Fix and re-enable dynamic scheduler scaling

Change dynamic scheduler scaling implementation in order to resolve
the hangs encountered in ponylang#2451.

The previous implementation assumed that signalling to wake a thread
was a reliable operation. Apparently, that's not necessarily true
(see https://en.wikipedia.org/wiki/Spurious_wakeup and
https://askldjd.com/2010/04/24/the-lost-wakeup-problem/). Seeing
as we couldn't find any other explanation for why the previous
implementation was experiencing hangs, I've assumed it is either
because of lost wake ups or spurious wake ups and redesigned the
logic accordingly.

Now, when a thread is about to suspend, it will decrement the
`active_scheduler_count` and then suspend. When it wakes up, it will
check to see if the `active_scheduler_count` is at least as big as
its `index`. If the `active_scheduler_count` isn't big enough, the
thread will suspend itself again immediately. If it is big enough,
it will resume. Threads no longer modify `active_scheduler_count`
when they wake up.

`active_scheduler_count` must now be modified by the thread that is
waking up another thread prior to sending the wake up notification.
Additionally, since we're now assuming that wake up signals can be
lost, we now send multiple wake up notifications just in case. While
this is somewhat wasteful, it is better than being in a situation
where some threads aren't woken up at all (i.e. a hang).

Additionally, only use `scheduler_count_changing` for `signals`
implementation of dynamic scheduler scaling. `pthreads`
implementation now uses a mutex (`sched_mut`) in its place.
We also now change logic to only unlock mutex in `pthreads`
implementation once threads have been woken to avoid potential
lost wake ups. This isn't an issue for the `signals` implementation
and the unlocking of `scheduler_count_changing` can remain where it
is prior to threads being woken up.

This commit also splits out scheduler block/unblock message handling
logic into their own functions (this is so that sched 0 can call those
functions directly instead of sending messages to itself).

This commit also includes a change inspired by ponylang#2474. Now, *all*
scheduler threads can suspend as long as there is at least one
noisy actor registered with the ASIO subsystem. If there are no
noisy actors registered with the ASIO subsystem then scheduler 0
is not allowed to suspend itself because it is reponsible for
quiescence detection.

Lastly, this commit adds logic to allow a scheduler thread to suspend
even if it has already sent a scheduler block message so that we can
now suspend scheduler threads in most scenarios.

src/common/threads.h

@@ -57,7 +57,11 @@ void ponyint_thread_detach(pony_thread_id_t thread);
 
 pony_thread_id_t ponyint_thread_self();
 
+#if defined(USE_SCHEDULER_SCALING_PTHREADS)
+void ponyint_thread_suspend(pony_signal_event_t signal, pthread_mutex_t* mut);
+#else
 void ponyint_thread_suspend(pony_signal_event_t signal);
+#endif
 
 int ponyint_thread_wake(pony_thread_id_t thread, pony_signal_event_t signal);
 

src/libponyc/options/options.c

@@ -209,7 +209,7 @@ static void usage(void)
     "  --ponythreads    Use N scheduler threads. Defaults to the number of\n"
     "                   cores (not hyperthreads) available.\n"
     "  --ponyminthreads Minimum number of active scheduler threads allowed.\n"
-    "                   Defaults to the number of '--ponythreads'.\n"
+    "                   Defaults to 0.\n"
     "  --ponycdmin      Defer cycle detection until 2^N actors have blocked.\n"
     "                   Defaults to 2^4.\n"
     "  --ponycdmax      Always cycle detect when 2^N actors have blocked.\n"

src/libponyrt/asio/event.c

@@ -2,6 +2,7 @@
 #include "asio.h"
 #include "../actor/actor.h"
 #include "../mem/pool.h"
+#include "../sched/scheduler.h"
 #include "ponyassert.h"
 #include <string.h>
 
@@ -122,4 +123,7 @@ PONY_API void pony_asio_event_send(asio_event_t* ev, uint32_t flags,
   // sender they aren't covered by backpressure. We pass false for an early
   // bailout in the backpressure code.
   pony_sendv(pony_ctx(), ev->owner, &m->msg, &m->msg, false);
+
+  // maybe wake up a scheduler thread if they've all fallen asleep
+  ponyint_sched_maybe_wakeup_if_all_asleep(-1);
 }

src/libponyrt/platform/threads.c

@@ -18,17 +18,6 @@
 typedef cpuset_t cpu_set_t;
 #endif
 
-#if defined(USE_SCHEDULER_SCALING_PTHREADS)
-static pthread_mutex_t sleep_mut;
-
-static pthread_once_t sleep_mut_once = PTHREAD_ONCE_INIT;
-
-void sleep_mut_init()
-{
-    pthread_mutex_init(&sleep_mut, NULL);
-}
-#endif
-
 #if defined(PLATFORM_IS_LINUX)
 
 #include <dlfcn.h>
@@ -213,10 +202,6 @@ bool ponyint_thread_create(pony_thread_id_t* thread, thread_fn start,
     return false;
 #endif
 
-#if !defined(PLATFORM_IS_WINDOWS) && defined(USE_SCHEDULER_SCALING_PTHREADS)
-  pthread_once(&sleep_mut_once, sleep_mut_init);
-#endif
-
   return true;
 }
 
@@ -249,22 +234,21 @@ pony_thread_id_t ponyint_thread_self()
 #endif
 }
 
+#if defined(USE_SCHEDULER_SCALING_PTHREADS)
+void ponyint_thread_suspend(pony_signal_event_t signal, pthread_mutex_t* mut)
+#else
 void ponyint_thread_suspend(pony_signal_event_t signal)
+#endif
 {
 #ifdef PLATFORM_IS_WINDOWS
   WaitForSingleObject(signal, INFINITE);
 #elif defined(USE_SCHEDULER_SCALING_PTHREADS)
   int ret;
-  // lock mutex
-  ret = pthread_mutex_lock(&sleep_mut);
 
   // wait for condition variable (will sleep and release mutex)
-  ret = pthread_cond_wait(signal, &sleep_mut);
+  ret = pthread_cond_wait(signal, mut);
   // TODO: What to do if `ret` is an unrecoverable error?
   (void) ret;
-
-  // unlock mutex
-  ret = pthread_mutex_unlock(&sleep_mut);
 #else
   int sig;
   sigset_t sigmask;