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

Mathieu Desnoyers
2009-09-23T22:32:06Z

* Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote:
> * Chris Friesen (cfriesen@nortel.com) wrote:
> > On 09/23/2009 11:48 AM, Mathieu Desnoyers wrote:
> >
> > > 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.
> >
> > > /*
> > > * 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);
> > > }
> >
> > Is it a problem if multiple threads all hit the defer_thread_futex==-1
> > case simultaneously?
>
> It should not be, since what we'll have is, e.g.:
>
> thread 1 calls wakeup
> thread 2 calls wakeup
> thread 3 calls wait
>
> (thread 3 is waiting on the futex, defer_thread_futex = -1)
> - thread 1 sees defer_thread_futex==-1
> - thread 2 sees defer_thread_futex==-1
> - thread 1 sets defer_thread_futex = 0
> - thread 2 sets defer_thread_futex = 0
> - thread 1 calls futex() to wake up the waiter, expect 0
> - thread 2 calls futex() to wake up the waiter, expect 0
>
> Basically, what happens in this scenario is that the first futex()
> call will wake up any waiter, and the second will be a no-op.
>
> Let's complicate this, if we have thread 3 running wait_defer()
> concurrently:
>
> - thread 3 decrements defer_thread_futex
> - thread 1 sees defer_thread_futex==-1
> - thread 2 sees defer_thread_futex==-1
> - thread 1 sets defer_thread_futex = 0
> - thread 2 sets defer_thread_futex = 0
> - thread 1 calls futex() to wake up the waiter, expect 0
> - thread 2 calls futex() to wake up the waiter, expect 0
> - thread 3 calls futex() to wait, expect -1
> Returns immediately because defer_thread_futex == 0
>
> Other scenario, where thread decrements defer_thread_futex a bit later:
>
> - thread 1 sees defer_thread_futex==0
> - thread 2 sees defer_thread_futex==0
> - thread 3 decrements defer_thread_futex
> - thread 3 tests defer_thread_futex==-1
> - thread 3 calls futex() to wait, expect -1
>
> In this scenario, we have to notice that if threads 1/2 enqueued tasks
> to do before checking defer_thread_futex, these tasks would not be seen
> by the waiter thread.
>
> So correct memory ordering of:
>
> - wake_up_defer:
> * queue callbacks to perform (1)
> * wake up (2)
>
> - wait_defer:
> * for (;;)
> * wait for futex (3)
> * sleep 100ms (wait for more callbacks to be enqueued)
> * dequeue callbacks, execute them (4)
>
>
> actually matters. I'll have to be really careful about that (unless we
> just accept that tasks to perform could be queued for a while, however,
> I'd like to give an upper bound to the delay between batch callback
> execution).
>
> Ensuring that 1 is written before 2, and that 4 is done before 3 seems a
> bit racy. (I have to got out for lunch now, so I'll have to review the
> ordering afterward)
>

I knew I needed to think about it a bit more. Here is the proposed
algorithm hopefully fixing the race identified in the 3rd scenario
above. The idea is to perform the "check for empty queue" between the
&defer_thread_futex decrement and the test in wait_defer. It skips the
futex call and proceed if the list is non-empty.

As I am drilling into the problem, it looks very much like an attempt to
implement efficient wait queues in userspace based on sys_futex().

/*
* 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);
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. */
atomic_set(&defer_thread_futex, 0);
} else {
smp_rmb(); /* Read queue before read futex */
if (atomic_read(&defer_thread_futex) == -1)
futex(&defer_thread_futex, FUTEX_WAIT, -1,
NULL, NULL, 0);
}
}

- call_rcu():
* queue callbacks to perform
* smp_mb()
* wake_up_defer()

- defer thread:
* for (;;)
* wait_defer()
* sleep 100ms (wait for more callbacks to be enqueued)
* dequeue callbacks, execute them

The goal here is that if call_rcu() enqueues a callback (even if it
races with defer thread going to sleep), there should not be a
potentially infinite delay before it gets executed. Therefore, being
blocked in sys_futex while there is a callback to execute, without any
hope to be woken up unless another callback is queued, would not meet
that design requirement. I think that checking the number of queued
callbacks within wait_defer() as I propose here should address this
situation.

Comments ?

Mathieu

>
> > If so, maybe this should use an atomic
> > test-and-set operation so that only one thread actually calls futex().
>
> It's not a matter if many threads wake up the waiter, so I don't think
> the test-and-set is required. The benefit of using a simple test here is
> that we don't have to bring the cache-line in exclusive mode to the
> local CPU to perform the test. It can stay shared.
>
> >
> > > /*
> > > * 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);
> > > }
> >
> > Is it a problem if the value of defer_thread_futex changes to zero after
> > the dec but before the test?
>
> No. That's not a problem, because this means there is a concurrent "wake
> up". Seeing a value of "0" here will skip over the futex wait and go on.
> The concurrent futex wakeup call will simply be a no-op in that case.
>
> Thanks for the comments,
>
> Mathieu
>
> >
> > Chris
>
> --
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68