Commit bcc6ec3d authored by Uros Bizjak's avatar Uros Bizjak Committed by Ingo Molnar
Browse files

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: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarIngo 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
parent 1ce19bf9

arch/x86/events/rapl.c

+4 −6
Original line number Diff line number Diff line
@@ -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);

	do {

		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;

	} 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