[ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost)

[Paul E. McKenney]

On Sat, Feb 07, 2009 at 05:56:31PM -0500, Kyle Moffett wrote:
> On Thu, Feb 5, 2009 at 11:58 PM, Mathieu Desnoyers
> <compudj at krystal.dyndns.org> wrote:
> > I figured out I needed some userspace RCU for the userspace tracing part
> > of LTTng (for quick read access to the control variables) to trace
> > userspace pthread applications. So I've done a quick-and-dirty userspace
> > RCU implementation.
> >
> > It works so far, but I have not gone through any formal verification
> > phase. It seems to work on paper, and the tests are also OK (so far),
> > but I offer no guarantee for this 300-lines-ish 1-day hack. :-) If you
> > want to comment on it, it would be welcome. It's a userland-only
> > library. It's also currently x86-only, but only a few basic definitions
> > must be adapted in urcu.h to port it.
> 
> I have actually been fiddling with an RCU-esque design for a
> multithreaded event-driven userspace server process.  Essentially all
> threads using RCU-protected data run through a central event loop
> which drives my entirely-userspace RCU state machine.  I actually have
> a cooperative scheduler for groups of events to allow me to
> load-balance a large number of clients without the full overhead of a
> kernel thread per client.  This does rely on
> clock_gettime(CLOCK_THREAD_CPUTIME_ID) returning a useful monotonic
> value, however.
> 
> By building the whole internal system as an
> event-driven-state-machine, I don't need to keep a stack for blocked
> events.  The events which do large amounts of work call a
> "need_resched()"-ish function every so often, and if it returns true
> they return up the stack.  Relatively few threads (1 per physical CPU,
> plus a few for blocking event polling) are needed to completely
> saturate the system.
> 
> For RCU I simply treat event-handler threads the way the kernel treats
> CPUs, I report a Quiescent State every so often in-between processing
> events.
> 
> The event-handling mechanism is entirely agnostic to the way that
> events are generated.  It has built-in mechanisms for FD, signal, and
> AIO-based events, and it's trivial to add another event-polling thread
> for GTK/Qt/etc.
> 
> I'm still only halfway through laying out the framework for this
> library, but once it's done I'll make sure to post it somewhere for
> those who are interested.

I look forward to seeing it!  Perhaps user-level RCU is an idea whose
time has come?  ;-)

							Thanx, Paul