From bcc6ec3d954bbcc8bec34a21c05ea536a2e96d6f Mon Sep 17 00:00:00 2001 From: Uros Bizjak Date: Mon, 7 Aug 2023 16:51:15 +0200 Subject: [PATCH] perf/x86/rapl: Use local64_try_cmpxchg in rapl_event_update() 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. Signed-off-by: Uros Bizjak Signed-off-by: Ingo Molnar Cc: Peter Zijlstra Cc: Linus Torvalds Cc: "H. Peter Anvin" Link: https://lore.kernel.org/r/20230807145134.3176-2-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 e8f53b2590a5537..6d3e738486437c8 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