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References: <20220322185709.141236-1-namhyung@kernel.org> <20220322185709.141236-3-namhyung@kernel.org>
 <20220328112921.GZ8939@worktop.programming.kicks-ass.net>
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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Mon, 28 Mar 2022 10:41:31 -0700
Message-ID: <CAM9d7cgaTdswDHKA364BN9Me-_LBxMfuZ70O=Y3_sZf3GNfgLw@mail.gmail.com>
Subject: Re: [PATCH 2/2] locking: Apply contention tracepoints in the slow path
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Byungchul Park <byungchul.park@lge.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Radoslaw Burny <rburny@google.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, Hyeonggon Yoo <42.hyeyoo@gmail.com>
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Hi Peter,

On Mon, Mar 28, 2022 at 4:29 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Mar 22, 2022 at 11:57:09AM -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.
> >
> > Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> > Signed-off-by: Namhyung Kim <namhyung@kernel.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    | 15 ++++++++++++++-
> >  8 files changed, 57 insertions(+), 1 deletion(-)
>
> I had conflicts in rwsem.c due to Waiman's patches, but that was simple
> enough to resolve. However, I had a good look at the other sites and
> ended up with the below...
>
> Yes, I know I'm the one that suggested the percpu thing, but upon
> looking again it missed the largest part of percpu_down_write(), which
> very much includes that RCU grace period and waiting for the readers to
> bugger off
>
> Also, rwbase_rt was missing the entire READ side -- yes, I see that's
> also covered by the rtmuex.c part, but that's on a different address and
> with different flags, and it's very confusing to not have it annotated.
>
> Anyway, I'll queue this patch with the below folded in for post -rc1.

Thanks for doing this, the changes look good.

Namhyung

>
> ---
>
> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c
> @@ -155,7 +155,6 @@ static void percpu_rwsem_wait(struct per
>         }
>         spin_unlock_irq(&sem->waiters.lock);
>
> -       trace_contention_begin(sem, LCB_F_PERCPU | (reader ? LCB_F_READ : LCB_F_WRITE));
>         while (wait) {
>                 set_current_state(TASK_UNINTERRUPTIBLE);
>                 if (!smp_load_acquire(&wq_entry.private))
> @@ -163,7 +162,6 @@ static void percpu_rwsem_wait(struct per
>                 schedule();
>         }
>         __set_current_state(TASK_RUNNING);
> -       trace_contention_end(sem, 0);
>  }
>
>  bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
> @@ -174,9 +172,11 @@ bool __sched __percpu_down_read(struct p
>         if (try)
>                 return false;
>
> +       trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_READ);
>         preempt_enable();
>         percpu_rwsem_wait(sem, /* .reader = */ true);
>         preempt_disable();
> +       trace_contention_end(sem, 0);
>
>         return true;
>  }
> @@ -219,6 +219,7 @@ void __sched percpu_down_write(struct pe
>  {
>         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);
> @@ -240,6 +241,7 @@ void __sched percpu_down_write(struct pe
>
>         /* Wait for all active readers to complete. */
>         rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
> +       trace_contention_end(sem, 0);
>  }
>  EXPORT_SYMBOL_GPL(percpu_down_write);
>
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -35,7 +35,7 @@ void queued_read_lock_slowpath(struct qr
>         }
>         atomic_sub(_QR_BIAS, &lock->cnts);
>
> -       trace_contention_begin(lock, LCB_F_READ | LCB_F_SPIN);
> +       trace_contention_begin(lock, LCB_F_SPIN | LCB_F_READ);
>
>         /*
>          * Put the reader into the wait queue
> @@ -67,7 +67,7 @@ void queued_write_lock_slowpath(struct q
>  {
>         int cnts;
>
> -       trace_contention_begin(lock, LCB_F_WRITE | LCB_F_SPIN);
> +       trace_contention_begin(lock, LCB_F_SPIN | LCB_F_WRITE);
>
>         /* Put the writer into the wait queue */
>         arch_spin_lock(&lock->wait_lock);
> --- a/kernel/locking/rwbase_rt.c
> +++ b/kernel/locking/rwbase_rt.c
> @@ -112,6 +112,8 @@ static int __sched __rwbase_read_lock(st
>          * Reader2 to call up_read(), which might be unbound.
>          */
>
> +       trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ);
> +
>         /*
>          * For rwlocks this returns 0 unconditionally, so the below
>          * !ret conditionals are optimized out.
> @@ -130,6 +132,8 @@ static int __sched __rwbase_read_lock(st
>         raw_spin_unlock_irq(&rtm->wait_lock);
>         if (!ret)
>                 rwbase_rtmutex_unlock(rtm);
> +
> +       trace_contention_end(rwb, ret);
>         return ret;
>  }
>
> @@ -247,7 +251,7 @@ static int __sched rwbase_write_lock(str
>                 goto out_unlock;
>
>         rwbase_set_and_save_current_state(state);
> -       trace_contention_begin(rwb, LCB_F_WRITE | LCB_F_RT);
> +       trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
>         for (;;) {
>                 /* Optimized out for rwlocks */
>                 if (rwbase_signal_pending_state(state, current)) {