btrfs: cache if we are using free space bitmaps for a block group

Every time we add free space to the free space tree or we remove free
space from the free space tree, we do a lookup for the block group's free
space info item in the free space tree. This takes time, navigating the
btree and we may block either on IO when reading extent buffers from disk
or on extent buffer lock contention due to concurrency.

Instead of doing this lookup every time, cache the result in the block
structure and use it after the first lookup. This adds two boolean members
to the block group structure but doesn't increase the structure's size.

The following script that runs fs_mark was used to measure the time spent
on run_delayed_tree_ref(), since down that call chain we have calls to
add and remove free space to/from the free space tree (calls to
btrfs_add_to_free_space_tree() and btrfs_remove_from_free_space_tree()):

  $ cat test.sh
  #!/bin/bash

  DEV=/dev/nullb0
  MNT=/mnt
  FILES=100000
  THREADS=$(nproc --all)

  echo "performance" | \
      tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

  umount $DEV &> /dev/null
  mkfs.btrfs -f $DEV
  mount -o ssd $DEV $MNT

  OPTS="-S 0 -L 5 -n $FILES -s 0 -t $THREADS -k"
  for ((i = 1; i <= $THREADS; i++)); do
      OPTS="$OPTS -d $MNT/d$i"
  done

  fs_mark $OPTS

  umount $MNT

This is a heavy metadata test as it's exercising only file creation, so a
lot of allocations of metadata extents, creating delayed refs for adding
new metadata extents and dropping existing ones due to COW. The results
of the times it took to execute run_delayed_tree_ref(), in nanoseconds,
are the following.

Before this change:

  Range: 1868.000 - 6482857.000; Mean: 10231.430; Median: 7005.000; Stddev: 27993.173
  Percentiles:  90th: 13342.000; 95th: 23279.000; 99th: 82448.000
     1868.000 -    4222.038:  270696 ############
     4222.038 -    9541.029: 1201327 #####################################################
     9541.029 -   21559.383:  385436 #################
    21559.383 -   48715.063:   64942 ###
    48715.063 -  110073.800:   31454 #
   110073.800 -  248714.944:    8218 |
   248714.944 -  561977.042:    1030 |
   561977.042 - 1269798.254:     295 |
  1269798.254 - 2869132.711:     116 |
  2869132.711 - 6482857.000:      28 |

After this change:

  Range: 1554.000 - 4557014.000; Mean: 9168.164; Median: 6391.000; Stddev: 21467.060
  Percentiles:  90th: 12478.000; 95th: 20964.000; 99th: 72234.000
     1554.000 -    3453.820: 219004 ############
     3453.820 -    7674.743: 980645 #####################################################
     7674.743 -   17052.574: 552486 ##############################
    17052.574 -   37887.762:  68558 ####
    37887.762 -   84178.322:  31557 ##
    84178.322 -  187024.331:  12102 #
   187024.331 -  415522.355:   1364 |
   415522.355 -  923187.626:    256 |
   923187.626 - 2051092.468:    125 |
  2051092.468 - 4557014.000:     21 |

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/block-group.h

@@ -246,6 +246,11 @@ struct btrfs_block_group {
 	/* Lock for free space tree operations. */
 	struct mutex free_space_lock;
 
+	/* Protected by @free_space_lock. */
+	bool using_free_space_bitmaps;
+	/* Protected by @free_space_lock. */
+	bool using_free_space_bitmaps_cached;
+
 	/*
 	 * Number of extents in this block group used for swap files.
 	 * All accesses protected by the spinlock 'lock'.

fs/btrfs/free-space-tree.c

@@ -287,6 +287,8 @@ int btrfs_convert_free_space_to_bitmaps(struct btrfs_trans_handle *trans,
 	leaf = path->nodes[0];
 	flags = btrfs_free_space_flags(leaf, info);
 	flags |= BTRFS_FREE_SPACE_USING_BITMAPS;
+	block_group->using_free_space_bitmaps = true;
+	block_group->using_free_space_bitmaps_cached = true;
 	btrfs_set_free_space_flags(leaf, info, flags);
 	expected_extent_count = btrfs_free_space_extent_count(leaf, info);
 	btrfs_release_path(path);
@@ -434,6 +436,8 @@ int btrfs_convert_free_space_to_extents(struct btrfs_trans_handle *trans,
 	leaf = path->nodes[0];
 	flags = btrfs_free_space_flags(leaf, info);
 	flags &= ~BTRFS_FREE_SPACE_USING_BITMAPS;
+	block_group->using_free_space_bitmaps = false;
+	block_group->using_free_space_bitmaps_cached = true;
 	btrfs_set_free_space_flags(leaf, info, flags);
 	expected_extent_count = btrfs_free_space_extent_count(leaf, info);
 	btrfs_release_path(path);
@@ -796,13 +800,19 @@ static int using_bitmaps(struct btrfs_block_group *bg, struct btrfs_path *path)
 	struct btrfs_free_space_info *info;
 	u32 flags;
 
+	if (bg->using_free_space_bitmaps_cached)
+		return bg->using_free_space_bitmaps;
+
 	info = btrfs_search_free_space_info(NULL, bg, path, 0);
 	if (IS_ERR(info))
 		return PTR_ERR(info);
 	flags = btrfs_free_space_flags(path->nodes[0], info);
 	btrfs_release_path(path);
 
-	return (flags & BTRFS_FREE_SPACE_USING_BITMAPS) ? 1 : 0;
+	bg->using_free_space_bitmaps = (flags & BTRFS_FREE_SPACE_USING_BITMAPS);
+	bg->using_free_space_bitmaps_cached = true;
+
+	return bg->using_free_space_bitmaps;
 }
 
 EXPORT_FOR_TESTS