The following commit has been merged into the perf/core branch of tip:
Commit-ID: bcc6ec3d954bbcc8bec34a21c05ea536a2e96d6f
Gitweb: https://git.kernel.org/tip/bcc6ec3d954bbcc8bec34a21c05ea536a2e96d6f
Author: Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate: Mon, 07 Aug 2023 16:51:15 +02:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
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 <ubizjak@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Link: https://lore.kernel.org/r/20230807145134.3176-2-ubizjak@xxxxxxxxx
---
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;
-again:
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
|