btrfs: switch extent buffer tree lock to rw_semaphore

Historically we've implemented our own locking because we wanted to be
able to selectively spin or sleep based on what we were doing in the
tree.  For instance, if all of our nodes were in cache then there's
rarely a reason to need to sleep waiting for node locks, as they'll
likely become available soon.  At the time this code was written the
rw_semaphore didn't do adaptive spinning, and thus was orders of
magnitude slower than our home grown locking.

However now the opposite is the case.  There are a few problems with how
we implement blocking locks, namely that we use a normal waitqueue and
simply wake everybody up in reverse sleep order.  This leads to some
suboptimal performance behavior, and a lot of context switches in highly
contended cases.  The rw_semaphores actually do this properly, and also
have adaptive spinning that works relatively well.

The locking code is also a bit of a bear to understand, and we lose the
benefit of lockdep for the most part because the blocking states of the
lock are simply ad-hoc and not mapped into lockdep.

So rework the locking code to drop all of this custom locking stuff, and
simply use a rw_semaphore for everything.  This makes the locking much
simpler for everything, as we can now drop a lot of cruft and blocking
transitions.  The performance numbers vary depending on the workload,
because generally speaking there doesn't tend to be a lot of contention
on the btree.  However, on my test system which is an 80 core single
socket system with 256GiB of RAM and a 2TiB NVMe drive I get the
following results (with all debug options off):

  dbench 200 baseline
  Throughput 216.056 MB/sec  200 clients  200 procs  max_latency=1471.197 ms

  dbench 200 with patch
  Throughput 737.188 MB/sec  200 clients  200 procs  max_latency=714.346 ms

Previously we also used fs_mark to test this sort of contention, and
those results are far less impressive, mostly because there's not enough
tasks to really stress the locking

  fs_mark -d /d[0-15] -S 0 -L 20 -n 100000 -s 0 -t 16

  baseline
    Average Files/sec:     160166.7
    p50 Files/sec:         165832
    p90 Files/sec:         123886
    p99 Files/sec:         123495

    real    3m26.527s
    user    2m19.223s
    sys     48m21.856s

  patched
    Average Files/sec:     164135.7
    p50 Files/sec:         171095
    p90 Files/sec:         122889
    p99 Files/sec:         113819

    real    3m29.660s
    user    2m19.990s
    sys     44m12.259s
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4946,12 +4946,8 @@ __alloc_extent_buffer(struct btrfs_fs_info *fs_info, u64 start,
 	eb->len = len;
 	eb->fs_info = fs_info;
 	eb->bflags = 0;
-	rwlock_init(&eb->lock);
-	atomic_set(&eb->blocking_readers, 0);
-	eb->blocking_writers = 0;
+	init_rwsem(&eb->lock);
 	eb->lock_recursed = false;
-	init_waitqueue_head(&eb->write_lock_wq);
-	init_waitqueue_head(&eb->read_lock_wq);
 
 	btrfs_leak_debug_add(&fs_info->eb_leak_lock, &eb->leak_list,
 			     &fs_info->allocated_ebs);
@@ -4967,13 +4963,6 @@ __alloc_extent_buffer(struct btrfs_fs_info *fs_info, u64 start,
 		> MAX_INLINE_EXTENT_BUFFER_SIZE);
 	BUG_ON(len > MAX_INLINE_EXTENT_BUFFER_SIZE);
 
-#ifdef CONFIG_BTRFS_DEBUG
-	eb->spinning_writers = 0;
-	atomic_set(&eb->spinning_readers, 0);
-	atomic_set(&eb->read_locks, 0);
-	eb->write_locks = 0;
-#endif
-
 	return eb;
 }
 

fs/btrfs/extent_io.h

@@ -87,31 +87,14 @@ struct extent_buffer {
 	int read_mirror;
 	struct rcu_head rcu_head;
 	pid_t lock_owner;
-
-	int blocking_writers;
-	atomic_t blocking_readers;
 	bool lock_recursed;
+	struct rw_semaphore lock;
+
 	/* >= 0 if eb belongs to a log tree, -1 otherwise */
 	short log_index;
 
-	/* protects write locks */
-	rwlock_t lock;
-
-	/* readers use lock_wq while they wait for the write
-	 * lock holders to unlock
-	 */
-	wait_queue_head_t write_lock_wq;
-
-	/* writers use read_lock_wq while they wait for readers
-	 * to unlock
-	 */
-	wait_queue_head_t read_lock_wq;
 	struct page *pages[INLINE_EXTENT_BUFFER_PAGES];
 #ifdef CONFIG_BTRFS_DEBUG
-	int spinning_writers;
-	atomic_t spinning_readers;
-	atomic_t read_locks;
-	int write_locks;
 	struct list_head leak_list;
 #endif
 };

fs/btrfs/locking.h

@@ -110,7 +110,7 @@ static inline struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root
 
 #ifdef CONFIG_BTRFS_DEBUG
 static inline void btrfs_assert_tree_locked(struct extent_buffer *eb) {
-	BUG_ON(!eb->write_locks);
+	lockdep_assert_held(&eb->lock);
 }
 #else
 static inline void btrfs_assert_tree_locked(struct extent_buffer *eb) { }

fs/btrfs/print-tree.c

@@ -191,15 +191,8 @@ static void print_uuid_item(struct extent_buffer *l, unsigned long offset,
 static void print_eb_refs_lock(struct extent_buffer *eb)
 {
 #ifdef CONFIG_BTRFS_DEBUG
-	btrfs_info(eb->fs_info,
-"refs %u lock (w:%d r:%d bw:%d br:%d sw:%d sr:%d) lock_owner %u current %u",
-		   atomic_read(&eb->refs), eb->write_locks,
-		   atomic_read(&eb->read_locks),
-		   eb->blocking_writers,
-		   atomic_read(&eb->blocking_readers),
-		   eb->spinning_writers,
-		   atomic_read(&eb->spinning_readers),
-		   eb->lock_owner, current->pid);
+	btrfs_info(eb->fs_info, "refs %u lock_owner %u current %u",
+		   atomic_read(&eb->refs), eb->lock_owner, current->pid);
 #endif
 }