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

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Message-ID: <CAM9d7cgCEsH6grH556Js6VX-cXAO_3hT7C+RSm+sxxBDgxHvig@mail.gmail.com>
Date:   Mon, 21 Mar 2022 22:31:30 -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 5:11 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> 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?

I've read the code and understood that it's a separate event that can
be used together.  Then I think we can leave the tracepoint with the
return value and let users customize it for their needs later.

Thanks,
Namhyung