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perf/core: Use local64_try_cmpxchg in perf_swevent_set_period

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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.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230708081129.45915-1-ubizjak@gmail.com
This commit is contained in:
Uros Bizjak 2023-07-08 10:10:57 +02:00 committed by Peter Zijlstra
parent 4c1c9dea20
commit 28fd85a10a
1 changed files with 9 additions and 9 deletions
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18
kernel/events/core.c
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@ -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;
}