[RFC PATCH 2/4] perf/core: Optimize a bit rotate_ctx()

[Frederic Weisbecker]

Don't use list_for_each_entry() just to swap the first and
last entry in the list.

Instead, use a direct list->next derefencing. This saves up a
useless prefetch.

Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 kernel/perf_event.c |   16 +++++++++-------
 1 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index b3a31c8..0432c1c 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -1432,7 +1432,7 @@ static void perf_ctx_adjust_freq(struct perf_event_context *ctx)
  */
 static void rotate_ctx(struct perf_event_context *ctx)
 {
-	struct perf_event *event;
+	struct list_head *list;
 
 	if (!ctx->nr_events)
 		return;
@@ -1442,15 +1442,17 @@ static void rotate_ctx(struct perf_event_context *ctx)
 	 * Rotate the first entry last (works just fine for group events too):
 	 */
 	perf_disable();
-	list_for_each_entry(event, &ctx->pinned_grp_list, group_entry) {
-		list_move_tail(&event->group_entry, &ctx->pinned_grp_list);
-		break;
+
+	if (!list_empty(&ctx->pinned_grp_list)) {
+		list = ctx->pinned_grp_list.next;
+		list_move_tail(list, &ctx->pinned_grp_list);
 	}
 
-	list_for_each_entry(event, &ctx->volatile_grp_list, group_entry) {
-		list_move_tail(&event->group_entry, &ctx->volatile_grp_list);
-		break;
+	if (!list_empty(&ctx->volatile_grp_list)) {
+		list = ctx->volatile_grp_list.next;
+		list_move_tail(list, &ctx->volatile_grp_list);
 	}
+
 	perf_enable();
 
 	spin_unlock(&ctx->lock);
-- 
1.6.2.3

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