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Subject[PATCH] perf/core: Use local64_try_cmpxchg in perf_swevent_set_period
Date
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.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
kernel/events/core.c | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 78ae7b6f90fd..f84e2640ea2f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -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);
- if (val < 0)
- return 0;
+ 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;
+ nr = div64_u64(period + val, period);
+ offset = nr * period;
+ val -= offset;
+ } while (!local64_try_cmpxchg(&hwc->period_left, &old, val));

return nr;
}
--
2.41.0
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