bcachefs: Have journal reclaim thread flush more aggressively

This adds a new watermark for the journal reclaim when flushing btree
key cache entries - it should try and stay ahead of where foreground
threads doing transaction commits will enter direct journal reclaim.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_key_cache.h

 #ifndef _BCACHEFS_BTREE_KEY_CACHE_H
 #define _BCACHEFS_BTREE_KEY_CACHE_H
 
+static inline size_t bch2_nr_btree_keys_want_flush(struct bch_fs *c)
+{
+	size_t nr_dirty = atomic_long_read(&c->btree_key_cache.nr_dirty);
+	size_t nr_keys = atomic_long_read(&c->btree_key_cache.nr_keys);
+	size_t max_dirty = nr_keys / 4;
+
+	return max_t(ssize_t, 0, nr_dirty - max_dirty);
+}
+
 static inline size_t bch2_nr_btree_keys_need_flush(struct bch_fs *c)
 {
 	size_t nr_dirty = atomic_long_read(&c->btree_key_cache.nr_dirty);

fs/bcachefs/journal_reclaim.c

@@ -602,7 +602,7 @@ static int __bch2_journal_reclaim(struct journal *j, bool direct)
 		if (fifo_free(&j->pin) <= 32)
 			min_nr = 1;
 
-		min_nr = max(min_nr, bch2_nr_btree_keys_need_flush(c));
+		min_nr = max(min_nr, bch2_nr_btree_keys_want_flush(c));
 
 		trace_journal_reclaim_start(c,
 				min_nr,