Commit 4c1c9dea authored by Uros Bizjak's avatar Uros Bizjak Committed by Peter Zijlstra
Browse files

perf/x86: Use local64_try_cmpxchg 



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.

Cc. "H. Peter Anvin" <hpa@zytor.com>
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/20230706141720.2672-1-ubizjak@gmail.com
parent 7c212823

arch/x86/events/amd/ibs.c

+2 −2
Original line number Diff line number Diff line
@@ -156,8 +156,8 @@ perf_event_try_update(struct perf_event *event, u64 new_raw_count, int width) 
	 * count to the generic event atomically:

	 */

	prev_raw_count = local64_read(&hwc->prev_count);

	if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,

					new_raw_count) != prev_raw_count)

	if (!local64_try_cmpxchg(&hwc->prev_count,

				 &prev_raw_count, new_raw_count))

		return 0;



	/*

arch/x86/events/core.c

+4 −6
Original line number Diff line number Diff line
@@ -129,13 +129,11 @@ u64 x86_perf_event_update(struct perf_event *event) 
	 * exchange a new raw count - then add that new-prev delta

	 * count to the generic event atomically:

	 */

again:

	prev_raw_count = local64_read(&hwc->prev_count);

	do {

		rdpmcl(hwc->event_base_rdpmc, 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

arch/x86/events/intel/cstate.c

+4 −6
Original line number Diff line number Diff line
@@ -365,13 +365,11 @@ static void cstate_pmu_event_update(struct perf_event *event) 
	struct hw_perf_event *hwc = &event->hw;

	u64 prev_raw_count, new_raw_count;



again:

	prev_raw_count = local64_read(&hwc->prev_count);

	do {

		new_raw_count = cstate_pmu_read_counter(event);



	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));



	local64_add(new_raw_count - prev_raw_count, &event->count);

}

arch/x86/events/msr.c

+3 −5
Original line number Diff line number Diff line
@@ -244,12 +244,10 @@ static void msr_event_update(struct perf_event *event) 
	s64 delta;



	/* Careful, an NMI might modify the previous event value: */

again:

	prev = local64_read(&event->hw.prev_count);

	do {

		now = msr_read_counter(event);



	if (local64_cmpxchg(&event->hw.prev_count, prev, now) != prev)

		goto again;

	} while (!local64_try_cmpxchg(&event->hw.prev_count, &prev, now));



	delta = now - prev;

	if (unlikely(event->hw.event_base == MSR_SMI_COUNT)) {