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FromFrederic Weisbecker <>
Subject[RFC PATCH 2/4] perf/core: Optimize a bit rotate_ctx()
DateSun, 8 Nov 2009 21:13:24 +0100
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@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
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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