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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 24 Mar 2009 10:59:26 +0530 Ananth N Mavinakayanahalli <ananth@in.ibm.com> wrote:

> On Sat, Mar 21, 2009 at 05:04:22AM -0700, Andrew Morton wrote:
> > On Sat, 21 Mar 2009 07:51:41 -0400 "Frank Ch. Eigler" <fche@redhat.com> wrote:
> > > On Sat, Mar 21, 2009 at 04:19:54AM -0700, Andrew Morton wrote:
> > 
> > I have strong memories of being traumatised by reading the uprobes code. 
> 
> That was a long time ago wasn't it? :-)
> 
> That approach was a carry over from an implementation from dprobes that
> used readdir hooks. Yes, that was not the most elegant approach, as such
> has long been shelved.
> 
> > What's the story on all of that nowadays?
> 
> Utrace makes implementing uprobes more cleaner. We have a prototype that
> implements uprobes over utrace. Its per process, doesn't use any
> in-kernel hooks, etc. It currently has a kprobes like interface (needs a
> kernel module), but it shouldn't be difficult to adapt it to use
> utrace's user interfaces (syscalls?) when those come around. The current
> generation of uprobes that has all the bells and whistles can be found at
> http://sources.redhat.com/git/gitweb.cgi?p=systemtap.git;a=tree;f=runtime/uprobes2
> 
> However, there are aspects of the current uprobes that can be useful to
> any other userspace tracer: instruction analysis, breakpoint insertion
> and removal, single-stepping support. With these layered on top of
> utrace, building userspace debug/trace tools that depend on utrace
> should be easier, outside of ptrace.
> 
> Work is currently on to factor these layers out. The intention is to
> upstream all the bits required for userspace tracing once utrace gets
> in, along with an easy to use interface for userspace developers
> (a /proc or /debugfs interface?) -- one should be able to use it on
> its own or with SystemTap, whatever they prefer. Details are still hazy
> at the moment.
> 
> But, utrace is the foundation to do all of that.
> 

The sticking point was uprobes's modification of live pagecache.  We said
"ick, COW the pages" and you said "too expensive".  And there things
remained.

Is that all going to happen again?