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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Mon, 20 Nov 2023 12:28:22 -0800
Message-ID: <CAM9d7chyJun57UV-6qRzgTzDEmUu5Z0mStgjRbrg2dcjUkMQzw@mail.gmail.com>
Subject: Re: [PATCH] locking/percpu-rwsem: Trigger contention tracepoints only
 if contended
To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>
Cc:     Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>
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Ping!

On Wed, Nov 8, 2023 at 1:53 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> It mistakenly fires lock contention tracepoints always in the writer path.
> It should be conditional on the try lock result.
>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  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
>