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

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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@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20230807145134.3176-2-ubizjak@gmail.com
This commit is contained in:
Uros Bizjak 2023-08-07 16:51:15 +02:00 committed by Ingo Molnar
parent 1ce19bf90b
commit bcc6ec3d95
1 changed files with 4 additions and 6 deletions
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10
arch/x86/events/rapl.c
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@ -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