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Message-Id: <20230305205628.27385-11-ubizjak@gmail.com>
Date: Sun, 5 Mar 2023 21:56:28 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org,
linux-perf-users@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Ian Rogers <irogers@...gle.com>
Subject: [PATCH 10/10] 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.
Also, local_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.
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Ian Rogers <irogers@...gle.com>
Signed-off-by: Uros Bizjak <ubizjak@...il.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 273a0fe7910a..e07c10f4d141 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;
--
2.39.2
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