bcachefs: Add a sysfs var for average btree write size

Useful number for performance tuning.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -796,6 +796,9 @@ mempool_t		bio_bounce_pages;
 	struct bio_set		dio_write_bioset;
 	struct bio_set		dio_read_bioset;
 
+
+	atomic64_t		btree_writes_nr;
+	atomic64_t		btree_writes_sectors;
 	struct bio_list		btree_write_error_list;
 	struct work_struct	btree_write_error_work;
 	spinlock_t		btree_write_error_lock;

fs/bcachefs/btree_io.c

@@ -1551,6 +1551,9 @@ void __bch2_btree_node_write(struct bch_fs *c, struct btree *b,
 
 	b->written += sectors_to_write;
 
+	atomic64_inc(&c->btree_writes_nr);
+	atomic64_add(sectors_to_write, &c->btree_writes_sectors);
+
 	/* XXX: submitting IO with btree locks held: */
 	bch2_submit_wbio_replicas(&wbio->wbio, c, BCH_DATA_btree, k.k);
 	bch2_bkey_buf_exit(&k, c);

fs/bcachefs/sysfs.c

@@ -153,6 +153,8 @@ read_attribute(io_latency_stats_read);
 read_attribute(io_latency_stats_write);
 read_attribute(congested);
 
+read_attribute(btree_avg_write_size);
+
 read_attribute(bucket_quantiles_last_read);
 read_attribute(bucket_quantiles_last_write);
 read_attribute(bucket_quantiles_fragmentation);
@@ -228,6 +230,14 @@ static size_t bch2_btree_cache_size(struct bch_fs *c)
 	return ret;
 }
 
+static size_t bch2_btree_avg_write_size(struct bch_fs *c)
+{
+	u64 nr = atomic64_read(&c->btree_writes_nr);
+	u64 sectors = atomic64_read(&c->btree_writes_sectors);
+
+	return nr ? div64_u64(sectors, nr) : 0;
+}
+
 static int fs_alloc_debug_to_text(struct printbuf *out, struct bch_fs *c)
 {
 	struct bch_fs_usage_online *fs_usage = bch2_fs_usage_read(c);
@@ -316,6 +326,7 @@ SHOW(bch2_fs)
 	sysfs_print(block_size,			block_bytes(c));
 	sysfs_print(btree_node_size,		btree_bytes(c));
 	sysfs_hprint(btree_cache_size,		bch2_btree_cache_size(c));
+	sysfs_hprint(btree_avg_write_size,	bch2_btree_avg_write_size(c));
 
 	sysfs_print(read_realloc_races,
 		    atomic_long_read(&c->read_realloc_races));
@@ -507,6 +518,7 @@ struct attribute *bch2_fs_files[] = {
 	&sysfs_block_size,
 	&sysfs_btree_node_size,
 	&sysfs_btree_cache_size,
+	&sysfs_btree_avg_write_size,
 
 	&sysfs_journal_write_delay_ms,
 	&sysfs_journal_reclaim_delay_ms,