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[31.46.240.84]) by smtp.gmail.com with ESMTPSA id cc13-20020a5d5c0d000000b0033db0bbc2ccsm15094458wrb.3.2024.02.28.04.50.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Feb 2024 04:50:22 -0800 (PST) Sender: Ingo Molnar Date: Wed, 28 Feb 2024 13:50:20 +0100 From: Ingo Molnar To: Waiman Long Cc: Namhyung Kim , Peter Zijlstra , Will Deacon , Boqun Feng , LKML Subject: Re: [PATCH] locking/percpu-rwsem: Trigger contention tracepoints only if contended Message-ID: References: <20231108215322.2845536-1-namhyung@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: * Waiman Long wrote: > > On 2/27/24 18:02, Namhyung Kim wrote: > > Hello, > > > > On Mon, Nov 20, 2023 at 12:28 PM Namhyung Kim wrote: > > > Ping! > > > > > > On Wed, Nov 8, 2023 at 1:53 PM Namhyung Kim wrote: > > > > It mistakenly fires lock contention tracepoints always in the writer path. > > > > It should be conditional on the try lock result. > > Can anybody take a look at this? This makes a large noise > > in the lock contention result. > > > > Thanks, > > Namhyung > > > > > > Signed-off-by: Namhyung Kim > > > > --- > > > > kernel/locking/percpu-rwsem.c | 11 ++++++++--- > > > > 1 file changed, 8 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c > > > > index 185bd1c906b0..6083883c4fe0 100644 > > > > --- a/kernel/locking/percpu-rwsem.c > > > > +++ b/kernel/locking/percpu-rwsem.c > > > > @@ -223,9 +223,10 @@ static bool readers_active_check(struct percpu_rw_semaphore *sem) > > > > > > > > void __sched percpu_down_write(struct percpu_rw_semaphore *sem) > > > > { > > > > + bool contended = false; > > > > + > > > > might_sleep(); > > > > rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_); > > > > - trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE); > > > > > > > > /* Notify readers to take the slow path. */ > > > > rcu_sync_enter(&sem->rss); > > > > @@ -234,8 +235,11 @@ void __sched percpu_down_write(struct percpu_rw_semaphore *sem) > > > > * Try set sem->block; this provides writer-writer exclusion. > > > > * Having sem->block set makes new readers block. > > > > */ > > > > - if (!__percpu_down_write_trylock(sem)) > > > > + if (!__percpu_down_write_trylock(sem)) { > > > > + trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE); > > > > percpu_rwsem_wait(sem, /* .reader = */ false); > > > > + contended = true; > > > > + } > > > > > > > > /* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */ > > > > > > > > @@ -247,7 +251,8 @@ void __sched percpu_down_write(struct percpu_rw_semaphore *sem) > > > > > > > > /* Wait for all active readers to complete. */ > > > > rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE); > > > > - trace_contention_end(sem, 0); > > > > + if (contended) > > > > + trace_contention_end(sem, 0); > > > > } > > > > EXPORT_SYMBOL_GPL(percpu_down_write); > > > > > > > > -- > > > > 2.42.0.869.gea05f2083d-goog > > Yes, that makes sense. Sorry for missing this patch. > > Reviewed-by: Waiman Long Applied to tip:locking/core, thanks guys! Ingo