maple_tree: add benchmarking for mas_prev()

Add some benchmarking functions in testing for mas_prev().  This is
useful to ensure there are no regressions added during modifications.

Link: https://lkml.kernel.org/r/20230724183157.3939892-3-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Peng Zhang <zhangpeng.00@bytedance.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/test_maple_tree.c

@@ -45,6 +45,7 @@ atomic_t maple_tree_tests_passed;
 /* #define BENCH_MT_FOR_EACH */
 /* #define BENCH_FORK */
 /* #define BENCH_MAS_FOR_EACH */
+/* #define BENCH_MAS_PREV */
 
 #ifdef __KERNEL__
 #define mt_set_non_kernel(x)		do {} while (0)
@@ -1801,7 +1802,36 @@ static noinline void __init bench_mas_for_each(struct maple_tree *mt)
 
 }
 #endif
+#if defined(BENCH_MAS_PREV)
+static noinline void __init bench_mas_prev(struct maple_tree *mt)
+{
+	int i, count = 1000000;
+	unsigned long max = 2500;
+	void *entry;
+	MA_STATE(mas, mt, 0, 0);
+
+	for (i = 0; i < max; i += 5) {
+		int gap = 4;
+
+		if (i % 30 == 0)
+			gap = 3;
+		mtree_store_range(mt, i, i + gap, xa_mk_value(i), GFP_KERNEL);
+	}
+
+	rcu_read_lock();
+	for (i = 0; i < count; i++) {
+		unsigned long j = 2495;
+
+		mas_set(&mas, ULONG_MAX);
+		while ((entry = mas_prev(&mas, 0)) != NULL) {
+			MT_BUG_ON(mt, entry != xa_mk_value(j));
+			j -= 5;
+		}
+	}
+	rcu_read_unlock();
 
+}
+#endif
 /* check_forking - simulate the kernel forking sequence with the tree. */
 static noinline void __init check_forking(struct maple_tree *mt)
 {
@@ -3537,6 +3567,13 @@ static int __init maple_tree_seed(void)
 	mtree_destroy(&tree);
 	goto skip;
 #endif
+#if defined(BENCH_MAS_PREV)
+#define BENCH
+	mt_init_flags(&tree, MT_FLAGS_ALLOC_RANGE);
+	bench_mas_prev(&tree);
+	mtree_destroy(&tree);
+	goto skip;
+#endif
 
 	mt_init_flags(&tree, MT_FLAGS_ALLOC_RANGE);
 	check_iteration(&tree);