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

[Ingo Molnar <mingo@kernel.org>]

* Waiman Long <longman@redhat.com> wrote:

> 
> On 2/27/24 18:02, Namhyung Kim wrote:
> > Hello,
> > 
> > On Mon, Nov 20, 2023 at 12:28 PM Namhyung Kim <namhyung@kernel.org> wrote:
> > > 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.
> > 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@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
> 
> Yes, that makes sense. Sorry for missing this patch.
> 
> Reviewed-by: Waiman Long <longman@redhat.com>

Applied to tip:locking/core, thanks guys!

	Ingo