linux-kernel - [tip: perf/core] perf/x86/rapl: Stop doing cpu_relax() in the local64_cmpxchg() loop in rapl_event

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Date:   Tue, 03 Oct 2023 07:45:55 -0000
From:   "tip-bot2 for Uros Bizjak" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: perf/core] perf/x86/rapl: Stop doing cpu_relax() in the
 local64_cmpxchg() loop in rapl_event_update()

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     df22fb4bcdd6f67c4f568e6321c9b0050819d213
Gitweb:        https://git.kernel.org/tip/df22fb4bcdd6f67c4f568e6321c9b0050819d213
Author:        Uros Bizjak <ubizjak@...il.com>
AuthorDate:    Mon, 07 Aug 2023 16:51:14 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 03 Oct 2023 09:36:17 +02:00

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@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: https://lore.kernel.org/r/20230807145134.3176-1-ubizjak@gmail.com

Cc. "H. Peter Anvin" <hpa@...or.com>
---
 arch/x86/events/rapl.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/arch/x86/events/rapl.c b/arch/x86/events/rapl.c
index 1579429..e8f53b2 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