linux-kernel - Re: [PATCH 3/3] utrace-based ftrace "process" engine, v2

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Date:	Sat, 21 Mar 2009 17:48:52 -0400
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>, utrace-devel@...hat.com,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/3] utrace-based ftrace "process" engine, v2

Hi -

On Sat, Mar 21, 2009 at 04:45:01PM +0100, Ingo Molnar wrote:
> [...]
> To me personally there are two big direct usability issues with 
> SystemTap:
> 
>  1) It relies on DEBUG_INFO for any reasonable level of utility.
>     Yes, it will limp along otherwise as well, but most of the
>     actual novel capabilities depend on debuginfo. Which is an
>     acceptable constraint for enterprise usage where kernels are
>     switched every few months and having a debuginfo package is not
>     a big issue. Not acceptable for upstream kernel development. 

In my own limited kernel-building experience, I find the debuginfo
data conveniently and instantly available after every "make".  Can you
elaborate how is it harder for you to incidentally make it than for
someone to download it?

>     It also puts way too trust into the compiler generating 1GB+ of
>     debuginfo correctly. I want to be able to rely on tools all the
>     time and thus i want tools to have some really simple and
>     predictable foundations.

Well, the data has to come from *somewhere*.  We know several
shortcomings (and have staff working on gcc debuginfo improvements),
but there is little alternative.  If not from the compiler, where are
you going to get detailed type/structure layouts?  Stack slot to
variable mappings?  Statement-level PC addresses?  Unwind data?

>  2) It's not upstream and folks using it seem to insist on not 
>     having it upstream ;-) This 'distance' to upstream seems to have 
>     grown during the past few years - instead of shrinking. [...]

Considering our upstream-bound assistance with foundation technologies
like markers, tracepoints, kprobes, utrace, and several other bits,
this does not seem entirely fair.

> If these fundamental problems are addressed then i'd even argue for
> the totality of SystemTap to be aimed upstreamed (including the
> scripting language, etc.), [...]

If consensus on this were plausible, we could seriously discuss it.

But I don't buy the package-deal that utrace must not attempt merging
on its own merits, just because it makes systemtap (as it is today)
useful to more people.

- FChE
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