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

From: Peter Zijlstra
Date: Wed Jun 05 2019 - 03:26:58 EST

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.