linux-kernel - Re: [PATCH 2/4] locking: Apply contention tracepoints in the slow path

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Message-ID: <CAM9d7ciHObwkvyy9Uz5NRb=KBY-HtXAtAJTgXocA6C42aBoAyg@mail.gmail.com>
Date:   Mon, 14 Mar 2022 14:44:04 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Byungchul Park <byungchul.park@....com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        bpf <bpf@...r.kernel.org>, Radoslaw Burny <rburny@...gle.com>
Subject: Re: [PATCH 2/4] locking: Apply contention tracepoints in the slow path

On Tue, Mar 1, 2022 at 1:04 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Feb 28, 2022 at 05:04:10PM -0800, Namhyung Kim wrote:
> > @@ -1718,9 +1726,11 @@ static __always_inline void __sched rtlock_slowlock(struct rt_mutex_base *lock)
> >  {
> >       unsigned long flags;
> >
> > +     trace_contention_begin(lock, _RET_IP_, LCB_F_RT | TASK_RTLOCK_WAIT);
> >       raw_spin_lock_irqsave(&lock->wait_lock, flags);
> >       rtlock_slowlock_locked(lock);
> >       raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
> > +     trace_contention_end(lock);
> >  }
>
> Same, if you do it one level in, you can have the tracepoint itself look
> at current->__state.

So I tried this by reading the state in the trace like below:

+       TP_fast_assign(
+               __entry->lock_addr = lock;
+               __entry->flags = flags | get_current_state();
+       ),

But I sometimes see unrelated values which contain
__TASK_TRACED or __TASK_STOPPED and some unexpected values
like TASK_UNINTERRUPTIBLE for rwlocks.  Maybe I missed something.

Anyway I think it's confusing and complicates things unnecessarily.
Probably it'd better using new flags like LCB_F_SPIN and LCB_F_WAIT.

Thanks,
Namhyung