Messages in this thread Patch in this message | | | From | Uros Bizjak <> | Subject | [PATCH] perf/core: Use local64_try_cmpxchg in perf_swevent_set_period | Date | Sat, 8 Jul 2023 10:10:57 +0200 |
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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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