[PATCH v2 0/5] btrfs: qgroup: address the performance penalty for subvolume dropping

[PATCH v2 0/5] btrfs: qgroup: address the performance penalty for subvolume dropping

[Qu Wenruo]

Btrfs qgroup has a long history of bringing huge performance penalty,
from subvolume dropping to balance.

Although we solved the problem for balance, but the subvolume dropping
problem is still unresolved, as we really need to do all the costly
backref for all the involved subtrees, or qgroup numbers will be
inconsistent.

But the performance penalty is sometimes too big, so big that it's
better just to disable qgroup, do the drop, then do the rescan.

This patchset will address the problem by introducing a user
configurable sysfs interface, to allow certain high subtree dropping to
mark qgroup inconsistent, and skip the whole accounting.

The following things are needed for this objective:

- New qgroups attributes

  Instead of plain qgroup kobjects, we need extra attributes like
  drop_subtree_threshold.

  This patchset will introduce two new attributes to the existing
  qgroups kobject:
  * qgroups_flags
    To indicate the qgroup status flags like ON, RESCAN, INCONSISTENT.

  * drop_subtree_threshold
    To show the subtree dropping level threshold.
    The default value is BTRFS_MAX_LEVEL (8), which means all subtree
    dropping will go through the qgroup accounting, while costly it will
    try to keep qgroup numbers as consistent as possible.

    Users can specify values like 3, meaning any subtree which is at
    level 3 or higher will mark qgroup inconsistent and skip all the
    costly accounting.

    NOTE: if a snapshot is create with tree root level 3, dropping the
    snapshot with drop_subtree_threshold 3 will not mark the qgroup
    inconsistent.
    Since the level threshold is for shared subtree node, not the
    snapshot root node.

    In the case of newly created snapshot, only its (root level - 1)
    tree blocks are shared subtrees.

    This only affects subvolume dropping.

- Skip qgroup accounting when the numbers are already inconsistent

  But still keeps the qgroup relationship correct, thus users can keep
  its qgroup organization while do the rescan later.


This sysfs interface needs user space tools to monitor and set the
values for each btrfs.
And it's also user space daemon's responsibility to save the
drop_subtree_threshold values.

As introducing a new on-disk format just for qgroup is a little
overkilled to an optional feature to me.

Currently the target user space tool is snapper, which by default
utilizes qgroups for its space-aware snapshots reclaim mechanism.

Changelog:
v2:
- Split /sys/fs/btrfs/<uuid>/qgroups/qgroup_flags into two
  entries

- Update the cover letter to explain the drop_subtree_threshold better

Qu Wenruo (5):
  btrfs: sysfs: introduce qgroup global attribute groups
  btrfs: introduce BTRFS_QGROUP_STATUS_FLAGS_MASK for later expansion
  btrfs: introduce BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN
  btrfs: introduce BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING to skip
    qgroup accounting
  btrfs: skip subtree scan if it's too high to avoid low stall in
    btrfs_commit_transaction()

 fs/btrfs/ctree.h                |   1 +
 fs/btrfs/disk-io.c              |   1 +
 fs/btrfs/qgroup.c               |  87 ++++++++++++++++++-------
 fs/btrfs/qgroup.h               |   3 +
 fs/btrfs/sysfs.c                | 112 ++++++++++++++++++++++++++++++--
 include/uapi/linux/btrfs_tree.h |   4 ++
 6 files changed, 182 insertions(+), 26 deletions(-)

-- 
2.34.1

[PATCH v2 1/5] btrfs: sysfs: introduce qgroup global attribute groups

[Qu Wenruo]

Although we already have info kobject for each qgroup, we don't have
global qgroup info attributes to show things like enabled or
inconsistent flags.

Add this qgroups attribute groups, and the first member is qgroup_flags,
which is a read-only attribute to show human readable qgroup flags.

The path is:
 /sys/fs/btrfs/<uuid>/qgroups/enabled
 /sys/fs/btrfs/<uuid>/qgroups/inconsistent

The output is pretty simple, just 1 or 0, with "\n".

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/sysfs.c | 73 +++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 69 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index f9eff3b0f77c..e48904666703 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -1820,11 +1820,72 @@ int btrfs_sysfs_add_mounted(struct btrfs_fs_info *fs_info)
 	return error;
 }
 
+static ssize_t qgroup_enabled_show(struct kobject *qgroups_kobj,
+				   struct kobj_attribute *a,
+				   char *buf)
+{
+	struct btrfs_fs_info *fs_info = to_fs_info(qgroups_kobj->parent);
+	bool enabled;
+	int ret = 0;
+
+	spin_lock(&fs_info->qgroup_lock);
+	enabled = fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON;
+	spin_unlock(&fs_info->qgroup_lock);
+
+	ret += scnprintf(buf, PAGE_SIZE, "%d\n", enabled);
+	return ret;
+}
+
+BTRFS_ATTR(qgroups, enabled, qgroup_enabled_show);
+
+static ssize_t qgroup_inconsistent_show(struct kobject *qgroups_kobj,
+					struct kobj_attribute *a,
+					char *buf)
+{
+	struct btrfs_fs_info *fs_info = to_fs_info(qgroups_kobj->parent);
+	bool inconsistent;
+	int ret = 0;
+
+	spin_lock(&fs_info->qgroup_lock);
+	inconsistent = fs_info->qgroup_flags &
+		       BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+	spin_unlock(&fs_info->qgroup_lock);
+
+	ret += scnprintf(buf, PAGE_SIZE, "%d\n", inconsistent);
+	return ret;
+}
+
+BTRFS_ATTR(qgroups, inconsistent, qgroup_inconsistent_show);
+
+/*
+ * Qgroups global info
+ *
+ * Path: /sys/fs/btrfs/<uuid>/qgroups/
+ */
+static struct attribute *qgroups_attrs[] = {
+	BTRFS_ATTR_PTR(qgroups, enabled),
+	BTRFS_ATTR_PTR(qgroups, inconsistent),
+	NULL
+};
+ATTRIBUTE_GROUPS(qgroups);
+
+static void qgroups_release(struct kobject *kobj)
+{
+	kfree(kobj);
+}
+
+static struct kobj_type qgroups_ktype = {
+	.sysfs_ops = &kobj_sysfs_ops,
+	.default_groups = qgroups_groups,
+	.release = qgroups_release,
+};
+
 static inline struct btrfs_fs_info *qgroup_kobj_to_fs_info(struct kobject *kobj)
 {
 	return to_fs_info(kobj->parent->parent);
 }
 
+
 #define QGROUP_ATTR(_member, _show_name)					\
 static ssize_t btrfs_qgroup_show_##_member(struct kobject *qgroup_kobj,		\
 					   struct kobj_attribute *a,		\
@@ -1945,11 +2006,15 @@ int btrfs_sysfs_add_qgroups(struct btrfs_fs_info *fs_info)
 	if (fs_info->qgroups_kobj)
 		return 0;
 
-	fs_info->qgroups_kobj = kobject_create_and_add("qgroups", fsid_kobj);
-	if (!fs_info->qgroups_kobj) {
-		ret = -ENOMEM;
+	fs_info->qgroups_kobj = kmalloc(sizeof(struct kobject), GFP_KERNEL);
+	if (!fs_info->qgroups_kobj)
+		return -ENOMEM;
+
+	ret = kobject_init_and_add(fs_info->qgroups_kobj, &qgroups_ktype,
+				   fsid_kobj, "qgroups");
+	if (ret < 0)
 		goto out;
-	}
+
 	rbtree_postorder_for_each_entry_safe(qgroup, next,
 					     &fs_info->qgroup_tree, node) {
 		ret = btrfs_sysfs_add_one_qgroup(fs_info, qgroup);
-- 
2.34.1

[PATCH v2 2/5] btrfs: introduce BTRFS_QGROUP_STATUS_FLAGS_MASK for later expansion

[Qu Wenruo]

Currently we only have 3 qgroup flags:
- BTRFS_QGROUP_STATUS_FLAG_ON
- BTRFS_QGROUP_STATUS_FLAG_RESCAN
- BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT

These flags matches the on-disk flags used in btrfs_qgroup_status.

But we're going to introduce extra runtime flags which will not reach
disks.

So here we introduce a new mask, BTRFS_QGROUP_STATUS_FLAGS_MASK, to
make sure only those flags can reach disks.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/qgroup.c               | 6 ++++--
 include/uapi/linux/btrfs_tree.h | 4 ++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index b126cc39ffd4..d4d81c8df999 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -867,7 +867,8 @@ static int update_qgroup_status_item(struct btrfs_trans_handle *trans)
 	l = path->nodes[0];
 	slot = path->slots[0];
 	ptr = btrfs_item_ptr(l, slot, struct btrfs_qgroup_status_item);
-	btrfs_set_qgroup_status_flags(l, ptr, fs_info->qgroup_flags);
+	btrfs_set_qgroup_status_flags(l, ptr, fs_info->qgroup_flags &
+				      BTRFS_QGROUP_STATUS_FLAGS_MASK);
 	btrfs_set_qgroup_status_generation(l, ptr, trans->transid);
 	btrfs_set_qgroup_status_rescan(l, ptr,
 				fs_info->qgroup_rescan_progress.objectid);
@@ -1035,7 +1036,8 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
 	btrfs_set_qgroup_status_version(leaf, ptr, BTRFS_QGROUP_STATUS_VERSION);
 	fs_info->qgroup_flags = BTRFS_QGROUP_STATUS_FLAG_ON |
 				BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
-	btrfs_set_qgroup_status_flags(leaf, ptr, fs_info->qgroup_flags);
+	btrfs_set_qgroup_status_flags(leaf, ptr, fs_info->qgroup_flags &
+				      BTRFS_QGROUP_STATUS_FLAGS_MASK);
 	btrfs_set_qgroup_status_rescan(leaf, ptr, 0);
 
 	btrfs_mark_buffer_dirty(leaf);
diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
index 5416f1f1a77a..5debe4b4e452 100644
--- a/include/uapi/linux/btrfs_tree.h
+++ b/include/uapi/linux/btrfs_tree.h
@@ -975,6 +975,10 @@ static inline __u16 btrfs_qgroup_level(__u64 qgroupid)
  */
 #define BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT	(1ULL << 2)
 
+#define BTRFS_QGROUP_STATUS_FLAGS_MASK	(BTRFS_QGROUP_STATUS_FLAG_ON |\
+					 BTRFS_QGROUP_STATUS_FLAG_RESCAN |\
+					 BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT)
+
 #define BTRFS_QGROUP_STATUS_VERSION        1
 
 struct btrfs_qgroup_status_item {
-- 
2.34.1

[PATCH v2 3/5] btrfs: introduce BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN

[Qu Wenruo]

Here we introduce a new runtime flag,
BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN, which will inform qgroup rescan
to cancel its work asynchronously.

This is to address the window when an operation makes qgroup numbers
inconsistent (like qgroup inheriting) while a qgroup rescan is running.

In that case, qgroup inconsistent flag will be cleared when qgroup
rescan finishes.
But we changed the ownership of some extents, which means the rescan is
already meaningless, and the qgroup inconsistent flag should not be
cleared.

With the new flag, each time we set INCONSISTENT flag, we also set this
new flag to inform any running qgroup rescan to exit immediately, and
leaving the INCONSISTENT flag there.

The new runtime flag can only be cleared when a new rescan is started.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/qgroup.c | 49 ++++++++++++++++++++++++++++++-----------------
 fs/btrfs/qgroup.h |  2 ++
 2 files changed, 33 insertions(+), 18 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index d4d81c8df999..d7c03b8ab926 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -322,6 +322,14 @@ int btrfs_verify_qgroup_counts(struct btrfs_fs_info *fs_info, u64 qgroupid,
 }
 #endif
 
+static void qgroup_mark_inconsistent(struct btrfs_fs_info *fs_info)
+{
+	BUILD_BUG_ON(BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN &
+		     BTRFS_QGROUP_STATUS_FLAGS_MASK);
+	fs_info->qgroup_flags |= (BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT |
+				  BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN);
+}
+
 /*
  * The full config is read in one go, only called from open_ctree()
  * It doesn't use any locking, as at this point we're still single-threaded
@@ -390,7 +398,7 @@ int btrfs_read_qgroup_config(struct btrfs_fs_info *fs_info)
 			}
 			if (btrfs_qgroup_status_generation(l, ptr) !=
 			    fs_info->generation) {
-				flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+				qgroup_mark_inconsistent(fs_info);
 				btrfs_err(fs_info,
 					"qgroup generation mismatch, marked as inconsistent");
 			}
@@ -408,7 +416,7 @@ int btrfs_read_qgroup_config(struct btrfs_fs_info *fs_info)
 		if ((qgroup && found_key.type == BTRFS_QGROUP_INFO_KEY) ||
 		    (!qgroup && found_key.type == BTRFS_QGROUP_LIMIT_KEY)) {
 			btrfs_err(fs_info, "inconsistent qgroup config");
-			flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+			qgroup_mark_inconsistent(fs_info);
 		}
 		if (!qgroup) {
 			qgroup = add_qgroup_rb(fs_info, found_key.offset);
@@ -1680,7 +1688,7 @@ int btrfs_limit_qgroup(struct btrfs_trans_handle *trans, u64 qgroupid,
 
 	ret = update_qgroup_limit_item(trans, qgroup);
 	if (ret) {
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+		qgroup_mark_inconsistent(fs_info);
 		btrfs_info(fs_info, "unable to update quota limit for %llu",
 		       qgroupid);
 	}
@@ -1756,7 +1764,7 @@ int btrfs_qgroup_trace_extent_post(struct btrfs_trans_handle *trans,
 	ret = btrfs_find_all_roots(NULL, trans->fs_info, bytenr, 0, &old_root,
 				   true);
 	if (ret < 0) {
-		trans->fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+		qgroup_mark_inconsistent(trans->fs_info);
 		btrfs_warn(trans->fs_info,
 "error accounting new delayed refs extent (err code: %d), quota inconsistent",
 			ret);
@@ -2232,7 +2240,7 @@ static int qgroup_trace_subtree_swap(struct btrfs_trans_handle *trans,
 out:
 	btrfs_free_path(dst_path);
 	if (ret < 0)
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+		qgroup_mark_inconsistent(fs_info);
 	return ret;
 }
 
@@ -2736,12 +2744,10 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans)
 		spin_unlock(&fs_info->qgroup_lock);
 		ret = update_qgroup_info_item(trans, qgroup);
 		if (ret)
-			fs_info->qgroup_flags |=
-					BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+			qgroup_mark_inconsistent(fs_info);
 		ret = update_qgroup_limit_item(trans, qgroup);
 		if (ret)
-			fs_info->qgroup_flags |=
-					BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+			qgroup_mark_inconsistent(fs_info);
 		spin_lock(&fs_info->qgroup_lock);
 	}
 	if (test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
@@ -2752,7 +2758,7 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans)
 
 	ret = update_qgroup_status_item(trans);
 	if (ret)
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+		qgroup_mark_inconsistent(fs_info);
 
 	return ret;
 }
@@ -2870,7 +2876,7 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
 
 		ret = update_qgroup_limit_item(trans, dstgroup);
 		if (ret) {
-			fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+			qgroup_mark_inconsistent(fs_info);
 			btrfs_info(fs_info,
 				   "unable to update quota limit for %llu",
 				   dstgroup->qgroupid);
@@ -2976,7 +2982,7 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
 	if (!committing)
 		mutex_unlock(&fs_info->qgroup_ioctl_lock);
 	if (need_rescan)
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+		qgroup_mark_inconsistent(fs_info);
 	return ret;
 }
 
@@ -3248,7 +3254,8 @@ static int qgroup_rescan_leaf(struct btrfs_trans_handle *trans,
 static bool rescan_should_stop(struct btrfs_fs_info *fs_info)
 {
 	return btrfs_fs_closing(fs_info) ||
-		test_bit(BTRFS_FS_STATE_REMOUNTING, &fs_info->fs_state);
+		test_bit(BTRFS_FS_STATE_REMOUNTING, &fs_info->fs_state) ||
+		fs_info->qgroup_flags & BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN;
 }
 
 static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
@@ -3296,7 +3303,8 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 	if (err > 0 &&
 	    fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT) {
 		fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
-	} else if (err < 0) {
+	} else if (err < 0 ||
+		   fs_info->qgroup_flags & BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN) {
 		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
 	}
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
@@ -3315,7 +3323,8 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 	}
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	if (!stopped)
+	if (!stopped ||
+	    fs_info->qgroup_flags & BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN)
 		fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN;
 	if (trans) {
 		ret = update_qgroup_status_item(trans);
@@ -3326,6 +3335,7 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 		}
 	}
 	fs_info->qgroup_rescan_running = false;
+	fs_info->qgroup_flags &= ~BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN;
 	complete_all(&fs_info->qgroup_rescan_completion);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
@@ -3336,6 +3346,9 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 
 	if (stopped) {
 		btrfs_info(fs_info, "qgroup scan paused");
+	} else if (fs_info->qgroup_flags &
+		   BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN) {
+		btrfs_info(fs_info, "qgroup scan cancelled");
 	} else if (err >= 0) {
 		btrfs_info(fs_info, "qgroup scan completed%s",
 			err > 0 ? " (inconsistency flag cleared)" : "");
@@ -3395,6 +3408,7 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 
 	memset(&fs_info->qgroup_rescan_progress, 0,
 		sizeof(fs_info->qgroup_rescan_progress));
+	fs_info->qgroup_flags &= ~BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN;
 	fs_info->qgroup_rescan_progress.objectid = progress_objectid;
 	init_completion(&fs_info->qgroup_rescan_completion);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
@@ -4191,8 +4205,7 @@ int btrfs_qgroup_add_swapped_blocks(struct btrfs_trans_handle *trans,
 	spin_unlock(&blocks->lock);
 out:
 	if (ret < 0)
-		fs_info->qgroup_flags |=
-			BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+		qgroup_mark_inconsistent(fs_info);
 	return ret;
 }
 
@@ -4279,7 +4292,7 @@ int btrfs_qgroup_trace_subtree_after_cow(struct btrfs_trans_handle *trans,
 		btrfs_err_rl(fs_info,
 			     "failed to account subtree at bytenr %llu: %d",
 			     subvol_eb->start, ret);
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+		qgroup_mark_inconsistent(fs_info);
 	}
 	return ret;
 }
diff --git a/fs/btrfs/qgroup.h b/fs/btrfs/qgroup.h
index 880e9df0dac1..f92e635bfb69 100644
--- a/fs/btrfs/qgroup.h
+++ b/fs/btrfs/qgroup.h
@@ -100,6 +100,8 @@
  *     subtree rescan for them.
  */
 
+#define BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN		(1UL << 3)
+
 /*
  * Record a dirty extent, and info qgroup to update quota on it
  * TODO: Use kmem cache to alloc it.
-- 
2.34.1

[PATCH v2 4/5] btrfs: introduce BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING to skip qgroup accounting

[Qu Wenruo]

The new flag will make btrfs qgroup to skip all its time consuming
qgroup accounting.

The lifespan is the same as BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN,
only get cleared after a new rescan.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/qgroup.c | 18 ++++++++++++++----
 fs/btrfs/qgroup.h |  1 +
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index d7c03b8ab926..6977ce093993 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -326,8 +326,11 @@ static void qgroup_mark_inconsistent(struct btrfs_fs_info *fs_info)
 {
 	BUILD_BUG_ON(BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN &
 		     BTRFS_QGROUP_STATUS_FLAGS_MASK);
+	BUILD_BUG_ON(BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING &
+		     BTRFS_QGROUP_STATUS_FLAGS_MASK);
 	fs_info->qgroup_flags |= (BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT |
-				  BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN);
+				  BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN |
+				  BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING);
 }
 
 /*
@@ -1761,6 +1764,10 @@ int btrfs_qgroup_trace_extent_post(struct btrfs_trans_handle *trans,
 	 */
 	ASSERT(trans != NULL);
 
+	if (trans->fs_info->qgroup_flags &
+	    BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING)
+		return 0;
+
 	ret = btrfs_find_all_roots(NULL, trans->fs_info, bytenr, 0, &old_root,
 				   true);
 	if (ret < 0) {
@@ -2575,7 +2582,8 @@ int btrfs_qgroup_account_extent(struct btrfs_trans_handle *trans, u64 bytenr,
 	 * If quotas get disabled meanwhile, the resources need to be freed and
 	 * we can't just exit here.
 	 */
-	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags) ||
+	    fs_info->qgroup_flags & BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING)
 		goto out_free;
 
 	if (new_roots) {
@@ -2671,7 +2679,8 @@ int btrfs_qgroup_account_extents(struct btrfs_trans_handle *trans)
 		num_dirty_extents++;
 		trace_btrfs_qgroup_account_extents(fs_info, record);
 
-		if (!ret) {
+		if (!ret && !(fs_info->qgroup_flags &
+			      BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING)) {
 			/*
 			 * Old roots should be searched when inserting qgroup
 			 * extent record
@@ -3408,7 +3417,8 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 
 	memset(&fs_info->qgroup_rescan_progress, 0,
 		sizeof(fs_info->qgroup_rescan_progress));
-	fs_info->qgroup_flags &= ~BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN;
+	fs_info->qgroup_flags &= ~(BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN |
+				   BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING);
 	fs_info->qgroup_rescan_progress.objectid = progress_objectid;
 	init_completion(&fs_info->qgroup_rescan_completion);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
diff --git a/fs/btrfs/qgroup.h b/fs/btrfs/qgroup.h
index f92e635bfb69..fe42975dcea7 100644
--- a/fs/btrfs/qgroup.h
+++ b/fs/btrfs/qgroup.h
@@ -101,6 +101,7 @@
  */
 
 #define BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN		(1UL << 3)
+#define BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING		(1UL << 4)
 
 /*
  * Record a dirty extent, and info qgroup to update quota on it
-- 
2.34.1

[PATCH v2 5/5] btrfs: skip subtree scan if it's too high to avoid low stall in btrfs_commit_transaction()

[Qu Wenruo]

Btrfs qgroup has a long history of bringing performance penalty in
btrfs_commit_transaction().

Although we tried our best to migrate such impact, there is still an
unsolved call site, btrfs_drop_snapshot().

This function will find the highest shared tree block and modify its
extent ownership to do a subvolume/snapshot dropping.

Such change will affect the whole subtree, and cause tons of qgroup
dirty extents and stall btrfs_commit_transaction().

To avoid such problem, here we introduce a new sysfs interface,
/sys/fs/btrfs/<uuid>/qgroups/drop_subptree_threshold, to determine at
whether and at which level we should skip qgroup accounting for subtree
dropping.

The default value is BTRFS_MAX_LEVEL, thus every subtree drop will go
through qgroup accounting, to ensure qgroup numbers are kept as
consistent as possible.

While for performance sensitive users, they can change the values to
more reasonable values like 3, to make any subtree, which is at or higher
than level 3, to mark qgroup inconsistent and skip the accounting.

The cost is obvious, the qgroup number is no longer consistent, but at
least performance is more reasonable, and users have the control.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/ctree.h   |  1 +
 fs/btrfs/disk-io.c |  1 +
 fs/btrfs/qgroup.c  | 18 ++++++++++++++++++
 fs/btrfs/sysfs.c   | 39 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 59 insertions(+)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index dfee4b403da1..79512a0f6a5b 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -952,6 +952,7 @@ struct btrfs_fs_info {
 	struct completion qgroup_rescan_completion;
 	struct btrfs_work qgroup_rescan_work;
 	bool qgroup_rescan_running;	/* protected by qgroup_rescan_lock */
+	u8 qgroup_drop_subtree_thres;
 
 	/* filesystem state */
 	unsigned long fs_state;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 5c598e124c25..a5167de302e8 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2385,6 +2385,7 @@ static void btrfs_init_qgroup(struct btrfs_fs_info *fs_info)
 	fs_info->qgroup_seq = 1;
 	fs_info->qgroup_ulist = NULL;
 	fs_info->qgroup_rescan_running = false;
+	fs_info->qgroup_drop_subtree_thres = BTRFS_MAX_LEVEL;
 	mutex_init(&fs_info->qgroup_rescan_lock);
 }
 
diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 6977ce093993..0d89c0dfd67d 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1230,6 +1230,7 @@ int btrfs_quota_disable(struct btrfs_fs_info *fs_info)
 	quota_root = fs_info->quota_root;
 	fs_info->quota_root = NULL;
 	fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_ON;
+	fs_info->qgroup_drop_subtree_thres = BTRFS_MAX_LEVEL;
 	spin_unlock(&fs_info->qgroup_lock);
 
 	btrfs_free_qgroup_config(fs_info);
@@ -2258,6 +2259,7 @@ int btrfs_qgroup_trace_subtree(struct btrfs_trans_handle *trans,
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 	int ret = 0;
 	int level;
+	u8 drop_subptree_thres;
 	struct extent_buffer *eb = root_eb;
 	struct btrfs_path *path = NULL;
 
@@ -2267,6 +2269,22 @@ int btrfs_qgroup_trace_subtree(struct btrfs_trans_handle *trans,
 	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
 		return 0;
 
+	spin_lock(&fs_info->qgroup_lock);
+	drop_subptree_thres = fs_info->qgroup_drop_subtree_thres;
+	spin_unlock(&fs_info->qgroup_lock);
+	/*
+	 * This function only get called for snapshot drop, if we hit a high
+	 * node here, it means we are going to change ownership for quite a lot
+	 * of extents, which will greatly slow down btrfs_commit_transaction().
+	 *
+	 * So here if we find a high tree here, we just skip the accounting and
+	 * mark qgroup inconsistent.
+	 */
+	if (root_level >= drop_subptree_thres) {
+		qgroup_mark_inconsistent(fs_info);
+		return 0;
+	}
+
 	if (!extent_buffer_uptodate(root_eb)) {
 		ret = btrfs_read_buffer(root_eb, root_gen, root_level, NULL);
 		if (ret)
diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index e48904666703..7bc9906ecbfa 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -1857,6 +1857,44 @@ static ssize_t qgroup_inconsistent_show(struct kobject *qgroups_kobj,
 
 BTRFS_ATTR(qgroups, inconsistent, qgroup_inconsistent_show);
 
+static ssize_t qgroup_drop_subtree_thres_show(struct kobject *qgroups_kobj,
+					      struct kobj_attribute *a,
+					      char *buf)
+{
+	struct btrfs_fs_info *fs_info = to_fs_info(qgroups_kobj->parent);
+	u8 result;
+
+	spin_lock(&fs_info->qgroup_lock);
+	result = fs_info->qgroup_drop_subtree_thres;
+	spin_unlock(&fs_info->qgroup_lock);
+
+	return scnprintf(buf, PAGE_SIZE, "%d\n", result);
+}
+
+static ssize_t qgroup_drop_subtree_thres_store(struct kobject *qgroups_kobj,
+					       struct kobj_attribute *a,
+					       const char *buf, size_t len)
+{
+	struct btrfs_fs_info *fs_info = to_fs_info(qgroups_kobj->parent);
+	u8 new_thres;
+	int ret;
+
+	ret = kstrtou8(buf, 10, &new_thres);
+	if (ret)
+		return -EINVAL;
+
+	if (new_thres > BTRFS_MAX_LEVEL)
+		return -EINVAL;
+
+	spin_lock(&fs_info->qgroup_lock);
+	fs_info->qgroup_drop_subtree_thres = new_thres;
+	spin_unlock(&fs_info->qgroup_lock);
+	return len;
+}
+
+BTRFS_ATTR_RW(qgroups, drop_subtree_threshold, qgroup_drop_subtree_thres_show,
+	      qgroup_drop_subtree_thres_store);
+
 /*
  * Qgroups global info
  *
@@ -1865,6 +1903,7 @@ BTRFS_ATTR(qgroups, inconsistent, qgroup_inconsistent_show);
 static struct attribute *qgroups_attrs[] = {
 	BTRFS_ATTR_PTR(qgroups, enabled),
 	BTRFS_ATTR_PTR(qgroups, inconsistent),
+	BTRFS_ATTR_PTR(qgroups, drop_subtree_threshold),
 	NULL
 };
 ATTRIBUTE_GROUPS(qgroups);
-- 
2.34.1