Commit 28fd85a1 authored by Uros Bizjak's avatar Uros Bizjak Committed by Peter Zijlstra
Browse files

perf/core: Use local64_try_cmpxchg in perf_swevent_set_period 



Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old
in perf_swevent_set_period.  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 avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230708081129.45915-1-ubizjak@gmail.com
parent 4c1c9dea

kernel/events/core.c

+9 −9
Original line number Diff line number Diff line
@@ -9595,16 +9595,16 @@ u64 perf_swevent_set_period(struct perf_event *event) 


	hwc->last_period = hwc->sample_period;



again:

	old = val = local64_read(&hwc->period_left);

	old = local64_read(&hwc->period_left);

	do {

		val = old;

		if (val < 0)

			return 0;



		nr = div64_u64(period + val, period);

		offset = nr * period;

		val -= offset;

	if (local64_cmpxchg(&hwc->period_left, old, val) != old)

		goto again;

	} while (!local64_try_cmpxchg(&hwc->period_left, &old, val));



	return nr;

}