linux-kernel - Re: [PATCH] perf/x86/amd: Use try_cmpxchg() in events/amd/{un,}core.c

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Date: Sat, 18 May 2024 11:15:38 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...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>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] perf/x86/amd: Use try_cmpxchg() in events/amd/{un,}core.c


* Uros Bizjak <ubizjak@...il.com> wrote:

> Replace this pattern in events/amd/{un,}core.c:
> 
>     cmpxchg(*ptr, old, new) == old
> 
> ... with the simpler and faster:
> 
>     try_cmpxchg(*ptr, &old, new)
> 
> The x86 CMPXCHG instruction returns success in the ZF flag, so this change
> saves a compare after the CMPXCHG.
> 
> No functional change intended.
> 
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> Cc: Jiri Olsa <jolsa@...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>
> ---
>  arch/x86/events/amd/core.c   | 3 ++-
>  arch/x86/events/amd/uncore.c | 6 ++++--
>  2 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
> index 1fc4ce44e743..c1b8aeff7bc0 100644
> --- a/arch/x86/events/amd/core.c
> +++ b/arch/x86/events/amd/core.c
> @@ -433,7 +433,8 @@ static void __amd_put_nb_event_constraints(struct cpu_hw_events *cpuc,
>  	 * when we come here
>  	 */
>  	for (i = 0; i < x86_pmu.num_counters; i++) {
> -		if (cmpxchg(nb->owners + i, event, NULL) == event)
> +		struct perf_event *tmp = event;
> +		if (try_cmpxchg(nb->owners + i, &tmp, NULL))
>  			break;
>  	}
>  }
> diff --git a/arch/x86/events/amd/uncore.c b/arch/x86/events/amd/uncore.c
> index 4ccb8fa483e6..cac7c20d1aee 100644
> --- a/arch/x86/events/amd/uncore.c
> +++ b/arch/x86/events/amd/uncore.c
> @@ -162,7 +162,8 @@ static int amd_uncore_add(struct perf_event *event, int flags)
>  	/* if not, take the first available counter */
>  	hwc->idx = -1;
>  	for (i = 0; i < pmu->num_counters; i++) {
> -		if (cmpxchg(&ctx->events[i], NULL, event) == NULL) {
> +		struct perf_event *tmp = NULL;
> +		if (try_cmpxchg(&ctx->events[i], &tmp, event)) {
>  			hwc->idx = i;
>  			break;
>  		}
> @@ -196,7 +197,8 @@ static void amd_uncore_del(struct perf_event *event, int flags)
>  	event->pmu->stop(event, PERF_EF_UPDATE);
>  
>  	for (i = 0; i < pmu->num_counters; i++) {
> -		if (cmpxchg(&ctx->events[i], event, NULL) == event)
> +		struct perf_event *tmp = event;
> +		if (try_cmpxchg(&ctx->events[i], &tmp, NULL))
>  			break;
>  	}

Note that's a newline missing between local variable declaration block and 
the first statement after that. I've added all 3 to this patch (see below), 
but please keep an eye on this in future patches.

Thanks,

	Ingo

================>
 arch/x86/events/amd/core.c   | 1 +
 arch/x86/events/amd/uncore.c | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
index c1b8aeff7bc0..18bfe3451f3a 100644
--- a/arch/x86/events/amd/core.c
+++ b/arch/x86/events/amd/core.c
@@ -434,6 +434,7 @@ static void __amd_put_nb_event_constraints(struct cpu_hw_events *cpuc,
 	 */
 	for (i = 0; i < x86_pmu.num_counters; i++) {
 		struct perf_event *tmp = event;
+
 		if (try_cmpxchg(nb->owners + i, &tmp, NULL))
 			break;
 	}
diff --git a/arch/x86/events/amd/uncore.c b/arch/x86/events/amd/uncore.c
index cac7c20d1aee..0fafe233bba4 100644
--- a/arch/x86/events/amd/uncore.c
+++ b/arch/x86/events/amd/uncore.c
@@ -163,6 +163,7 @@ static int amd_uncore_add(struct perf_event *event, int flags)
 	hwc->idx = -1;
 	for (i = 0; i < pmu->num_counters; i++) {
 		struct perf_event *tmp = NULL;
+
 		if (try_cmpxchg(&ctx->events[i], &tmp, event)) {
 			hwc->idx = i;
 			break;
@@ -198,6 +199,7 @@ static void amd_uncore_del(struct perf_event *event, int flags)
 
 	for (i = 0; i < pmu->num_counters; i++) {
 		struct perf_event *tmp = event;
+
 		if (try_cmpxchg(&ctx->events[i], &tmp, NULL))
 			break;
 	}