linux-kernel - [PATCH 1/2] perf/x86/rapl: Stop doing cpu

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Date:   Mon,  7 Aug 2023 16:51:14 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: [PATCH 1/2] perf/x86/rapl: Stop doing cpu_relax in the cmpxchg loop

According to the 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
cmpxchg loop of rapl_event_update.

Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Ian Rogers <irogers@...gle.com>
Cc: Adrian Hunter <adrian.hunter@...el.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc. "H. Peter Anvin" <hpa@...or.com>
Signed-off-by: Uros Bizjak <ubizjak@...il.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 52e6e7ed4f78..e8e26733e17b 100644
--- a/arch/x86/events/rapl.c
+++ b/arch/x86/events/rapl.c
@@ -184,10 +184,8 @@ static u64 rapl_event_update(struct perf_event *event)
 	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
-- 
2.41.0