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

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

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);
}

Thanks,

Mathieu


-- 
Mathieu Desnoyers
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