Re: [RFC] Userspace RCU: (ab)using futexes to save cpu cycles and energy

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Wed, Sep 23, 2009 at 01:48:20PM -0400, Mathieu Desnoyers wrote:
> > Hi,
> > 
> > When implementing the call_rcu() "worker thread" in userspace, I ran
> > into the problem that it had to be woken up periodically to check if
> > there are any callbacks to execute. However, I easily imagine that this
> > does not fit well with the "green computing" definition.
> > 
> > Therefore, I've looked at ways to have the call_rcu() callers waking up
> > this worker thread when callbacks are enqueued. However, I don't want to
> > take any lock and the fast path (when no wake up is required) should not
> > cause any cache-line exchange.
> > 
> > Here are the primitives I've created. I'd like to have feedback on my
> > futex use, just to make sure I did not do any incorrect assumptions.
> > 
> > This could also be eventually used in the QSBR Userspace RCU quiescent
> > state and in mb/signal userspace RCU when exiting RCU read-side C.S. to
> > ensure synchronize_rcu() does not busy-wait for too long.
> > 
> > /*
> >  * Wake-up any waiting defer thread. Called from many concurrent threads.
> >  */
> > static void wake_up_defer(void)
> > {
> >         if (unlikely(atomic_read(&defer_thread_futex) == -1))
> >                 atomic_set(&defer_thread_futex, 0);
> >                 futex(&defer_thread_futex, FUTEX_WAKE,
> >                       0, NULL, NULL, 0);
> > }
> > 
> > /*
> >  * Defer thread waiting. Single thread.
> >  */
> > static void wait_defer(void)
> > {
> >         atomic_dec(&defer_thread_futex);
> >         if (atomic_read(&defer_thread_futex) == -1)
> >                 futex(&defer_thread_futex, FUTEX_WAIT, -1,
> >                       NULL, NULL, 0);
> > }
> 
> The standard approach would be to use pthread_cond_wait() and
> pthread_cond_broadcast().  Unfortunately, this would require holding a
> pthread_mutex_lock across both operations, which would not necessarily
> be so good for wake-up-side scalability.

The pthread_cond_broadcast() mutex is really a bugger when it comes to
execute it at each rcu_read_unlock(). We could as well use a mutex to
protect the whole read-side.. :-(

> 
> That said, without this sort of heavy-locking approach, wakeup races
> are quite difficult to avoid.
> 

I did a formal model of my futex-based wait/wakeup. The main idea is
that the waiter:

- Set itself to "waiting"
- Checks the "real condition" for which it will wait (e.g. queues empty
  when used for rcu callbacks, no more ongoing old reader thread C.S.
  when used in synchronize_rcu())
- Calls sys_futex if the variable have not changed.

And the waker:
- sets the "real condition" waking up the waiter (enqueuing, or
  rcu_read_unlock())
- check if the waiter must be woken up, if so, wake it up by setting the
  state to "running" and calling sys_futex.

But as you say, wakeup races are difficult (but not impossible!) to
avoid. This is why I resorted to a formal model of the wait/wakeup
scheme to ensure that we cannot end up in a situation where a waker
races with the waiter and does not wake it up when it should. This is
nothing fancy (does not model memory and instruction reordering
automatically), but I figure that memory barriers are required between
almost every steps of this algorithm, so by adding smp_mb() I end up
ensure sequential behavior. I added test cases in the model to ensure
that incorrect memory reordering _would_ cause errors by doing the
reordering by hand in error-injection runs.

The model is available at:
http://www.lttng.org/cgi-bin/gitweb.cgi?p=userspace-rcu.git;a=tree;f=futex-wakeup;h=4ddeaeb2784165cb0465d4ca9f7d27acb562eae3;hb=refs/heads/formal-model

(this is in the formal-model branch of the urcu tree, futex-wakeup
subdir)

This is modeling this snippet of code :

static int defer_thread_futex;

/*
 * Wake-up any waiting defer thread. Called from many concurrent threads.
 */
static void wake_up_defer(void)
{
        if (unlikely(uatomic_read(&defer_thread_futex) == -1)) {
                uatomic_set(&defer_thread_futex, 0);
                futex(&defer_thread_futex, FUTEX_WAKE, 1,
                      NULL, NULL, 0);
        }
}

static void enqueue(void *callback)	/* not the actual types */
{
	add_to_queue(callback);
	smp_mb();
	wake_up_defer();
}

/*
 * rcu_defer_num_callbacks() returns the total number of callbacks
 * enqueued.
 */

/*
 * Defer thread waiting. Single thread.
 */
static void wait_defer(void)
{
        uatomic_dec(&defer_thread_futex);
        smp_mb();       /* Write futex before read queue */
        if (rcu_defer_num_callbacks()) {
                smp_mb();       /* Read queue before write futex */
                /* Callbacks are queued, don't wait. */
                uatomic_set(&defer_thread_futex, 0);
        } else {
                smp_rmb();      /* Read queue before read futex */
                if (uatomic_read(&defer_thread_futex) == -1)
                        futex(&defer_thread_futex, FUTEX_WAIT, -1,
                              NULL, NULL, 0);
        }
}


Comments are welcome,

Thanks,

Mathieu

> 							Thanx, Paul

-- 
Mathieu Desnoyers
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