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

[Mathieu Desnoyers]

* Kyle Moffett (kyle at moffetthome.net) 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.
> 

That would be interesting to look at. It would indeed be very efficient
at the reader site, because no barriers would be required. However, it
might not be appropriate for use-cases like userspace tracing, where we
ideally want to add tracing functionnality to applications as a library,
without requiring to modify the application behavior (e.g. adding a
"quiescent state" call into the application loop). I also think Paul
already has such application quiescent state notification implementation
in the links he gave us, we might want to compare those two.

Mathieu

> Cheers,
> Kyle Moffett
> 

-- 
Mathieu Desnoyers
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