Messages in this thread Patch in this message | | | From | Uros Bizjak <> | Subject | [PATCH 2/2] perf/x86/rapl: Use local64_try_cmpxchg in rapl_event_update | Date | Mon, 7 Aug 2023 16:51:15 +0200 |
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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: 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> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Borislav Petkov <bp@alien8.de> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc. "H. Peter Anvin" <hpa@zytor.com> Signed-off-by: Uros Bizjak <ubizjak@gmail.com> --- arch/x86/events/rapl.c | 10 ++++------ 1 file changed, 4 insertions(+), 6 deletions(-)
diff --git a/arch/x86/events/rapl.c b/arch/x86/events/rapl.c index e8e26733e17b..7c2376dae79d 100644 --- a/arch/x86/events/rapl.c +++ b/arch/x86/events/rapl.c @@ -179,13 +179,11 @@ static u64 rapl_event_update(struct perf_event *event) s64 delta, sdelta; int shift = RAPL_CNTR_WIDTH; -again: prev_raw_count = local64_read(&hwc->prev_count); - rdmsrl(event->hw.event_base, new_raw_count); - - if (local64_cmpxchg(&hwc->prev_count, prev_raw_count, - new_raw_count) != prev_raw_count) - goto again; + do { + rdmsrl(event->hw.event_base, new_raw_count); + } 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 -- 2.41.0
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