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

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Date: Tue, 27 Feb 2024 15:02:57 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, 
	Will Deacon <will@...nel.org>
Cc: Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] locking/percpu-rwsem: Trigger contention tracepoints only
 if contended

Hello,

On Mon, Nov 20, 2023 at 12:28 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Ping!
>
> On Wed, Nov 8, 2023 at 1:53 PM Namhyung Kim <namhyung@...nel.org> 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 <namhyung@...nel.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
> >