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Date:   Tue, 03 Oct 2023 19:19:05 -0000
From:   "tip-bot2 for Uros Bizjak" <>
Cc:     Uros Bizjak <>, Ingo Molnar <>,
        Peter Zijlstra <>,
        Linus Torvalds <>,
        "H. Peter Anvin" <>,,
Subject: [tip: perf/core] perf/x86/rapl: Use local64_try_cmpxchg in

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     bcc6ec3d954bbcc8bec34a21c05ea536a2e96d6f
Author:        Uros Bizjak <>
AuthorDate:    Mon, 07 Aug 2023 16:51:15 +02:00
Committer:     Ingo Molnar <>
CommitterDate: Tue, 03 Oct 2023 21:13:45 +02:00

perf/x86/rapl: Use local64_try_cmpxchg in rapl_event_update()

Use local64_try_cmpxchg() instead of local64_cmpxchg(*ptr, old, new) == old.

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: Ingo Molnar <>
Cc: Peter Zijlstra <>
Cc: Linus Torvalds <>
Cc: "H. Peter Anvin" <>
 arch/x86/events/rapl.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/x86/events/rapl.c b/arch/x86/events/rapl.c
index e8f53b2..6d3e738 100644
--- a/arch/x86/events/rapl.c
+++ b/arch/x86/events/rapl.c
@@ -179,13 +179,11 @@ static u64 rapl_event_update(struct perf_event *event)
 	s64 delta, sdelta;
 	int shift = RAPL_CNTR_WIDTH;
 	prev_raw_count = local64_read(&hwc->prev_count);
-	rdmsrl(event->hw.event_base, new_raw_count);
-	if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
-			    new_raw_count) != prev_raw_count)
-		goto again;
+	do {
+		rdmsrl(event->hw.event_base, new_raw_count);
+	} while (!local64_try_cmpxchg(&hwc->prev_count,
+				      &prev_raw_count, new_raw_count));
 	 * Now we have the new raw value and have updated the prev

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