bcachefs: Btree key cache shrinker fix

The shrinker assumes freed key cache items are ordered by age, so that
it doesn't have to scan the full list to find items that are old enough
(according to the srcu code) to be freed.

But percpu freelists broke this ordering; this patch fixes this by
ensuring we insert items into the proper position.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_key_cache.c

@@ -103,16 +103,34 @@ static void bkey_cached_free(struct btree_key_cache *bc,
 	six_unlock_intent(&ck->c.lock);
 }
 
+#ifdef __KERNEL__
+static void __bkey_cached_move_to_freelist_ordered(struct btree_key_cache *bc,
+						   struct bkey_cached *ck)
+{
+	struct bkey_cached *pos;
+
+	list_for_each_entry_reverse(pos, &bc->freed_nonpcpu, list) {
+		if (ULONG_CMP_GE(ck->btree_trans_barrier_seq,
+				 pos->btree_trans_barrier_seq)) {
+			list_move(&ck->list, &pos->list);
+			return;
+		}
+	}
+
+	list_move(&ck->list, &bc->freed_nonpcpu);
+}
+#endif
+
 static void bkey_cached_move_to_freelist(struct btree_key_cache *bc,
 					 struct bkey_cached *ck)
 {
-	struct btree_key_cache_freelist *f;
-	bool freed = false;
-
 	BUG_ON(test_bit(BKEY_CACHED_DIRTY, &ck->flags));
 
 	if (!ck->c.lock.readers) {
 #ifdef __KERNEL__
+		struct btree_key_cache_freelist *f;
+		bool freed = false;
+
 		preempt_disable();
 		f = this_cpu_ptr(bc->pcpu_freed);
 
@@ -130,11 +148,11 @@ static void bkey_cached_move_to_freelist(struct btree_key_cache *bc,
 			while (f->nr > ARRAY_SIZE(f->objs) / 2) {
 				struct bkey_cached *ck2 = f->objs[--f->nr];
 
-				list_move_tail(&ck2->list, &bc->freed_nonpcpu);
+				__bkey_cached_move_to_freelist_ordered(bc, ck2);
 			}
 			preempt_enable();
 
-			list_move_tail(&ck->list, &bc->freed_nonpcpu);
+			__bkey_cached_move_to_freelist_ordered(bc, ck);
 			mutex_unlock(&bc->lock);
 		}
 #else
@@ -176,11 +194,12 @@ bkey_cached_alloc(struct btree_trans *trans, struct btree_path *path)
 	struct bch_fs *c = trans->c;
 	struct btree_key_cache *bc = &c->btree_key_cache;
 	struct bkey_cached *ck = NULL;
-	struct btree_key_cache_freelist *f;
 	bool pcpu_readers = btree_uses_pcpu_readers(path->btree_id);
 
 	if (!pcpu_readers) {
 #ifdef __KERNEL__
+		struct btree_key_cache_freelist *f;
+
 		preempt_disable();
 		f = this_cpu_ptr(bc->pcpu_freed);
 		if (f->nr)
@@ -982,7 +1001,7 @@ int bch2_fs_btree_key_cache_init(struct btree_key_cache *bc)
 
 	bc->table_init_done = true;
 
-	bc->shrink.seeks		= 1;
+	bc->shrink.seeks		= 0;
 	bc->shrink.count_objects	= bch2_btree_key_cache_count;
 	bc->shrink.scan_objects		= bch2_btree_key_cache_scan;
 	return register_shrinker(&bc->shrink, "%s/btree_key_cache", c->name);
@@ -990,9 +1009,12 @@ int bch2_fs_btree_key_cache_init(struct btree_key_cache *bc)
 
 void bch2_btree_key_cache_to_text(struct printbuf *out, struct btree_key_cache *c)
 {
-	prt_printf(out, "nr_freed:\t%zu\n",	atomic_long_read(&c->nr_freed));
-	prt_printf(out, "nr_keys:\t%lu\n",	atomic_long_read(&c->nr_keys));
-	prt_printf(out, "nr_dirty:\t%lu\n",	atomic_long_read(&c->nr_dirty));
+	prt_printf(out, "nr_freed:\t%zu",	atomic_long_read(&c->nr_freed));
+	prt_newline(out);
+	prt_printf(out, "nr_keys:\t%lu",	atomic_long_read(&c->nr_keys));
+	prt_newline(out);
+	prt_printf(out, "nr_dirty:\t%lu",	atomic_long_read(&c->nr_dirty));
+	prt_newline(out);
 }
 
 void bch2_btree_key_cache_exit(void)