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

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Date:   Fri, 18 Mar 2022 17:11:54 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 Fri, Mar 18, 2022 at 3:07 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Fri, 18 Mar 2022 14:55:27 -0700
> Namhyung Kim <namhyung@...nel.org> wrote:
>
> > > > This looks a littl ugly ;-/ Maybe we can rename __down_common() to
> > > > ___down_common() and implement __down_common() as:
> > > >
> > > >       static inline int __sched __down_common(...)
> > > >       {
> > > >               int ret;
> > > >               trace_contention_begin(sem, 0);
> > > >               ret = ___down_common(...);
> > > >               trace_contention_end(sem, ret);
> > > >               return ret;
> > > >       }
> > > >
> > > > Thoughts?
> > >
> > > Yeah, that works, except I think he wants a few extra
> > > __set_current_state()'s like so:
> >
> > Not anymore, I decided not to because of noise in the task state.
> >
> > Also I'm considering two tracepoints for the return path to reduce
> > the buffer size as Mathieu suggested.  Normally it'd return with 0
> > so we can ignore it in the contention_end.  For non-zero cases,
> > we can add a new tracepoint to save the return value.
>
> I don't think you need two tracepoints, but one that you can override.
>
> We have eprobes that let you create a trace event on top of a current trace
> event that can limit or extend what is traced in the buffer.
>
> And I also have custom events that can be placed on top of any existing
> tracepoint that has full access to what is sent into the tracepoint on not
> just what is available to the trace event:
>
>   https://lore.kernel.org/all/20220312232551.181178712@goodmis.org/

Thanks for the info.  But it's unclear to me if it provides the custom
event with the same or different name.  Can I use both of the original
and the custom events at the same time?

Thanks,
Namhyung