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

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Date:   Mon, 28 Nov 2016 09:42:08 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 Fri, Nov 25, 2016 at 11:51:26AM -0800, Linus Torvalds wrote:
> On Thu, Nov 24, 2016 at 11:37 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > My impression is that nobody (at least kernel-side) wants them to be
> > a stable ABI, so long as nobody in userland screams about their code
> > being broken, everything is fine.  As usual, if nobody notices an ABI
> > change, it hasn't happened.  The question is what happens when somebody
> > does.
> 
> Right. There is basically _no_ "stable API" for the kernel anywhere,
> it's just an issue of "you can't break workflow for normal people".
> 
> And if somebody writes his own trace scripts, and some random trace
> point goes away (or changes semantics), that's easy: he can just fix
> his script. Tracepoints aren't ever going to be stable in that sense.
> 
> But when then somebody writes a trace script that is so useful that
> distros pick it up, and people start using it and depending on it,
> then _that_ trace point may well have become effectively locked in
> stone.

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? And
how are we going to know why it's considered stable if there isn't a
commit message that explains why it was made stable?

> We do have filesystem code that is just disgusting. As an example:
> fs/afs/ tends to have these crazy "_enter()/_exit()" macros in every
> single function. If you want that, use the function tracer. That seems
> to be just debugging code that has been left around for others to
> stumble over. I do *not* believe that we should encourage that kind of
> "machine gun spray" use of tracepoints.

Inappropriate use of tracepoints is a different problem. The issue
here is getting rid of the uncertainty caused by the handwavy
"tracepoints a mutable until someone, somewhere decides to use it in
userspace" policy.

> But tracing actual high-level things like IO and faults? I think that
> makes perfect sense, as long as the data that is collected is also the
> actual event data, and not so much a random implementation issue of
> the day.

IME, a tracepoint that doesn't expose detailed context specific
information isn't really useful for complex problem diagnosis...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com