);
 
 			p = k;
 		} while (p && !ret);
@@ -1903,7 +1903,7 @@ int bch_btree_check(struct cache_set *c)
 
 	bch_btree_op_init(&op, SHRT_MAX);
 
-	return btree_root(check_recurse, c, &op);
+	return bcache_btree_root(check_recurse, c, &op);
 }
 
 void bch_initial_gc_finish(struct cache_set *c)
@@ -2343,7 +2343,7 @@ static int bch_btree_map_nodes_recurse(struct btree *b, struct btree_op *op,
 
 		while ((k = bch_btree_iter_next_filter(&iter, &b->keys,
 						       bch_ptr_bad))) {
-			ret = btree(map_nodes_recurse, k, b,
+			ret = bcache_btree(map_nodes_recurse, k, b,
 				    op, from, fn, flags);
 			from = NULL;
 
@@ -2361,7 +2361,7 @@ static int bch_btree_map_nodes_recurse(struct btree *b, struct btree_op *op,
 int __bch_btree_map_nodes(struct btree_op *op, struct cache_set *c,
 			  struct bkey *from, btree_map_nodes_fn *fn, int flags)
 {
-	return btree_root(map_nodes_recurse, c, op, from, fn, flags);
+	return bcache_btree_root(map_nodes_recurse, c, op, from, fn, flags);
 }
 
 int bch_btree_map_keys_recurse(struct btree *b, struct btree_op *op,
@@ -2377,7 +2377,8 @@ int bch_btree_map_keys_recurse(struct btree *b, struct btree_op *op,
 	while ((k = bch_btree_iter_next_filter(&iter, &b->keys, bch_ptr_bad))) {
 		ret = !b->level
 			? fn(op, b, k)
-			: btree(map_keys_recurse, k, b, op, from, fn, flags);
+			: bcache_btree(map_keys_recurse, k,
+				       b, op, from, fn, flags);
 		from = NULL;
 
 		if (ret != MAP_CONTINUE)
@@ -2394,7 +2395,7 @@ int bch_btree_map_keys_recurse(struct btree *b, struct btree_op *op,
 int bch_btree_map_keys(struct btree_op *op, struct cache_set *c,
 		       struct bkey *from, btree_map_keys_fn *fn, int flags)
 {
-	return btree_root(map_keys_recurse, c, op, from, fn, flags);
+	return bcache_btree_root(map_keys_recurse, c, op, from, fn, flags);
 }
 
 /* Keybuf code */
diff --git a/drivers/md/bcache/btree.h b/drivers/md/bcache/btree.h
index f37153db3f6c..19e30266070a 100644
--- a/drivers/md/bcache/btree.h
+++ b/drivers/md/bcache/btree.h
@@ -309,7 +309,7 @@ static inline void force_wake_up_gc(struct cache_set *c)
  * @b:		parent btree node
  * @op:		pointer to struct btree_op
  */
-#define btree(fn, key, b, op, ...)					\
+#define bcache_btree(fn, key, b, op, ...)				\
 ({									\
 	int _r, l = (b)->level - 1;					\
 	bool _w = l <= (op)->lock;					\
@@ -329,7 +329,7 @@ static inline void force_wake_up_gc(struct cache_set *c)
  * @c:		cache set
  * @op:		pointer to struct btree_op
  */
-#define btree_root(fn, c, op, ...)					\
+#define bcache_btree_root(fn, c, op, ...)				\
 ({									\
 	int _r = -EINTR;						\
 	do {								\
-- 
2.25.0

[PATCH 3/7] bcache: make bch_btree_check() to be multithreaded

[Coly Li]

When registering a cache device, bch_btree_check() is called to check
all btree node, to make sure the btree is consistency and no corruption.

bch_btree_check() is recursively executed in single thread, when there
are a lot of data cached and the btree is huge, it may take very long
time to check all the btree nodes. In my testing, I observed it took
around 50 minutes to finish bch_btree_check().

When checking the bcache btree nodes, the cache set is not running yet,
and indeed the whole tree is in read-only state, it is safe to create
multiple threads to check the btree in parallel.

This patch tries to create multiple threads, and each thread tries to
one-by-one check the sub-tree indexed by a key from the btree root node.
The parallel thread number depends on how many keys in the btree root
node. At most BCH_BTR_CHKTHREAD_MAX (64) threads can be created, but in
practice is should be min(cpu-number/2, root-node-keys-number).

Signed-off-by: Coly Li <colyli@suse.de>
Cc: Christoph Hellwig <hch@infradead.org>
---
 drivers/md/bcache/btree.c | 169 +++++++++++++++++++++++++++++++++++++-
 drivers/md/bcache/btree.h |  22 +++++
 2 files changed, 188 insertions(+), 3 deletions(-)

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index faf152524a16..74d66b641169 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1897,13 +1897,176 @@ static int bch_btree_check_recurse(struct btree *b, struct btree_op *op)
 	return ret;
 }
 
+
+static int bch_btree_check_thread(void *arg)
+{
+	int ret;
+	struct btree_check_info *info = arg;
+	struct btree_check_state *check_state = info->state;
+	struct cache_set *c = check_state->c;
+	struct btree_iter iter;
+	struct bkey *k, *p;
+	int cur_idx, prev_idx, skip_nr;
+	int i, n;
+
+	k = p = NULL;
+	i = n = 0;
+	cur_idx = prev_idx = 0;
+	ret = 0;
+
+	/* root node keys are checked before thread created */
+	bch_btree_iter_init(&c->root->keys, &iter, NULL);
+	k = bch_btree_iter_next_filter(&iter, &c->root->keys, bch_ptr_bad);
+	BUG_ON(!k);
+
+	p = k;
+	while (k) {
+		/*
+		 * Fetch a root node key index, skip the keys which
+		 * should be fetched by other threads, then check the
+		 * sub-tree indexed by the fetched key.
+		 */
+		spin_lock(&check_state->idx_lock);
+		cur_idx = check_state->key_idx;
+		check_state->key_idx++;
+		spin_unlock(&check_state->idx_lock);
+
+		skip_nr = cur_idx - prev_idx;
+
+		while (skip_nr) {
+			k = bch_btree_iter_next_filter(&iter,
+						       &c->root->keys,
+						       bch_ptr_bad);
+			if (k)
+				p = k;
+			else {
+				/*
+				 * No more keys to check in root node,
+				 * current checking threads are enough,
+				 * stop creating more.
+				 */
+				atomic_set(&check_state->enough, 1);
+				/* Update check_state->enough earlier */
+				smp_mb();
+				goto out;
+			}
+			skip_nr--;
+			cond_resched();
+		}
+
+		if (p) {
+			struct btree_op op;
+
+			btree_node_prefetch(c->root, p);
+			c->gc_stats.nodes++;
+			bch_btree_op_init(&op, 0);
+			ret = bcache_btree(check_recurse, p, c->root, &op);
+			if (ret)
+				goto out;
+		}
+		p = NULL;
+		prev_idx = cur_idx;
+		cond_resched();
+	}
+
+out:
+	info->result = ret;
+	/* update check_state->started among all CPUs */
+	smp_mb();
+	if (atomic_dec_and_test(&check_state->started))
+		wake_up(&check_state->wait);
+
+	return ret;
+}
+
+
+
+static int bch_btree_chkthread_nr(void)
+{
+	int n = num_online_cpus()/2;
+
+	if (n == 0)
+		n = 1;
+	else if (n > BCH_BTR_CHKTHREAD_MAX)
+		n = BCH_BTR_CHKTHREAD_MAX;
+
+	return n;
+}
+
 int bch_btree_check(struct cache_set *c)
 {
-	struct btree_op op;
+	int ret = 0;
+	int i;
+	struct bkey *k = NULL;
+	struct btree_iter iter;
+	struct btree_check_state *check_state;
+	char name[32];
 
-	bch_btree_op_init(&op, SHRT_MAX);
+	/* check and mark root node keys */
+	for_each_key_filter(&c->root->keys, k, &iter, bch_ptr_invalid)
+		bch_initial_mark_key(c, c->root->level, k);
+
+	bch_initial_mark_key(c, c->root->level + 1, &c->root->key);
+
+	if (c->root->level == 0)
+		return 0;
+
+	check_state = kzalloc(sizeof(struct btree_check_state), GFP_KERNEL);
+	if (!check_state)
+		return -ENOMEM;
+
+	check_state->c = c;
+	check_state->total_threads = bch_btree_chkthread_nr();
+	check_state->key_idx = 0;
+	spin_lock_init(&check_state->idx_lock);
+	atomic_set(&check_state->started, 0);
+	atomic_set(&check_state->enough, 0);
+	init_waitqueue_head(&check_state->wait);
 
-	return bcache_btree_root(check_recurse, c, &op);
+	/*
+	 * Run multiple threads to check btree nodes in parallel,
+	 * if check_state->enough is non-zero, it means current
+	 * running check threads are enough, unncessary to create
+	 * more.
+	 */
+	for (i = 0; i < check_state->total_threads; i++) {
+		/* fetch latest check_state->enough earlier */
+		smp_mb();
+		if (atomic_read(&check_state->enough))
+			break;
+
+		check_state->infos[i].result = 0;
+		check_state->infos[i].state = check_state;
+		snprintf(name, sizeof(name), "bch_btrchk[%u]", i);
+		atomic_inc(&check_state->started);
+
+		check_state->infos[i].thread =
+			kthread_run(bch_btree_check_thread,
+				    &check_state->infos[i],
+				    name);
+		if (IS_ERR(check_state->infos[i].thread)) {
+			pr_err("fails to run thread bch_btrchk[%d]", i);
+			for (--i; i >= 0; i--)
+				kthread_stop(check_state->infos[i].thread);
+			ret = -ENOMEM;
+			goto out;
+		}
+	}
+
+	wait_event_interruptible(check_state->wait,
+				 atomic_read(&check_state->started) == 0 ||
+				  test_bit(CACHE_SET_IO_DISABLE, &c->flags));
+
+	for (i = 0; i < check_state->total_threads; i++) {
+		if (check_state->infos[i].result) {
+			ret = check_state->infos[i].result;
+			goto out;
+		}
+	}
+
+out:
+	kfree(check_state);
+	return ret;
 }
 
 void bch_initial_gc_finish(struct cache_set *c)
diff --git a/drivers/md/bcache/btree.h b/drivers/md/bcache/btree.h
index 19e30266070a..7c884f278da8 100644
--- a/drivers/md/bcache/btree.h
+++ b/drivers/md/bcache/btree.h
@@ -145,6 +145,9 @@ struct btree {
 	struct bio		*bio;
 };
 
+
+
+
 #define BTREE_FLAG(flag)						\
 static inline bool btree_node_ ## flag(struct btree *b)			\
 {	return test_bit(BTREE_NODE_ ## flag, &b->flags); }		\
@@ -216,6 +219,25 @@ struct btree_op {
 	unsigned int		insert_collision:1;
 };
 
+struct btree_check_state;
+struct btree_check_info {
+	struct btree_check_state	*state;
+	struct task_struct		*thread;
+	int				result;
+};
+
+#define BCH_BTR_CHKTHREAD_MAX	64
+struct btree_check_state {
+	struct cache_set		*c;
+	int				total_threads;
+	int				key_idx;
+	spinlock_t			idx_lock;
+	atomic_t			started;
+	atomic_t			enough;
+	wait_queue_head_t		wait;
+	struct btree_check_info		infos[BCH_BTR_CHKTHREAD_MAX];
+};
+
 static inline void bch_btree_op_init(struct btree_op *op, int write_lock_level)
 {
 	memset(op, 0, sizeof(struct btree_op));
-- 
2.25.0

[PATCH 4/7] bcache: make bch_sectors_dirty_init() to be multithreaded

[Coly Li]

When attaching a cached device (a.k.a backing device) to a cache
device, bch_sectors_dirty_init() is called to count dirty sectors
and stripes (see what bcache_dev_sectors_dirty_add() does) on the
cache device.

The counting is done by a single thread recursive function
bch_btree_map_keys() to iterate all the bcache btree nodes.
If the btree has huge number of nodes, bch_sectors_dirty_init() will
take quite long time. In my testing, if the registering cache set has
a existed UUID which matches a already registered cached device, the
automatical attachment during the registration may take more than
55 minutes. This is too long for waiting the bcache to work in real
deployment.

Fortunately when bch_sectors_dirty_init() is called, no other thread
will access the btree yet, it is safe to do a read-only parallelized
dirty sectors counting by multiple threads.

This patch tries to create multiple threads, and each thread tries to
one-by-one count dirty sectors from the sub-tree indexed by a root
node key which the thread fetched. After the sub-tree is counted, the
counting thread will continue to fetch another root node key, until
the fetched key is NULL. How many threads in parallel depends on
the number of keys from the btree root node, and the number of online
CPU core. The thread number will be the less number but no more than
BCH_DIRTY_INIT_THRD_MAX. If there are only 2 keys in root node, it
can only be 2x times faster by this patch. But if there are 10 keys
in the root node, with this patch it can be 10x times faster.

Signed-off-by: Coly Li <colyli@suse.de>
Cc: Christoph Hellwig <hch@infradead.org>
---
 drivers/md/bcache/writeback.c | 158 +++++++++++++++++++++++++++++++++-
 drivers/md/bcache/writeback.h |  19 ++++
 2 files changed, 174 insertions(+), 3 deletions(-)

diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 4a40f9eadeaf..6673a37c8bd2 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -785,7 +785,9 @@ static int sectors_dirty_init_fn(struct btree_op *_op, struct btree *b,
 	return MAP_CONTINUE;
 }
 
-void bch_sectors_dirty_init(struct bcache_device *d)
+static int bch_root_node_dirty_init(struct cache_set *c,
+				     struct bcache_device *d,
+				     struct bkey *k)
 {
 	struct sectors_dirty_init op;
 	int ret;
@@ -796,8 +798,13 @@ void bch_sectors_dirty_init(struct bcache_device *d)
 	op.start = KEY(op.inode, 0, 0);
 
 	do {
-		ret = bch_btree_map_keys(&op.op, d->c, &op.start,
-					 sectors_dirty_init_fn, 0);
+		ret = bcache_btree(map_keys_recurse,
+				   k,
+				   c->root,
+				   &op.op,
+				   &op.start,
+				   sectors_dirty_init_fn,
+				   0);
 		if (ret == -EAGAIN)
 			schedule_timeout_interruptible(
 				msecs_to_jiffies(INIT_KEYS_SLEEP_MS));
@@ -806,6 +813,151 @@ void bch_sectors_dirty_init(struct bcache_device *d)
 			break;
 		}
 	} while (ret == -EAGAIN);
+
+	return ret;
+}
+
+static int bch_dirty_init_thread(void *arg)
+{
+	struct dirty_init_thrd_info *info = arg;
+	struct bch_dirty_init_state *state = info->state;
+	struct cache_set *c = state->c;
+	struct btree_iter iter;
+	struct bkey *k, *p;
+	int cur_idx, prev_idx, skip_nr;
+	int i;
+
+	k = p = NULL;
+	i = 0;
+	cur_idx = prev_idx = 0;
+
+	bch_btree_iter_init(&c->root->keys, &iter, NULL);
+	k = bch_btree_iter_next_filter(&iter, &c->root->keys, bch_ptr_bad);
+	BUG_ON(!k);
+
+	p = k;
+
+	while (k) {
+		spin_lock(&state->idx_lock);
+		cur_idx = state->key_idx;
+		state->key_idx++;
+		spin_unlock(&state->idx_lock);
+
+		skip_nr = cur_idx - prev_idx;
+
+		while (skip_nr) {
+			k = bch_btree_iter_next_filter(&iter,
+						       &c->root->keys,
+						       bch_ptr_bad);
+			if (k)
+				p = k;
+			else {
+				atomic_set(&state->enough, 1);
+				/* Update state->enough earlier */
+				smp_mb();
+				goto out;
+			}
+			skip_nr--;
+			cond_resched();
+		}
+
+		if (p) {
+			if (bch_root_node_dirty_init(c, state->d, p) < 0)
+				goto out;
+		}
+
+		p = NULL;
+		prev_idx = cur_idx;
+		cond_resched();
+	}
+
+out:
+	/* In order to wake up state->wait in time */
+	smp_mb();
+	if (atomic_dec_and_test(&state->started))
+		wake_up(&state->wait);
+
+	return 0;
+}
+
+static int bch_btre_dirty_init_thread_nr(void)
+{
+	int n = num_online_cpus()/2;
+
+	if (n == 0)
+		n = 1;
+	else if (n > BCH_DIRTY_INIT_THRD_MAX)
+		n = BCH_DIRTY_INIT_THRD_MAX;
+
+	return n;
+}
+
+void bch_sectors_dirty_init(struct bcache_device *d)
+{
+	int i;
+	struct bkey *k = NULL;
+	struct btree_iter iter;
+	struct sectors_dirty_init op;
+	struct cache_set *c = d->c;
+	struct bch_dirty_init_state *state;
+	char name[32];
+
+	/* Just count root keys if no leaf node */
+	if (c->root->level == 0) {
+		bch_btree_op_init(&op.op, -1);
+		op.inode = d->id;
+		op.count = 0;
+		op.start = KEY(op.inode, 0, 0);
+
+		for_each_key_filter(&c->root->keys,
+				    k, &iter, bch_ptr_invalid)
+			sectors_dirty_init_fn(&op.op, c->root, k);
+		return;
+	}
+
+	state = kzalloc(sizeof(struct bch_dirty_init_state), GFP_KERNEL);
+	if (!state) {
+		pr_warn("sectors dirty init failed: cannot allocate memory");
+		return;
+	}
+
+	state->c = c;
+	state->d = d;
+	state->total_threads = bch_btre_dirty_init_thread_nr();
+	state->key_idx = 0;
+	spin_lock_init(&state->idx_lock);
+	atomic_set(&state->started, 0);
+	atomic_set(&state->enough, 0);
+	init_waitqueue_head(&state->wait);
+
+	for (i = 0; i < state->total_threads; i++) {
+		/* Fetch latest state->enough earlier */
+		smp_mb();
+		if (atomic_read(&state->enough))
+			break;
+
+		state->infos[i].state = state;
+		atomic_inc(&state->started);
+		snprintf(name, sizeof(name), "bch_dirty_init[%d]", i);
+
+		state->infos[i].thread =
+			kthread_run(bch_dirty_init_thread,
+				    &state->infos[i],
+				    name);
+		if (IS_ERR(state->infos[i].thread)) {
+			pr_err("fails to run thread bch_dirty_init[%d]", i);
+			for (--i; i >= 0; i--)
+				kthread_stop(state->infos[i].thread);
+			goto out;
+		}
+	}
+
+	wait_event_interruptible(state->wait,
+		 atomic_read(&state->started) == 0 ||
+		 test_bit(CACHE_SET_IO_DISABLE, &c->flags));
+
+out:
+	kfree(state);
 }
 
 void bch_cached_dev_writeback_init(struct cached_dev *dc)
diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h
index 4e4c6810dc3c..b029843ce5b6 100644
--- a/drivers/md/bcache/writeback.h
+++ b/drivers/md/bcache/writeback.h
@@ -16,6 +16,7 @@
 
 #define BCH_AUTO_GC_DIRTY_THRESHOLD	50
 
+#define BCH_DIRTY_INIT_THRD_MAX	64
 /*
  * 14 (16384ths) is chosen here as something that each backing device
  * should be a reasonable fraction of the share, and not to blow up
@@ -23,6 +24,24 @@
  */
 #define WRITEBACK_SHARE_SHIFT   14
 
+struct bch_dirty_init_state;
+struct dirty_init_thrd_info {
+	struct bch_dirty_init_state	*state;
+	struct task_struct		*thread;
+};
+
+struct bch_dirty_init_state {
+	struct cache_set		*c;
+	struct bcache_device		*d;
+	int				total_threads;
+	int				key_idx;
+	spinlock_t			idx_lock;
+	atomic_t			started;
+	atomic_t			enough;
+	wait_queue_head_t		wait;
+	struct dirty_init_thrd_info	infos[BCH_DIRTY_INIT_THRD_MAX];
+};
+
 static inline uint64_t bcache_dev_sectors_dirty(struct bcache_device *d)
 {
 	uint64_t i, ret = 0;
-- 
2.25.0

[PATCH 5/7] bcache: Use scnprintf() for avoiding potential buffer overflow

[Coly Li]

From: Takashi Iwai <tiwai@suse.de>

Since snprintf() returns the would-be-output size instead of the
actual output size, the succeeding calls may go beyond the given
buffer limit.  Fix it by replacing with scnprintf().

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Coly Li <colyli@suse.de>
---
 drivers/md/bcache/sysfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index 3470fae4eabc..323276994aab 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -154,7 +154,7 @@ static ssize_t bch_snprint_string_list(char *buf,
 	size_t i;
 
 	for (i = 0; list[i]; i++)
-		out += snprintf(out, buf + size - out,
+		out += scnprintf(out, buf + size - out,
 				i == selected ? "[%s] " : "%s ", list[i]);
 
 	out[-1] = '\n';
-- 
2.25.0

[PATCH 6/7] bcache: optimize barrier usage for Rmw atomic bitops

[Coly Li]

From: Davidlohr Bueso <dave@stgolabs.net>

We can avoid the unnecessary barrier on non LL/SC architectures,
such as x86. Instead, use the smp_mb__after_atomic().

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Coly Li <colyli@suse.de>
---
 drivers/md/bcache/writeback.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 6673a37c8bd2..72ba6d015786 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -183,7 +183,7 @@ static void update_writeback_rate(struct work_struct *work)
 	 */
 	set_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
 	/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
-	smp_mb();
+	smp_mb__after_atomic();
 
 	/*
 	 * CACHE_SET_IO_DISABLE might be set via sysfs interface,
@@ -193,7 +193,7 @@ static void update_writeback_rate(struct work_struct *work)
 	    test_bit(CACHE_SET_IO_DISABLE, &c->flags)) {
 		clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
 		/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
-		smp_mb();
+		smp_mb__after_atomic();
 		return;
 	}
 
@@ -229,7 +229,7 @@ static void update_writeback_rate(struct work_struct *work)
 	 */
 	clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
 	/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
-	smp_mb();
+	smp_mb__after_atomic();
 }
 
 static unsigned int writeback_delay(struct cached_dev *dc,
-- 
2.25.0

[PATCH 7/7] bcache: optimize barrier usage for atomic operations

[Coly Li]

The idea of this patch is from Davidlohr Bueso, he posts a patch
for bcache to optimize barrier usage for read-modify-write atomic
bitops. Indeed such optimization can also apply on other locations
where smp_mb() is used before or after an atomic operation.

This patch replaces smp_mb() with smp_mb__before_atomic() or
smp_mb__after_atomic() in btree.c and writeback.c,  where it is used
to synchronize memory cache just earlier on other cores. Although
the locations are not on hot code path, it is always not bad to mkae
things a little better.

Signed-off-by: Coly Li <colyli@suse.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
---
 drivers/md/bcache/btree.c     | 6 +++---
 drivers/md/bcache/writeback.c | 6 +++---
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 74d66b641169..72856e5f23a3 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1947,7 +1947,7 @@ static int bch_btree_check_thread(void *arg)
 				 */
 				atomic_set(&check_state->enough, 1);
 				/* Update check_state->enough earlier */
-				smp_mb();
+				smp_mb__after_atomic();
 				goto out;
 			}
 			skip_nr--;
@@ -1972,7 +1972,7 @@ static int bch_btree_check_thread(void *arg)
 out:
 	info->result = ret;
 	/* update check_state->started among all CPUs */
-	smp_mb();
+	smp_mb__before_atomic();
 	if (atomic_dec_and_test(&check_state->started))
 		wake_up(&check_state->wait);
 
@@ -2031,7 +2031,7 @@ int bch_btree_check(struct cache_set *c)
 	 */
 	for (i = 0; i < check_state->total_threads; i++) {
 		/* fetch latest check_state->enough earlier */
-		smp_mb();
+		smp_mb__before_atomic();
 		if (atomic_read(&check_state->enough))
 			break;
 
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 72ba6d015786..3f7641fb28d5 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -854,7 +854,7 @@ static int bch_dirty_init_thread(void *arg)
 			else {
 				atomic_set(&state->enough, 1);
 				/* Update state->enough earlier */
-				smp_mb();
+				smp_mb__after_atomic();
 				goto out;
 			}
 			skip_nr--;
@@ -873,7 +873,7 @@ static int bch_dirty_init_thread(void *arg)
 
 out:
 	/* In order to wake up state->wait in time */
-	smp_mb();
+	smp_mb__before_atomic();
 	if (atomic_dec_and_test(&state->started))
 		wake_up(&state->wait);
 
@@ -932,7 +932,7 @@ void bch_sectors_dirty_init(struct bcache_device *d)
 
 	for (i = 0; i < state->total_threads; i++) {
 		/* Fetch latest state->enough earlier */
-		smp_mb();
+		smp_mb__before_atomic();
 		if (atomic_read(&state->enough))
 			break;
 
-- 
2.25.0

Re: [PATCH 0/7] bcache patches for Linux v5.7-rc1

[Jens Axboe]

On 3/22/20 12:02 AM, Coly Li wrote:
> Hi Jens,
> 
> These are bcache patches for Linux v5.7-rc1.
> 
> The major change is to make bcache btree check and dirty secrtors
> counting being multithreaded, then the registration time can be
> much less. My first four patches are for this purpose.
> 
> Davidlohr Bueso contributes a patch to optimize barrier usage for
> atomic operations. By his inspiration I also compose a patch for
> the rested locations to change.
> 
> Takashi Iwai contributes a helpful patch to avoid potential
> buffer overflow in bcache sysfs code path.
> 
> Please take them, and thank you in advance.

Applied, thanks.

-- 
Jens Axboe

Re: [PATCH 3/7] bcache: make bch_btree_check() to be multithreaded

[Hannes Reinecke]

On 3/22/20 7:03 AM, Coly Li wrote:
> When registering a cache device, bch_btree_check() is called to check
> all btree node, to make sure the btree is consistency and no corruption.
> 
> bch_btree_check() is recursively executed in single thread, when there
> are a lot of data cached and the btree is huge, it may take very long
> time to check all the btree nodes. In my testing, I observed it took
> around 50 minutes to finish bch_btree_check().
> 
> When checking the bcache btree nodes, the cache set is not running yet,
> and indeed the whole tree is in read-only state, it is safe to create
> multiple threads to check the btree in parallel.
> 
> This patch tries to create multiple threads, and each thread tries to
> one-by-one check the sub-tree indexed by a key from the btree root node.
> The parallel thread number depends on how many keys in the btree root
> node. At most BCH_BTR_CHKTHREAD_MAX (64) threads can be created, but in
> practice is should be min(cpu-number/2, root-node-keys-number).
> 
> Signed-off-by: Coly Li <colyli@suse.de>
> Cc: Christoph Hellwig <hch@infradead.org>
> ---
>   drivers/md/bcache/btree.c | 169 +++++++++++++++++++++++++++++++++++++-
>   drivers/md/bcache/btree.h |  22 +++++
>   2 files changed, 188 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> index faf152524a16..74d66b641169 100644
> --- a/drivers/md/bcache/btree.c
> +++ b/drivers/md/bcache/btree.c
> @@ -1897,13 +1897,176 @@ static int bch_btree_check_recurse(struct btree *b, struct btree_op *op)
>   	return ret;
>   }
>   
> +
> +static int bch_btree_check_thread(void *arg)
> +{
> +	int ret;
> +	struct btree_check_info *info = arg;
> +	struct btree_check_state *check_state = info->state;
> +	struct cache_set *c = check_state->c;
> +	struct btree_iter iter;
> +	struct bkey *k, *p;
> +	int cur_idx, prev_idx, skip_nr;
> +	int i, n;
> +
> +	k = p = NULL;
> +	i = n = 0;
> +	cur_idx = prev_idx = 0;
> +	ret = 0;
> +
> +	/* root node keys are checked before thread created */
> +	bch_btree_iter_init(&c->root->keys, &iter, NULL);
> +	k = bch_btree_iter_next_filter(&iter, &c->root->keys, bch_ptr_bad);
> +	BUG_ON(!k);
> +
> +	p = k;
> +	while (k) {
> +		/*
> +		 * Fetch a root node key index, skip the keys which
> +		 * should be fetched by other threads, then check the
> +		 * sub-tree indexed by the fetched key.
> +		 */
> +		spin_lock(&check_state->idx_lock);
> +		cur_idx = check_state->key_idx;
> +		check_state->key_idx++;
> +		spin_unlock(&check_state->idx_lock);
> +
> +		skip_nr = cur_idx - prev_idx;
> +
> +		while (skip_nr) {
> +			k = bch_btree_iter_next_filter(&iter,
> +						       &c->root->keys,
> +						       bch_ptr_bad);
> +			if (k)
> +				p = k;
> +			else {
> +				/*
> +				 * No more keys to check in root node,
> +				 * current checking threads are enough,
> +				 * stop creating more.
> +				 */
> +				atomic_set(&check_state->enough, 1);
> +				/* Update check_state->enough earlier */
> +				smp_mb();
> +				goto out;
> +			}
> +			skip_nr--;
> +			cond_resched();
> +		}
> +
> +		if (p) {
> +			struct btree_op op;
> +
> +			btree_node_prefetch(c->root, p);
> +			c->gc_stats.nodes++;
> +			bch_btree_op_init(&op, 0);
> +			ret = bcache_btree(check_recurse, p, c->root, &op);
> +			if (ret)
> +				goto out;
> +		}
> +		p = NULL;
> +		prev_idx = cur_idx;
> +		cond_resched();
> +	}
> +
> +out:
> +	info->result = ret;
> +	/* update check_state->started among all CPUs */
> +	smp_mb();
> +	if (atomic_dec_and_test(&check_state->started))
> +		wake_up(&check_state->wait);
> +
> +	return ret;
> +}
> +
> +
> +
> +static int bch_btree_chkthread_nr(void)
> +{
> +	int n = num_online_cpus()/2;
> +
> +	if (n == 0)
> +		n = 1;
> +	else if (n > BCH_BTR_CHKTHREAD_MAX)
> +		n = BCH_BTR_CHKTHREAD_MAX;
> +
> +	return n;
> +}
> +
>   int bch_btree_check(struct cache_set *c)
>   {
> -	struct btree_op op;
> +	int ret = 0;
> +	int i;
> +	struct bkey *k = NULL;
> +	struct btree_iter iter;
> +	struct btree_check_state *check_state;
> +	char name[32];
>   
> -	bch_btree_op_init(&op, SHRT_MAX);
> +	/* check and mark root node keys */
> +	for_each_key_filter(&c->root->keys, k, &iter, bch_ptr_invalid)
> +		bch_initial_mark_key(c, c->root->level, k);
> +
> +	bch_initial_mark_key(c, c->root->level + 1, &c->root->key);
> +
> +	if (c->root->level == 0)
> +		return 0;
> +
> +	check_state = kzalloc(sizeof(struct btree_check_state), GFP_KERNEL);
> +	if (!check_state)
> +		return -ENOMEM;
> +
> +	check_state->c = c;
> +	check_state->total_threads = bch_btree_chkthread_nr();
> +	check_state->key_idx = 0;
> +	spin_lock_init(&check_state->idx_lock);
> +	atomic_set(&check_state->started, 0);
> +	atomic_set(&check_state->enough, 0);
> +	init_waitqueue_head(&check_state->wait);
>   
> -	return bcache_btree_root(check_recurse, c, &op);
> +	/*
> +	 * Run multiple threads to check btree nodes in parallel,
> +	 * if check_state->enough is non-zero, it means current
> +	 * running check threads are enough, unncessary to create
> +	 * more.
> +	 */
> +	for (i = 0; i < check_state->total_threads; i++) {
> +		/* fetch latest check_state->enough earlier */
> +		smp_mb();
> +		if (atomic_read(&check_state->enough))
> +			break;
> +
> +		check_state->infos[i].result = 0;
> +		check_state->infos[i].state = check_state;
> +		snprintf(name, sizeof(name), "bch_btrchk[%u]", i);
> +		atomic_inc(&check_state->started);
> +
> +		check_state->infos[i].thread =
> +			kthread_run(bch_btree_check_thread,
> +				    &check_state->infos[i],
> +				    name);
> +		if (IS_ERR(check_state->infos[i].thread)) {
> +			pr_err("fails to run thread bch_btrchk[%d]", i);
> +			for (--i; i >= 0; i--)
> +				kthread_stop(check_state->infos[i].thread);
> +			ret = -ENOMEM;
> +			goto out;
> +		}
> +	}
> +
> +	wait_event_interruptible(check_state->wait,
> +				 atomic_read(&check_state->started) == 0 ||
> +				  test_bit(CACHE_SET_IO_DISABLE, &c->flags));
> +
> +	for (i = 0; i < check_state->total_threads; i++) {
> +		if (check_state->infos[i].result) {
> +			ret = check_state->infos[i].result;
> +			goto out;
> +		}
> +	}
> +
> +out:
> +	kfree(check_state);
> +	return ret;
>   }
>   
>   void bch_initial_gc_finish(struct cache_set *c)
> diff --git a/drivers/md/bcache/btree.h b/drivers/md/bcache/btree.h
> index 19e30266070a..7c884f278da8 100644
> --- a/drivers/md/bcache/btree.h
> +++ b/drivers/md/bcache/btree.h
> @@ -145,6 +145,9 @@ struct btree {
>   	struct bio		*bio;
>   };
>   
> +
> +
> +
>   #define BTREE_FLAG(flag)						\
>   static inline bool btree_node_ ## flag(struct btree *b)			\
>   {	return test_bit(BTREE_NODE_ ## flag, &b->flags); }		\
> @@ -216,6 +219,25 @@ struct btree_op {
>   	unsigned int		insert_collision:1;
>   };
>   
> +struct btree_check_state;
> +struct btree_check_info {
> +	struct btree_check_state	*state;
> +	struct task_struct		*thread;
> +	int				result;
> +};
> +
> +#define BCH_BTR_CHKTHREAD_MAX	64
> +struct btree_check_state {
> +	struct cache_set		*c;
> +	int				total_threads;
> +	int				key_idx;
> +	spinlock_t			idx_lock;
> +	atomic_t			started;
> +	atomic_t			enough;
> +	wait_queue_head_t		wait;
> +	struct btree_check_info		infos[BCH_BTR_CHKTHREAD_MAX];
> +};
> +
>   static inline void bch_btree_op_init(struct btree_op *op, int write_lock_level)
>   {
>   	memset(op, 0, sizeof(struct btree_op));
> 
Any particular reason why you are not using workqueues?
Kthreads is a bit of an overkill here I think...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

Re: [PATCH 4/7] bcache: make bch_sectors_dirty_init() to be multithreaded

[Hannes Reinecke]

On 3/22/20 7:03 AM, Coly Li wrote:
> When attaching a cached device (a.k.a backing device) to a cache
> device, bch_sectors_dirty_init() is called to count dirty sectors
> and stripes (see what bcache_dev_sectors_dirty_add() does) on the
> cache device.
> 
> The counting is done by a single thread recursive function
> bch_btree_map_keys() to iterate all the bcache btree nodes.
> If the btree has huge number of nodes, bch_sectors_dirty_init() will
> take quite long time. In my testing, if the registering cache set has
> a existed UUID which matches a already registered cached device, the
> automatical attachment during the registration may take more than
> 55 minutes. This is too long for waiting the bcache to work in real
> deployment.
> 
> Fortunately when bch_sectors_dirty_init() is called, no other thread
> will access the btree yet, it is safe to do a read-only parallelized
> dirty sectors counting by multiple threads.
> 
> This patch tries to create multiple threads, and each thread tries to
> one-by-one count dirty sectors from the sub-tree indexed by a root
> node key which the thread fetched. After the sub-tree is counted, the
> counting thread will continue to fetch another root node key, until
> the fetched key is NULL. How many threads in parallel depends on
> the number of keys from the btree root node, and the number of online
> CPU core. The thread number will be the less number but no more than
> BCH_DIRTY_INIT_THRD_MAX. If there are only 2 keys in root node, it
> can only be 2x times faster by this patch. But if there are 10 keys
> in the root node, with this patch it can be 10x times faster.
> 
> Signed-off-by: Coly Li <colyli@suse.de>
> Cc: Christoph Hellwig <hch@infradead.org>
> ---
>   drivers/md/bcache/writeback.c | 158 +++++++++++++++++++++++++++++++++-
>   drivers/md/bcache/writeback.h |  19 ++++
>   2 files changed, 174 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> index 4a40f9eadeaf..6673a37c8bd2 100644
> --- a/drivers/md/bcache/writeback.c
> +++ b/drivers/md/bcache/writeback.c
> @@ -785,7 +785,9 @@ static int sectors_dirty_init_fn(struct btree_op *_op, struct btree *b,
>   	return MAP_CONTINUE;
>   }
>   
> -void bch_sectors_dirty_init(struct bcache_device *d)
> +static int bch_root_node_dirty_init(struct cache_set *c,
> +				     struct bcache_device *d,
> +				     struct bkey *k)
>   {
>   	struct sectors_dirty_init op;
>   	int ret;
> @@ -796,8 +798,13 @@ void bch_sectors_dirty_init(struct bcache_device *d)
>   	op.start = KEY(op.inode, 0, 0);
>   
>   	do {
> -		ret = bch_btree_map_keys(&op.op, d->c, &op.start,
> -					 sectors_dirty_init_fn, 0);
> +		ret = bcache_btree(map_keys_recurse,
> +				   k,
> +				   c->root,
> +				   &op.op,
> +				   &op.start,
> +				   sectors_dirty_init_fn,
> +				   0);
>   		if (ret == -EAGAIN)
>   			schedule_timeout_interruptible(
>   				msecs_to_jiffies(INIT_KEYS_SLEEP_MS));
> @@ -806,6 +813,151 @@ void bch_sectors_dirty_init(struct bcache_device *d)
>   			break;
>   		}
>   	} while (ret == -EAGAIN);
> +
> +	return ret;
> +}
> +
> +static int bch_dirty_init_thread(void *arg)
> +{
> +	struct dirty_init_thrd_info *info = arg;
> +	struct bch_dirty_init_state *state = info->state;
> +	struct cache_set *c = state->c;
> +	struct btree_iter iter;
> +	struct bkey *k, *p;
> +	int cur_idx, prev_idx, skip_nr;
> +	int i;
> +
> +	k = p = NULL;
> +	i = 0;
> +	cur_idx = prev_idx = 0;
> +
> +	bch_btree_iter_init(&c->root->keys, &iter, NULL);
> +	k = bch_btree_iter_next_filter(&iter, &c->root->keys, bch_ptr_bad);
> +	BUG_ON(!k);
> +
> +	p = k;
> +
> +	while (k) {
> +		spin_lock(&state->idx_lock);
> +		cur_idx = state->key_idx;
> +		state->key_idx++;
> +		spin_unlock(&state->idx_lock);
> +
> +		skip_nr = cur_idx - prev_idx;
> +
> +		while (skip_nr) {
> +			k = bch_btree_iter_next_filter(&iter,
> +						       &c->root->keys,
> +						       bch_ptr_bad);
> +			if (k)
> +				p = k;
> +			else {
> +				atomic_set(&state->enough, 1);
> +				/* Update state->enough earlier */
> +				smp_mb();
> +				goto out;
> +			}
> +			skip_nr--;
> +			cond_resched();
> +		}
> +
> +		if (p) {
> +			if (bch_root_node_dirty_init(c, state->d, p) < 0)
> +				goto out;
> +		}
> +
> +		p = NULL;
> +		prev_idx = cur_idx;
> +		cond_resched();
> +	}
> +
> +out:
> +	/* In order to wake up state->wait in time */
> +	smp_mb();
> +	if (atomic_dec_and_test(&state->started))
> +		wake_up(&state->wait);
> +
> +	return 0;
> +}
> +
> +static int bch_btre_dirty_init_thread_nr(void)
> +{
> +	int n = num_online_cpus()/2;
> +
> +	if (n == 0)
> +		n = 1;
> +	else if (n > BCH_DIRTY_INIT_THRD_MAX)
> +		n = BCH_DIRTY_INIT_THRD_MAX;
> +
> +	return n;
> +}
> +
> +void bch_sectors_dirty_init(struct bcache_device *d)
> +{
> +	int i;
> +	struct bkey *k = NULL;
> +	struct btree_iter iter;
> +	struct sectors_dirty_init op;
> +	struct cache_set *c = d->c;
> +	struct bch_dirty_init_state *state;
> +	char name[32];
> +
> +	/* Just count root keys if no leaf node */
> +	if (c->root->level == 0) {
> +		bch_btree_op_init(&op.op, -1);
> +		op.inode = d->id;
> +		op.count = 0;
> +		op.start = KEY(op.inode, 0, 0);
> +
> +		for_each_key_filter(&c->root->keys,
> +				    k, &iter, bch_ptr_invalid)
> +			sectors_dirty_init_fn(&op.op, c->root, k);
> +		return;
> +	}
> +
> +	state = kzalloc(sizeof(struct bch_dirty_init_state), GFP_KERNEL);
> +	if (!state) {
> +		pr_warn("sectors dirty init failed: cannot allocate memory");
> +		return;
> +	}
> +
> +	state->c = c;
> +	state->d = d;
> +	state->total_threads = bch_btre_dirty_init_thread_nr();
> +	state->key_idx = 0;
> +	spin_lock_init(&state->idx_lock);
> +	atomic_set(&state->started, 0);
> +	atomic_set(&state->enough, 0);
> +	init_waitqueue_head(&state->wait);
> +
> +	for (i = 0; i < state->total_threads; i++) {
> +		/* Fetch latest state->enough earlier */
> +		smp_mb();
> +		if (atomic_read(&state->enough))
> +			break;
> +
> +		state->infos[i].state = state;
> +		atomic_inc(&state->started);
> +		snprintf(name, sizeof(name), "bch_dirty_init[%d]", i);
> +
> +		state->infos[i].thread =
> +			kthread_run(bch_dirty_init_thread,
> +				    &state->infos[i],
> +				    name);
> +		if (IS_ERR(state->infos[i].thread)) {
> +			pr_err("fails to run thread bch_dirty_init[%d]", i);
> +			for (--i; i >= 0; i--)
> +				kthread_stop(state->infos[i].thread);
> +			goto out;
> +		}
> +	}
> +
> +	wait_event_interruptible(state->wait,
> +		 atomic_read(&state->started) == 0 ||
> +		 test_bit(CACHE_SET_IO_DISABLE, &c->flags));
> +
> +out:
> +	kfree(state);
>   }
>   
>   void bch_cached_dev_writeback_init(struct cached_dev *dc)
> diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h
> index 4e4c6810dc3c..b029843ce5b6 100644
> --- a/drivers/md/bcache/writeback.h
> +++ b/drivers/md/bcache/writeback.h
> @@ -16,6 +16,7 @@
>   
>   #define BCH_AUTO_GC_DIRTY_THRESHOLD	50
>   
> +#define BCH_DIRTY_INIT_THRD_MAX	64
>   /*
>    * 14 (16384ths) is chosen here as something that each backing device
>    * should be a reasonable fraction of the share, and not to blow up
> @@ -23,6 +24,24 @@
>    */
>   #define WRITEBACK_SHARE_SHIFT   14
>   
> +struct bch_dirty_init_state;
> +struct dirty_init_thrd_info {
> +	struct bch_dirty_init_state	*state;
> +	struct task_struct		*thread;
> +};
> +
> +struct bch_dirty_init_state {
> +	struct cache_set		*c;
> +	struct bcache_device		*d;
> +	int				total_threads;
> +	int				key_idx;
> +	spinlock_t			idx_lock;
> +	atomic_t			started;
> +	atomic_t			enough;
> +	wait_queue_head_t		wait;
> +	struct dirty_init_thrd_info	infos[BCH_DIRTY_INIT_THRD_MAX];
> +};
> +
>   static inline uint64_t bcache_dev_sectors_dirty(struct bcache_device *d)
>   {
>   	uint64_t i, ret = 0;
> 
Same here; why not workqueues?
You could even be extra sneaky and have a workqueue element per node.
Then you would not descend into the leaf nodes, but rather call the 
workqueue element for the leaf node.
That way you get automatic parallelism...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

Re: [PATCH 5/7] bcache: Use scnprintf() for avoiding potential buffer overflow

[Hannes Reinecke]

On 3/22/20 7:03 AM, Coly Li wrote:
> From: Takashi Iwai <tiwai@suse.de>
> 
> Since snprintf() returns the would-be-output size instead of the
> actual output size, the succeeding calls may go beyond the given
> buffer limit.  Fix it by replacing with scnprintf().
> 
> Signed-off-by: Takashi Iwai <tiwai@suse.de>
> Signed-off-by: Coly Li <colyli@suse.de>
> ---
>   drivers/md/bcache/sysfs.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
> index 3470fae4eabc..323276994aab 100644
> --- a/drivers/md/bcache/sysfs.c
> +++ b/drivers/md/bcache/sysfs.c
> @@ -154,7 +154,7 @@ static ssize_t bch_snprint_string_list(char *buf,
>   	size_t i;
>   
>   	for (i = 0; list[i]; i++)
> -		out += snprintf(out, buf + size - out,
> +		out += scnprintf(out, buf + size - out,
>   				i == selected ? "[%s] " : "%s ", list[i]);
>   
>   	out[-1] = '\n';
> 
Well, if you already consider a possible overflow here, why don't you 
abort the loop once 'out == buf + size' ?

Cheers,

Hannes

-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

Re: [PATCH 1/7] bcache: move macro btree() and btree_root() into btree.h

[Hannes Reinecke]

On 3/22/20 7:02 AM, Coly Li wrote:
> In order to accelerate bcache registration speed, the macro btree()
> and btree_root() will be referenced out of btree.c. This patch moves
> them from btree.c into btree.h with other relative function declaration
> in btree.h, for the following changes.
> 
> Signed-off-by: Coly Li <colyli@suse.de>
> ---
>   drivers/md/bcache/btree.c | 60 +----------------------------------
>   drivers/md/bcache/btree.h | 66 +++++++++++++++++++++++++++++++++++++++
>   2 files changed, 67 insertions(+), 59 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

Re: [PATCH 2/7] bcache: add bcache_ prefix to btree_root() and btree() macros

[Hannes Reinecke]

On 3/22/20 7:03 AM, Coly Li wrote:
> This patch changes macro btree_root() and btree() to bcache_btree_root()
> and bcache_btree(), to avoid potential generic name clash in future.
> 
> NOTE: for porduct kernel maintainers, this patch can be skipped if
> you feel the rename stuffs introduce inconvenince to patch backport.
> 
> Suggested-by: Christoph Hellwig <hch@infradead.org>
> Signed-off-by: Coly Li <colyli@suse.de>
> ---
>   drivers/md/bcache/btree.c | 15 ++++++++-------
>   drivers/md/bcache/btree.h |  4 ++--
>   2 files changed, 10 insertions(+), 9 deletions(-)
> Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

Re: [PATCH 6/7] bcache: optimize barrier usage for Rmw atomic bitops

[Hannes Reinecke]

On 3/22/20 7:03 AM, Coly Li wrote:
> From: Davidlohr Bueso <dave@stgolabs.net>
> 
> We can avoid the unnecessary barrier on non LL/SC architectures,
> such as x86. Instead, use the smp_mb__after_atomic().
> 
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> Signed-off-by: Coly Li <colyli@suse.de>
> ---
>   drivers/md/bcache/writeback.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> index 6673a37c8bd2..72ba6d015786 100644
> --- a/drivers/md/bcache/writeback.c
> +++ b/drivers/md/bcache/writeback.c
> @@ -183,7 +183,7 @@ static void update_writeback_rate(struct work_struct *work)
>   	 */
>   	set_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
>   	/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
> -	smp_mb();
> +	smp_mb__after_atomic();
>   
>   	/*
>   	 * CACHE_SET_IO_DISABLE might be set via sysfs interface,
> @@ -193,7 +193,7 @@ static void update_writeback_rate(struct work_struct *work)
>   	    test_bit(CACHE_SET_IO_DISABLE, &c->flags)) {
>   		clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
>   		/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
> -		smp_mb();
> +		smp_mb__after_atomic();
>   		return;
>   	}
>   
> @@ -229,7 +229,7 @@ static void update_writeback_rate(struct work_struct *work)
>   	 */
>   	clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
>   	/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
> -	smp_mb();
> +	smp_mb__after_atomic();
>   }
>   
>   static unsigned int writeback_delay(struct cached_dev *dc,
> 
Personally, I'd typically tend to use 'test_and_set_bit', as this not 
only implies a barrier, but also captures any errors; if you just use
'set_bit' you'll never know if the bit had been set already.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

Re: [PATCH 7/7] bcache: optimize barrier usage for atomic operations

[Hannes Reinecke]

On 3/22/20 7:03 AM, Coly Li wrote:
> The idea of this patch is from Davidlohr Bueso, he posts a patch
> for bcache to optimize barrier usage for read-modify-write atomic
> bitops. Indeed such optimization can also apply on other locations
> where smp_mb() is used before or after an atomic operation.
> 
> This patch replaces smp_mb() with smp_mb__before_atomic() or
> smp_mb__after_atomic() in btree.c and writeback.c,  where it is used
> to synchronize memory cache just earlier on other cores. Although
> the locations are not on hot code path, it is always not bad to mkae
> things a little better.
> 
> Signed-off-by: Coly Li <colyli@suse.de>
> Cc: Davidlohr Bueso <dave@stgolabs.net>
> ---
>   drivers/md/bcache/btree.c     | 6 +++---
>   drivers/md/bcache/writeback.c | 6 +++---
>   2 files changed, 6 insertions(+), 6 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

Re: [PATCH 5/7] bcache: Use scnprintf() for avoiding potential buffer overflow

[Takashi Iwai]

On Mon, 23 Mar 2020 08:04:39 +0100,
Hannes Reinecke wrote:
> 
> On 3/22/20 7:03 AM, Coly Li wrote:
> > From: Takashi Iwai <tiwai@suse.de>
> >
> > Since snprintf() returns the would-be-output size instead of the
> > actual output size, the succeeding calls may go beyond the given
> > buffer limit.  Fix it by replacing with scnprintf().
> >
> > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > Signed-off-by: Coly Li <colyli@suse.de>
> > ---
> >   drivers/md/bcache/sysfs.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
> > index 3470fae4eabc..323276994aab 100644
> > --- a/drivers/md/bcache/sysfs.c
> > +++ b/drivers/md/bcache/sysfs.c
> > @@ -154,7 +154,7 @@ static ssize_t bch_snprint_string_list(char *buf,
> >   	size_t i;
> >     	for (i = 0; list[i]; i++)
> > -		out += snprintf(out, buf + size - out,
> > +		out += scnprintf(out, buf + size - out,
> >   				i == selected ? "[%s] " : "%s ", list[i]);
> >     	out[-1] = '\n';
> >
> Well, if you already consider a possible overflow here, why don't you
> abort the loop once 'out == buf + size' ?

Sure, feel free to optimize.  But no need to mix two things into a
single patch :)


thanks,

Takashi

Re: [PATCH 6/7] bcache: optimize barrier usage for Rmw atomic bitops

[Coly Li]

On 2020/3/23 3:08 下午, Hannes Reinecke wrote:
> On 3/22/20 7:03 AM, Coly Li wrote:
>> From: Davidlohr Bueso <dave@stgolabs.net>
>>
>> We can avoid the unnecessary barrier on non LL/SC architectures,
>> such as x86. Instead, use the smp_mb__after_atomic().
>>
>> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
>> Signed-off-by: Coly Li <colyli@suse.de>
>> ---
>>   drivers/md/bcache/writeback.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/md/bcache/writeback.c
>> b/drivers/md/bcache/writeback.c
>> index 6673a37c8bd2..72ba6d015786 100644
>> --- a/drivers/md/bcache/writeback.c
>> +++ b/drivers/md/bcache/writeback.c
>> @@ -183,7 +183,7 @@ static void update_writeback_rate(struct
>> work_struct *work)
>>        */
>>       set_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
>>       /* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
>> -    smp_mb();
>> +    smp_mb__after_atomic();
>>         /*
>>        * CACHE_SET_IO_DISABLE might be set via sysfs interface,
>> @@ -193,7 +193,7 @@ static void update_writeback_rate(struct
>> work_struct *work)
>>           test_bit(CACHE_SET_IO_DISABLE, &c->flags)) {
>>           clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
>>           /* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
>> -        smp_mb();
>> +        smp_mb__after_atomic();
>>           return;
>>       }
>>   @@ -229,7 +229,7 @@ static void update_writeback_rate(struct
>> work_struct *work)
>>        */
>>       clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
>>       /* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
>> -    smp_mb();
>> +    smp_mb__after_atomic();
>>   }
>>     static unsigned int writeback_delay(struct cached_dev *dc,
>>
> Personally, I'd typically tend to use 'test_and_set_bit', as this not
> only implies a barrier, but also captures any errors; if you just use
> 'set_bit' you'll never know if the bit had been set already.

Copied. There are other locations may optimize with test_and_set_bit(),
for example the cannibalize_lock stuffs. I will post all the
test_and_set_bit()/test_and_clear_bit() optimization as another series
later.

Thanks.

-- 

Coly Li

Re: [PATCH 4/7] bcache: make bch_sectors_dirty_init() to be multithreaded

[Coly Li]

On 2020/3/23 3:03 下午, Hannes Reinecke wrote:
> On 3/22/20 7:03 AM, Coly Li wrote:
>> When attaching a cached device (a.k.a backing device) to a cache
>> device, bch_sectors_dirty_init() is called to count dirty sectors
>> and stripes (see what bcache_dev_sectors_dirty_add() does) on the
>> cache device.
>>
>> The counting is done by a single thread recursive function
>> bch_btree_map_keys() to iterate all the bcache btree nodes.
>> If the btree has huge number of nodes, bch_sectors_dirty_init() will
>> take quite long time. In my testing, if the registering cache set has
>> a existed UUID which matches a already registered cached device, the
>> automatical attachment during the registration may take more than
>> 55 minutes. This is too long for waiting the bcache to work in real
>> deployment.
>>
>> Fortunately when bch_sectors_dirty_init() is called, no other thread
>> will access the btree yet, it is safe to do a read-only parallelized
>> dirty sectors counting by multiple threads.
>>
>> This patch tries to create multiple threads, and each thread tries to
>> one-by-one count dirty sectors from the sub-tree indexed by a root
>> node key which the thread fetched. After the sub-tree is counted, the
>> counting thread will continue to fetch another root node key, until
>> the fetched key is NULL. How many threads in parallel depends on
>> the number of keys from the btree root node, and the number of online
>> CPU core. The thread number will be the less number but no more than
>> BCH_DIRTY_INIT_THRD_MAX. If there are only 2 keys in root node, it
>> can only be 2x times faster by this patch. But if there are 10 keys
>> in the root node, with this patch it can be 10x times faster.
>>
>> Signed-off-by: Coly Li <colyli@suse.de>
>> Cc: Christoph Hellwig <hch@infradead.org>
>> ---
>>   drivers/md/bcache/writeback.c | 158 +++++++++++++++++++++++++++++++++-
>>   drivers/md/bcache/writeback.h |  19 ++++
>>   2 files changed, 174 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/md/bcache/writeback.c
>> b/drivers/md/bcache/writeback.c
>> index 4a40f9eadeaf..6673a37c8bd2 100644
>> --- a/drivers/md/bcache/writeback.c
>> +++ b/drivers/md/bcache/writeback.c
>> @@ -785,7 +785,9 @@ static int sectors_dirty_init_fn(struct btree_op
>> *_op, struct btree *b,
>>       return MAP_CONTINUE;
>>   }
>>   -void bch_sectors_dirty_init(struct bcache_device *d)
>> +static int bch_root_node_dirty_init(struct cache_set *c,
>> +                     struct bcache_device *d,
>> +                     struct bkey *k)
>>   {
>>       struct sectors_dirty_init op;
>>       int ret;
>> @@ -796,8 +798,13 @@ void bch_sectors_dirty_init(struct bcache_device *d)
>>       op.start = KEY(op.inode, 0, 0);
>>         do {
>> -        ret = bch_btree_map_keys(&op.op, d->c, &op.start,
>> -                     sectors_dirty_init_fn, 0);
>> +        ret = bcache_btree(map_keys_recurse,
>> +                   k,
>> +                   c->root,
>> +                   &op.op,
>> +                   &op.start,
>> +                   sectors_dirty_init_fn,
>> +                   0);
>>           if (ret == -EAGAIN)
>>               schedule_timeout_interruptible(
>>                   msecs_to_jiffies(INIT_KEYS_SLEEP_MS));
>> @@ -806,6 +813,151 @@ void bch_sectors_dirty_init(struct bcache_device
>> *d)
>>               break;
>>           }
>>       } while (ret == -EAGAIN);
>> +
>> +    return ret;
>> +}
>> +
>> +static int bch_dirty_init_thread(void *arg)
>> +{
>> +    struct dirty_init_thrd_info *info = arg;
>> +    struct bch_dirty_init_state *state = info->state;
>> +    struct cache_set *c = state->c;
>> +    struct btree_iter iter;
>> +    struct bkey *k, *p;
>> +    int cur_idx, prev_idx, skip_nr;
>> +    int i;
>> +
>> +    k = p = NULL;
>> +    i = 0;
>> +    cur_idx = prev_idx = 0;
>> +
>> +    bch_btree_iter_init(&c->root->keys, &iter, NULL);
>> +    k = bch_btree_iter_next_filter(&iter, &c->root->keys, bch_ptr_bad);
>> +    BUG_ON(!k);
>> +
>> +    p = k;
>> +
>> +    while (k) {
>> +        spin_lock(&state->idx_lock);
>> +        cur_idx = state->key_idx;
>> +        state->key_idx++;
>> +        spin_unlock(&state->idx_lock);
>> +
>> +        skip_nr = cur_idx - prev_idx;
>> +
>> +        while (skip_nr) {
>> +            k = bch_btree_iter_next_filter(&iter,
>> +                               &c->root->keys,
>> +                               bch_ptr_bad);
>> +            if (k)
>> +                p = k;
>> +            else {
>> +                atomic_set(&state->enough, 1);
>> +                /* Update state->enough earlier */
>> +                smp_mb();
>> +                goto out;
>> +            }
>> +            skip_nr--;
>> +            cond_resched();
>> +        }
>> +
>> +        if (p) {
>> +            if (bch_root_node_dirty_init(c, state->d, p) < 0)
>> +                goto out;
>> +        }
>> +
>> +        p = NULL;
>> +        prev_idx = cur_idx;
>> +        cond_resched();
>> +    }
>> +
>> +out:
>> +    /* In order to wake up state->wait in time */
>> +    smp_mb();
>> +    if (atomic_dec_and_test(&state->started))
>> +        wake_up(&state->wait);
>> +
>> +    return 0;
>> +}
>> +
>> +static int bch_btre_dirty_init_thread_nr(void)
>> +{
>> +    int n = num_online_cpus()/2;
>> +
>> +    if (n == 0)
>> +        n = 1;
>> +    else if (n > BCH_DIRTY_INIT_THRD_MAX)
>> +        n = BCH_DIRTY_INIT_THRD_MAX;
>> +
>> +    return n;
>> +}
>> +
>> +void bch_sectors_dirty_init(struct bcache_device *d)
>> +{
>> +    int i;
>> +    struct bkey *k = NULL;
>> +    struct btree_iter iter;
>> +    struct sectors_dirty_init op;
>> +    struct cache_set *c = d->c;
>> +    struct bch_dirty_init_state *state;
>> +    char name[32];
>> +
>> +    /* Just count root keys if no leaf node */
>> +    if (c->root->level == 0) {
>> +        bch_btree_op_init(&op.op, -1);
>> +        op.inode = d->id;
>> +        op.count = 0;
>> +        op.start = KEY(op.inode, 0, 0);
>> +
>> +        for_each_key_filter(&c->root->keys,
>> +                    k, &iter, bch_ptr_invalid)
>> +            sectors_dirty_init_fn(&op.op, c->root, k);
>> +        return;
>> +    }
>> +
>> +    state = kzalloc(sizeof(struct bch_dirty_init_state), GFP_KERNEL);
>> +    if (!state) {
>> +        pr_warn("sectors dirty init failed: cannot allocate memory");
>> +        return;
>> +    }
>> +
>> +    state->c = c;
>> +    state->d = d;
>> +    state->total_threads = bch_btre_dirty_init_thread_nr();
>> +    state->key_idx = 0;
>> +    spin_lock_init(&state->idx_lock);
>> +    atomic_set(&state->started, 0);
>> +    atomic_set(&state->enough, 0);
>> +    init_waitqueue_head(&state->wait);
>> +
>> +    for (i = 0; i < state->total_threads; i++) {
>> +        /* Fetch latest state->enough earlier */
>> +        smp_mb();
>> +        if (atomic_read(&state->enough))
>> +            break;
>> +
>> +        state->infos[i].state = state;
>> +        atomic_inc(&state->started);
>> +        snprintf(name, sizeof(name), "bch_dirty_init[%d]", i);
>> +
>> +        state->infos[i].thread =
>> +            kthread_run(bch_dirty_init_thread,
>> +                    &state->infos[i],
>> +                    name);
>> +        if (IS_ERR(state->infos[i].thread)) {
>> +            pr_err("fails to run thread bch_dirty_init[%d]", i);
>> +            for (--i; i >= 0; i--)
>> +                kthread_stop(state->infos[i].thread);
>> +            goto out;
>> +        }
>> +    }
>> +
>> +    wait_event_interruptible(state->wait,
>> +         atomic_read(&state->started) == 0 ||
>> +         test_bit(CACHE_SET_IO_DISABLE, &c->flags));
>> +
>> +out:
>> +    kfree(state);
>>   }
>>     void bch_cached_dev_writeback_init(struct cached_dev *dc)
>> diff --git a/drivers/md/bcache/writeback.h
>> b/drivers/md/bcache/writeback.h
>> index 4e4c6810dc3c..b029843ce5b6 100644
>> --- a/drivers/md/bcache/writeback.h
>> +++ b/drivers/md/bcache/writeback.h
>> @@ -16,6 +16,7 @@
>>     #define BCH_AUTO_GC_DIRTY_THRESHOLD    50
>>   +#define BCH_DIRTY_INIT_THRD_MAX    64
>>   /*
>>    * 14 (16384ths) is chosen here as something that each backing device
>>    * should be a reasonable fraction of the share, and not to blow up
>> @@ -23,6 +24,24 @@
>>    */
>>   #define WRITEBACK_SHARE_SHIFT   14
>>   +struct bch_dirty_init_state;
>> +struct dirty_init_thrd_info {
>> +    struct bch_dirty_init_state    *state;
>> +    struct task_struct        *thread;
>> +};
>> +
>> +struct bch_dirty_init_state {
>> +    struct cache_set        *c;
>> +    struct bcache_device        *d;
>> +    int                total_threads;
>> +    int                key_idx;
>> +    spinlock_t            idx_lock;
>> +    atomic_t            started;
>> +    atomic_t            enough;
>> +    wait_queue_head_t        wait;
>> +    struct dirty_init_thrd_info    infos[BCH_DIRTY_INIT_THRD_MAX];
>> +};
>> +
>>   static inline uint64_t bcache_dev_sectors_dirty(struct bcache_device
>> *d)
>>   {
>>       uint64_t i, ret = 0;
>>

Hi Hannes,

> Same here; why not workqueues?

The major reason is, there are too many kworkers already. I am not able
to tell which one is working the btree nodes checking. And there is no
difference between kthread and workqueue for this condition.

> You could even be extra sneaky and have a workqueue element per node.
> Then you would not descend into the leaf nodes, but rather call the
> workqueue element for the leaf node.
> That way you get automatic parallelism...

I though about kworker, but it cannot be that ideal parallelism. The
whole btree check only happens in bcache registration time, and it is in
strong order from parent node to children nodes. It means if the parent
node is corrupted, its unreliable children node won't be checked.

The only chance is to check sub-tree indexed by each root node element
in parallel, and in each sub-tree, the checking is still in depth-first
order.

In this case, no difference between kworker queue or kthread.

Thanks.
-- 

Coly Li