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

[Kyle Moffett]

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.

Cheers,
Kyle Moffett