linux-kernel - Re: [PATCH 2/2] locking: Apply contention tracepoints in the slow path

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Date:   Fri, 18 Mar 2022 14:43:16 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Byungchul Park <byungchul.park@....com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnd Bergmann <arnd@...db.de>,
        Radoslaw Burny <rburny@...gle.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH 2/2] locking: Apply contention tracepoints in the slow path

On Thu, Mar 17, 2022 at 11:19 AM Hyeonggon Yoo <42.hyeyoo@...il.com> wrote:
>
> On Wed, Mar 16, 2022 at 03:45:48PM -0700, Namhyung Kim wrote:
> > Adding the lock contention tracepoints in various lock function slow
> > paths.  Note that each arch can define spinlock differently, I only
> > added it only to the generic qspinlock for now.
> >
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> >  kernel/locking/mutex.c        |  3 +++
> >  kernel/locking/percpu-rwsem.c |  3 +++
> >  kernel/locking/qrwlock.c      |  9 +++++++++
> >  kernel/locking/qspinlock.c    |  5 +++++
> >  kernel/locking/rtmutex.c      | 11 +++++++++++
> >  kernel/locking/rwbase_rt.c    |  3 +++
> >  kernel/locking/rwsem.c        |  9 +++++++++
> >  kernel/locking/semaphore.c    | 14 +++++++++++++-
> >  8 files changed, 56 insertions(+), 1 deletion(-)
> >
>
> [...]
>
> > diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
> > index 9ee381e4d2a4..e3c19668dfee 100644
> > --- a/kernel/locking/semaphore.c
> > +++ b/kernel/locking/semaphore.c
> > @@ -32,6 +32,7 @@
> >  #include <linux/semaphore.h>
> >  #include <linux/spinlock.h>
> >  #include <linux/ftrace.h>
> > +#include <trace/events/lock.h>
> >
> >  static noinline void __down(struct semaphore *sem);
> >  static noinline int __down_interruptible(struct semaphore *sem);
> > @@ -209,6 +210,7 @@ static inline int __sched __down_common(struct semaphore *sem, long state,
> >                                                               long timeout)
> >  {
> >       struct semaphore_waiter waiter;
> > +     bool tracing = false;
> >
> >       list_add_tail(&waiter.list, &sem->wait_list);
> >       waiter.task = current;
> > @@ -220,18 +222,28 @@ static inline int __sched __down_common(struct semaphore *sem, long state,
> >               if (unlikely(timeout <= 0))
> >                       goto timed_out;
> >               __set_current_state(state);
> > +             if (!tracing) {
> > +                     trace_contention_begin(sem, 0);
> > +                     tracing = true;
> > +             }
> >               raw_spin_unlock_irq(&sem->lock);
> >               timeout = schedule_timeout(timeout);
> >               raw_spin_lock_irq(&sem->lock);
> > -             if (waiter.up)
> > +             if (waiter.up) {
> > +                     trace_contention_end(sem, 0);
> >                       return 0;
> > +             }
> >       }
> >
> >   timed_out:
> > +     if (tracing)
> > +             trace_contention_end(sem, -ETIME);
> >       list_del(&waiter.list);
> >       return -ETIME;
> >
> >   interrupted:
> > +     if (tracing)
> > +             trace_contention_end(sem, -EINTR);
> >       list_del(&waiter.list);
> >       return -EINTR;
> >  }
>
> why not simply remove tracing variable and call trace_contention_begin()
> earlier like in rwsem? we can ignore it if ret != 0.

Right, will change.  But we should not ignore the return value.

Thanks,
Namhyung