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

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Message-ID: <20090321125706.GB3566@redhat.com>
Date:	Sat, 21 Mar 2009 08:57:06 -0400
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, 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 05:04:22AM -0700, Andrew Morton wrote:
> [...]
> > There have been many mixed messages from LKML on the topic - sometimes
> > mentioning systemtap is forbidden, other times necessary.  Sorry about
> > that.
> 
> heh.  We all love systemtap and want it to get better.

Great!

> [...]
> I have strong memories of being traumatised by reading the uprobes
> code.  What's the story on all of that nowadays?

uprobes, being a layer upon utrace that provides a kprobes-like
breakpointing API for user threads, is being refactored into several
parts.  I don't know about the aesthetics of it all, but I believe the
general future plan is this:

One piece would perform machine code analysis (to classify
instructions for ideal/safe placement of breakpoints or for code
patching), and another thin layer that uses this and utrace to manage
user-space breakpoints.  (Systemtap would interface at this point.)
Then a user-space syscallish interface could come along to expose this
to a super-ptrace client (to speed up gdb; perhaps to allow multiple
debuggers).  Plus one might as well add an ftrace-engine for it
(directly analogous to the recent kprobe-based one that ftrace people
found "cool".)

> > > Actually it seems that the whole utrace-ftrace thing is a big
> > > distraction and could/should just be omitted.  This is a systemtap
> > > feature and should be viewed as such. [...]
> > 
> > utrace is a better way to perform user thread management than what is
> > there now, and the utrace-ftrace widget shows how to *hook* thread
> > events such as syscalls in a lighter weight / more managed way than
> > the first one proposed.  (That's one reason we've been participating
> > in the ftrace discussions.)  Of course it can be made to use the fine
> > syscall pretty-printing code recently added.
> 
> eh.  Boring.  Let's fix systemtap?

There are several constituencies here, some of which find the above
exciting.  That's OK and we'd like to help them too.

- FChE
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