[PATCH v2 4/4] lib: list_sort.c: Limit number of unused cmp callbacks

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

The helper merge_and_restore_back_links() makes sure to call the
caller's cmp function during the final ->prev pointer fixup, so that
the cmp function may call cond_resched(). However, if the cmp function
does not call cond_resched() at all, this is entirely redundant. If it
does, doing at least two function calls for every two pointer
assignments is a bit excessive. This patch limits the calls to once
for every 256 iterations.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 lib/list_sort.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/list_sort.c b/lib/list_sort.c
index a34c78c..6b9fdaf 100644
--- a/lib/list_sort.c
+++ b/lib/list_sort.c
@@ -47,6 +47,7 @@ static void merge_and_restore_back_links(void *priv,
 				struct list_head *a, struct list_head *b)
 {
 	struct list_head *tail = head;
+	u8 count = 0;
 
 	while (a && b) {
 		/* if equal, take 'a' -- important for sort stability */
@@ -70,7 +71,8 @@ static void merge_and_restore_back_links(void *priv,
 		 * element comparison is needed, so the client's cmp()
 		 * routine can invoke cond_resched() periodically.
 		 */
-		(*cmp)(priv, tail->next, tail->next);
+		if (unlikely(!(++count)))
+			(*cmp)(priv, tail->next, tail->next);
 
 		tail->next->prev = tail;
 		tail = tail->next;
-- 
1.9.2