Re: [PATCH] locking/percpu-rwsem: Trigger contention tracepoints only if contended

From: Namhyung Kim
Date: Mon Nov 20 2023 - 15:28:43 EST


Ping!

On Wed, Nov 8, 2023 at 1:53 PM Namhyung Kim <namhyung@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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
>