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[31.46.241.174]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a5a1789247csm1199183466b.82.2024.05.18.02.15.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 18 May 2024 02:15:40 -0700 (PDT) Sender: Ingo Molnar Date: Sat, 18 May 2024 11:15:38 +0200 From: Ingo Molnar To: Uros Bizjak Cc: x86@kernel.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, Peter Zijlstra , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Thomas Gleixner , Borislav Petkov , Dave Hansen , "H. Peter Anvin" Subject: Re: [PATCH] perf/x86/amd: Use try_cmpxchg() in events/amd/{un,}core.c Message-ID: References: <20240425101708.5025-1-ubizjak@gmail.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240425101708.5025-1-ubizjak@gmail.com> * Uros Bizjak 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 > Cc: Peter Zijlstra > Cc: Ingo Molnar > Cc: Arnaldo Carvalho de Melo > Cc: Namhyung Kim > Cc: Mark Rutland > Cc: Alexander Shishkin > Cc: Jiri Olsa > Cc: Ian Rogers > Cc: Adrian Hunter > Cc: Thomas Gleixner > Cc: Borislav Petkov > Cc: Dave Hansen > Cc: "H. Peter Anvin" > --- > 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; }