[PATCH] lib/plist.c: Avoid worst case scenario in plist_add

[I Hsin Cheng]

Worst case scenario of plist_add() happens when the priority of the
inserted plist_node is going to be the largest after the insertion is
done. The cost is going to be more significant when the original plist
is longer, because the iterator is going to traverse the whole plist to
find the correct position to insert the new node.

The situation can be avoided by using a reverse iterator at the same
time, doing so the maximum possible number of iteration is going to
shrink from N to N/2.

The proposed change of plist_add pasts the test in lib/plist.c to
validate its correctness, also add the worst case scenario test for
plist_add() in plist_test().

The worst case test are tested with the size of test_data and test_node
growing from 200 to 1000. The result are showned in the following table,
in which we can observed that the proposed change of plist_add performs
better than the original version, and the difference between these two
implementations are more significant with the size of N growing.

The random case test [1], and best case test [2] are also provided, with
result showing the proposed change performs slightly better in random
case test while the original implementation performs slightly better in
best case test, while the difference in both test are minor, we can see
them as even in those two situations.

 -----------------------------------------------------------
| Test size      |   200 |   400 |    600 |    800 |   1000 |
 -----------------------------------------------------------
| new_plist_add  | 140911| 548681| 1220512| 2048493| 3763755|
 -----------------------------------------------------------
| old_plist_add  | 188198| 774222| 1643547| 3008929| 4947435|
 -----------------------------------------------------------

Signed-off-by: I Hsin Cheng <richard120310@xxxxxxxxx>
---
[1]:
The random cases are created via the following code
test_data[i] = get_random_u64();

[2]:
The best cases are created via the following code
test_data[i] = (ARRAY_SIZE(test_data) - i);
---
 lib/plist.c | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/lib/plist.c b/lib/plist.c
index 0d86ed7a7..06cf2169b 100644
--- a/lib/plist.c
+++ b/lib/plist.c
@@ -72,7 +72,7 @@ static void plist_check_head(struct plist_head *head)
  */
 void plist_add(struct plist_node *node, struct plist_head *head)
 {
-	struct plist_node *first, *iter, *prev = NULL;
+	struct plist_node *first, *iter, *prev = NULL, *last, *reverse_iter;
 	struct list_head *node_next = &head->node_list;
 
 	plist_check_head(head);
@@ -83,16 +83,26 @@ void plist_add(struct plist_node *node, struct plist_head *head)
 		goto ins_node;
 
 	first = iter = plist_first(head);
+	last = reverse_iter = list_entry(first->prio_list.prev, struct plist_node, prio_list);
 
 	do {
 		if (node->prio < iter->prio) {
 			node_next = &iter->node_list;
 			break;
+		} else if (node->prio >= reverse_iter->prio) {
+			prev = reverse_iter;
+			iter = list_entry(reverse_iter->prio_list.next,
+				struct plist_node, prio_list);
+			if (likely(reverse_iter != last))
+				node_next = &iter->node_list;
+			break;
 		}
 
 		prev = iter;
 		iter = list_entry(iter->prio_list.next,
 				struct plist_node, prio_list);
+		reverse_iter = list_entry(reverse_iter->prio_list.prev,
+				struct plist_node, prio_list);
 	} while (iter != first);
 
 	if (!prev || prev->prio != node->prio)
@@ -255,6 +265,32 @@ static int  __init plist_test(void)
 	}
 
 	printk(KERN_DEBUG "end plist test\n");
+
+	/* Worst case test for plist_add() */
+	unsigned int test_data[241];
+
+	for (i = 0; i < ARRAY_SIZE(test_data); i++)
+		test_data[i] = i;
+
+	ktime_t start, end, time_elapsed = 0;
+
+	plist_head_init(&test_head);
+
+	for (i = 0; i < ARRAY_SIZE(test_node); i++) {
+		plist_node_init(test_node + i, 0);
+		test_node[i].prio = test_data[i];
+	}
+
+	for (i = 0; i < ARRAY_SIZE(test_node); i++) {
+		if (plist_node_empty(test_node + i)) {
+			start = ktime_get();
+			plist_add(test_node + i, &test_head);
+			end = ktime_get();
+			time_elapsed += (end - start);
+		}
+	}
+
+	pr_debug("plist_add worst case test time elapsed %lld\n", time_elapsed);
 	return 0;
 }
 
-- 
2.34.1