linux-ext4 - Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing

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Message-ID: <CA+55aFwmCVZECoMszXZkJ8tSpG5+Ynt-5EKxKqDepNtjUv5vkg@mail.gmail.com>
Date:   Sun, 27 Nov 2016 16:58:43 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing

On Sun, Nov 27, 2016 at 2:42 PM, Dave Chinner <david@...morbit.com> wrote:
>
> And that's exactly why we need a method of marking tracepoints as
> stable. How else are we going to know whether a specific tracepoint
> is stable if the kernel code doesn't document that it's stable?

You are living in some unrealistic dream-world where you think you can
get the right tracepoint on the first try.

So there is no way in hell I would ever mark any tracepoint "stable"
until it has had a fair amount of use, and there are useful tools that
actually make use of it, and it has shown itself to be the right
trace-point.

And once that actually happens, what's the advantage of marking it
stable? None. It's a catch-22. Before it has uses and has been tested
and found to be good, it's not stable. And after, it's pointless.

So at no point does such a "stable" tracepoint marking make sense. At
most, you end up adding a comment saying "this tracepoint is used by
tools such-and-such".

                   Linus
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