From 1af61adb3a23192023fec1733bd4c8500f53e546 Mon Sep 17 00:00:00 2001 From: Uros Bizjak Date: Sat, 8 Jul 2023 11:00:37 +0200 Subject: [PATCH] perf/ring_buffer: Use local_try_cmpxchg in __perf_output_begin Use local_try_cmpxchg instead of local_cmpxchg (*ptr, old, new) == old in __perf_output_begin. x86 CMPXCHG instruction returns success in ZF flag, so this change saves a compare after cmpxchg (and related move instruction in front of cmpxchg). Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg fails. There is no need to re-read the value in the loop. No functional change intended. Signed-off-by: Uros Bizjak Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20230708090048.63046-2-ubizjak@gmail.com --- kernel/events/ring_buffer.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index a0433f3..fb1e180 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -191,9 +191,10 @@ __perf_output_begin(struct perf_output_handle *handle, perf_output_get_handle(handle); + offset = local_read(&rb->head); do { + head = offset; tail = READ_ONCE(rb->user_page->data_tail); - offset = head = local_read(&rb->head); if (!rb->overwrite) { if (unlikely(!ring_buffer_has_space(head, tail, perf_data_size(rb), @@ -217,7 +218,7 @@ __perf_output_begin(struct perf_output_handle *handle, head += size; else head -= size; - } while (local_cmpxchg(&rb->head, offset, head) != offset); + } while (!local_try_cmpxchg(&rb->head, &offset, head)); if (backward) { offset = head; -- 2.7.4