linux-kernel - Re: [PATCH 2/2] sched/debug: add sched_update_nr

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Message-ID: <20190904154000.GJ240514@google.com>
Date:   Wed, 4 Sep 2019 11:40:00 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Qais Yousef <qais.yousef@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jirka Hladký <jhladky@...hat.com>,
        Jiří Vozár <jvozar@...hat.com>,
        X86 ML <x86@...nel.org>
Subject: Re: [PATCH 2/2] sched/debug: add sched_update_nr_running tracepoint

On Wed, Sep 04, 2019 at 08:25:27AM -0700, Alexei Starovoitov wrote:
> On Wed, Sep 4, 2019 at 6:10 AM Joel Fernandes <joel@...lfernandes.org> wrote:
> >
> > I wonder if this distinction of "tracepoint" being non-ABI can be documented
> > somewhere. I would be happy to do that if there is a place for the same. I
> > really want some general "policy" in the kernel on where we draw a line in
> > the sand with respect to tracepoints and ABI :).
> 
> It's been discussed millions times. tracepoints are not abi.
> Example: android folks started abusing tracepoints inside bpf core
> and we _deleted_ them.

This is news to me, which ones?

> Same thing can be done with _any_ tracepoint.
> Do not abuse them and stop the fud about abi.

I don't know what FUD you are referring to. At least it is not coming from
me. This thread is dealing with the issue about ABI specifically, I jumped in
just now. As I was saying earlier, I don't have a strong opinion about this.
I just want to know what is the agreed upon approach so that we can stick to
it.

It sounds like the agreement here is tracepoints can be added and used
without ABI guarantees, however the same is not true with trace events.
Where's the FUD in that?

thanks,

 - Joel