From: Uros Bizjak <ubizjak@gmail.com>
Date: Mon, 7 Aug 2023 14:51:14 +0000 (+0200)
Subject: perf/x86/rapl: Stop doing cpu_relax() in the local64_cmpxchg() loop in rapl_event_upd... 
X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=1ce19bf90bd55bf54f9ed75d594029db63d395b0;p=linux.git

perf/x86/rapl: Stop doing cpu_relax() in the local64_cmpxchg() loop in rapl_event_update()

According to the following commit:

   f5fe24ef17b5 ("lockref: stop doing cpu_relax in the cmpxchg loop")

   "On the x86-64 architecture even a failing cmpxchg grants exclusive
    access to the cacheline, making it preferable to retry the failed op
    immediately instead of stalling with the pause instruction."

Based on the above observation, remove cpu_relax() from the
local64_cmpxchg() loop of rapl_event_update().

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20230807145134.3176-1-ubizjak@gmail.com
---

diff --git a/arch/x86/events/rapl.c b/arch/x86/events/rapl.c
index 1579429846ccf..e8f53b2590a55 100644
--- a/arch/x86/events/rapl.c
+++ b/arch/x86/events/rapl.c
@@ -184,10 +184,8 @@ again:
 	rdmsrl(event->hw.event_base, new_raw_count);
 
 	if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
-			    new_raw_count) != prev_raw_count) {
-		cpu_relax();
+			    new_raw_count) != prev_raw_count)
 		goto again;
-	}
 
 	/*
 	 * Now we have the new raw value and have updated the prev