linux-kernel - Re: [PATCH v3 0/6] sched: Add new tracepoints required for EAS testing

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Date:   Wed, 5 Jun 2019 09:22:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Qais Yousef <qais.yousef@....com>, Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Pavankumar Kondeti <pkondeti@...eaurora.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Uwe Kleine-Konig <u.kleine-koenig@...gutronix.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Quentin Perret <quentin.perret@....com>
Subject: Re: [PATCH v3 0/6] sched: Add new tracepoints required for EAS
 testing

On Tue, Jun 04, 2019 at 11:17:48PM -0700, Christoph Hellwig wrote:
> > The following patches add the bare minimum tracepoints required to perform EAS
> > testing in Lisa[1].
> 
> What is EAS?  Whhy is "Lisa" not part of the patch submission?
> submission.
> 
> > It is done in this way because adding new TRACE_EVENTS() is no longer accepted
> > AFAIU.
> 
> Huh?  We keep adding trace events all the time.  And they actually
> are useful because they are testable.

They also form an implicit API/ABI with userspace, and I've been bitten
by that crap before. No more tracepoints. IIRC viro is also not having
tracepoints in the vfs.

> This series on the other hand adds exports not used in tree, which is
> a big no-go.

I much prefer a few unused exports that expose data in a controlled
fashion than commit to an implicit ABI through tracepoints. By keeping
it all in kernel, we're punting to the no-in-kernel-ABI rule.

Basically nobody gives a crap if we break (out-of-tree) modules, but the
moment we break something userspace we're fscked.