[Feature #18339] GVL Instrumentation API

Ref: https://bugs.ruby-lang.org/issues/18339

Design:

- This tries to minimize the overhead when no hook is registered.
  It should only incur an extra unsynchronized boolean check.
- The hook list is protected with a read-write lock as to cause
  contention when some hooks are registered.
- The hooks MUST be thread safe, and MUST NOT call into Ruby as they
  are executed outside the GVL.
- It's simply a noop on Windows.

API:

```
rb_internal_thread_event_hook_t * rb_internal_thread_add_event_hook(rb_internal_thread_event_callback callback, rb_event_flag_t internal_event, void *user_data);
bool rb_internal_thread_remove_event_hook(rb_internal_thread_event_hook_t * hook);
```

You can subscribe to 3 events:

  - READY: called right before attempting to acquire the GVL
  - RESUMED: called right after successfully acquiring the GVL
  - SUSPENDED: called right after releasing the GVL.

The hooks MUST be threadsafe, as they are executed outside of the GVL, they also MUST NOT call any Ruby API.

ext/-test-/thread/instrumentation/depend

@@ -0,0 +1,164 @@
+# AUTOGENERATED DEPENDENCIES START
+instrumentation.o: $(RUBY_EXTCONF_H)
+instrumentation.o: $(arch_hdrdir)/ruby/config.h
+instrumentation.o: $(hdrdir)/ruby/assert.h
+instrumentation.o: $(hdrdir)/ruby/atomic.h
+instrumentation.o: $(hdrdir)/ruby/backward.h
+instrumentation.o: $(hdrdir)/ruby/backward/2/assume.h
+instrumentation.o: $(hdrdir)/ruby/backward/2/attributes.h
+instrumentation.o: $(hdrdir)/ruby/backward/2/bool.h
+instrumentation.o: $(hdrdir)/ruby/backward/2/inttypes.h
+instrumentation.o: $(hdrdir)/ruby/backward/2/limits.h
+instrumentation.o: $(hdrdir)/ruby/backward/2/long_long.h
+instrumentation.o: $(hdrdir)/ruby/backward/2/stdalign.h
+instrumentation.o: $(hdrdir)/ruby/backward/2/stdarg.h
+instrumentation.o: $(hdrdir)/ruby/defines.h
+instrumentation.o: $(hdrdir)/ruby/intern.h
+instrumentation.o: $(hdrdir)/ruby/internal/abi.h
+instrumentation.o: $(hdrdir)/ruby/internal/anyargs.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/char.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/double.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/fixnum.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/gid_t.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/int.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/intptr_t.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/long.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/long_long.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/mode_t.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/off_t.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/pid_t.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/short.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/size_t.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/st_data_t.h
+instrumentation.o: $(hdrdir)/ruby/internal/arithmetic/uid_t.h
+instrumentation.o: $(hdrdir)/ruby/internal/assume.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/alloc_size.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/artificial.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/cold.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/const.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/constexpr.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/deprecated.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/diagnose_if.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/enum_extensibility.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/error.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/flag_enum.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/forceinline.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/format.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/maybe_unused.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/noalias.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/nodiscard.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/noexcept.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/noinline.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/nonnull.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/noreturn.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/pure.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/restrict.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/returns_nonnull.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/warning.h
+instrumentation.o: $(hdrdir)/ruby/internal/attr/weakref.h
+instrumentation.o: $(hdrdir)/ruby/internal/cast.h
+instrumentation.o: $(hdrdir)/ruby/internal/compiler_is.h
+instrumentation.o: $(hdrdir)/ruby/internal/compiler_is/apple.h
+instrumentation.o: $(hdrdir)/ruby/internal/compiler_is/clang.h
+instrumentation.o: $(hdrdir)/ruby/internal/compiler_is/gcc.h
+instrumentation.o: $(hdrdir)/ruby/internal/compiler_is/intel.h
+instrumentation.o: $(hdrdir)/ruby/internal/compiler_is/msvc.h
+instrumentation.o: $(hdrdir)/ruby/internal/compiler_is/sunpro.h
+instrumentation.o: $(hdrdir)/ruby/internal/compiler_since.h
+instrumentation.o: $(hdrdir)/ruby/internal/config.h
+instrumentation.o: $(hdrdir)/ruby/internal/constant_p.h
+instrumentation.o: $(hdrdir)/ruby/internal/core.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rarray.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rbasic.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rbignum.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rclass.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rdata.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rfile.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rhash.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/robject.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rregexp.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rstring.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rstruct.h
+instrumentation.o: $(hdrdir)/ruby/internal/core/rtypeddata.h
+instrumentation.o: $(hdrdir)/ruby/internal/ctype.h
+instrumentation.o: $(hdrdir)/ruby/internal/dllexport.h
+instrumentation.o: $(hdrdir)/ruby/internal/dosish.h
+instrumentation.o: $(hdrdir)/ruby/internal/error.h
+instrumentation.o: $(hdrdir)/ruby/internal/eval.h
+instrumentation.o: $(hdrdir)/ruby/internal/event.h
+instrumentation.o: $(hdrdir)/ruby/internal/fl_type.h
+instrumentation.o: $(hdrdir)/ruby/internal/gc.h
+instrumentation.o: $(hdrdir)/ruby/internal/glob.h
+instrumentation.o: $(hdrdir)/ruby/internal/globals.h
+instrumentation.o: $(hdrdir)/ruby/internal/has/attribute.h
+instrumentation.o: $(hdrdir)/ruby/internal/has/builtin.h
+instrumentation.o: $(hdrdir)/ruby/internal/has/c_attribute.h
+instrumentation.o: $(hdrdir)/ruby/internal/has/cpp_attribute.h
+instrumentation.o: $(hdrdir)/ruby/internal/has/declspec_attribute.h
+instrumentation.o: $(hdrdir)/ruby/internal/has/extension.h
+instrumentation.o: $(hdrdir)/ruby/internal/has/feature.h
+instrumentation.o: $(hdrdir)/ruby/internal/has/warning.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/array.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/bignum.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/class.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/compar.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/complex.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/cont.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/dir.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/enum.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/enumerator.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/error.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/eval.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/file.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/gc.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/hash.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/io.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/load.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/marshal.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/numeric.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/object.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/parse.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/proc.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/process.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/random.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/range.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/rational.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/re.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/ruby.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/select.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/select/largesize.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/signal.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/sprintf.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/string.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/struct.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/thread.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/time.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/variable.h
+instrumentation.o: $(hdrdir)/ruby/internal/intern/vm.h
+instrumentation.o: $(hdrdir)/ruby/internal/interpreter.h
+instrumentation.o: $(hdrdir)/ruby/internal/iterator.h
+instrumentation.o: $(hdrdir)/ruby/internal/memory.h
+instrumentation.o: $(hdrdir)/ruby/internal/method.h
+instrumentation.o: $(hdrdir)/ruby/internal/module.h
+instrumentation.o: $(hdrdir)/ruby/internal/newobj.h
+instrumentation.o: $(hdrdir)/ruby/internal/rgengc.h
+instrumentation.o: $(hdrdir)/ruby/internal/scan_args.h
+instrumentation.o: $(hdrdir)/ruby/internal/special_consts.h
+instrumentation.o: $(hdrdir)/ruby/internal/static_assert.h
+instrumentation.o: $(hdrdir)/ruby/internal/stdalign.h
+instrumentation.o: $(hdrdir)/ruby/internal/stdbool.h
+instrumentation.o: $(hdrdir)/ruby/internal/symbol.h
+instrumentation.o: $(hdrdir)/ruby/internal/value.h
+instrumentation.o: $(hdrdir)/ruby/internal/value_type.h
+instrumentation.o: $(hdrdir)/ruby/internal/variable.h
+instrumentation.o: $(hdrdir)/ruby/internal/warning_push.h
+instrumentation.o: $(hdrdir)/ruby/internal/xmalloc.h
+instrumentation.o: $(hdrdir)/ruby/missing.h
+instrumentation.o: $(hdrdir)/ruby/ruby.h
+instrumentation.o: $(hdrdir)/ruby/st.h
+instrumentation.o: $(hdrdir)/ruby/subst.h
+instrumentation.o: $(hdrdir)/ruby/thread.h
+instrumentation.o: $(hdrdir)/ruby/thread_native.h
+instrumentation.o: instrumentation.c
+# AUTOGENERATED DEPENDENCIES END

ext/-test-/thread/instrumentation/extconf.rb

@@ -0,0 +1,2 @@
+# frozen_string_literal: false
+create_makefile("-test-/thread/instrumentation")

ext/-test-/thread/instrumentation/instrumentation.c

@@ -0,0 +1,95 @@
+#include "ruby/ruby.h"
+#include "ruby/atomic.h"
+#include "ruby/thread.h"
+
+static rb_atomic_t acquire_enter_count = 0;
+static rb_atomic_t acquire_exit_count = 0;
+static rb_atomic_t release_count = 0;
+
+void
+ex_callback(rb_event_flag_t event, const rb_internal_thread_event_data_t *event_data, void *user_data)
+{
+    switch(event) {
+      case RUBY_INTERNAL_THREAD_EVENT_READY:
+        RUBY_ATOMIC_INC(acquire_enter_count);
+        break;
+      case RUBY_INTERNAL_THREAD_EVENT_RESUMED:
+        RUBY_ATOMIC_INC(acquire_exit_count);
+        break;
+      case RUBY_INTERNAL_THREAD_EVENT_SUSPENDED:
+        RUBY_ATOMIC_INC(release_count);
+        break;
+    }
+}
+
+static rb_internal_thread_event_hook_t * single_hook = NULL;
+
+static VALUE
+thread_counters(VALUE thread)
+{
+    VALUE array = rb_ary_new2(3);
+    rb_ary_push(array, UINT2NUM(acquire_enter_count));
+    rb_ary_push(array, UINT2NUM(acquire_exit_count));
+    rb_ary_push(array, UINT2NUM(release_count));
+    return array;
+}
+
+static VALUE
+thread_reset_counters(VALUE thread)
+{
+    RUBY_ATOMIC_SET(acquire_enter_count, 0);
+    RUBY_ATOMIC_SET(acquire_exit_count, 0);
+    RUBY_ATOMIC_SET(release_count, 0);
+    return Qtrue;
+}
+
+static VALUE
+thread_register_callback(VALUE thread)
+{
+    single_hook = rb_internal_thread_add_event_hook(
+        *ex_callback,
+        RUBY_INTERNAL_THREAD_EVENT_READY | RUBY_INTERNAL_THREAD_EVENT_RESUMED | RUBY_INTERNAL_THREAD_EVENT_SUSPENDED,
+        NULL
+    );
+
+    return Qnil;
+}
+
+static VALUE
+thread_unregister_callback(VALUE thread)
+{
+    if (single_hook) {
+        rb_internal_thread_remove_event_hook(single_hook);
+        single_hook = NULL;
+    }
+
+    return Qnil;
+}
+
+static VALUE
+thread_register_and_unregister_callback(VALUE thread)
+{
+    rb_internal_thread_event_hook_t * hooks[5];
+    for (int i = 0; i < 5; i++) {
+        hooks[i] = rb_internal_thread_add_event_hook(*ex_callback, RUBY_INTERNAL_THREAD_EVENT_READY, NULL);
+    }
+
+    if (!rb_internal_thread_remove_event_hook(hooks[4])) return Qfalse;
+    if (!rb_internal_thread_remove_event_hook(hooks[0])) return Qfalse;
+    if (!rb_internal_thread_remove_event_hook(hooks[3])) return Qfalse;
+    if (!rb_internal_thread_remove_event_hook(hooks[2])) return Qfalse;
+    if (!rb_internal_thread_remove_event_hook(hooks[1])) return Qfalse;
+    return Qtrue;
+}
+
+void
+Init_instrumentation(void)
+{
+    VALUE mBug = rb_define_module("Bug");
+    VALUE klass = rb_define_module_under(mBug, "ThreadInstrumentation");
+    rb_define_singleton_method(klass, "counters", thread_counters, 0);
+    rb_define_singleton_method(klass, "reset_counters", thread_reset_counters, 0);
+    rb_define_singleton_method(klass, "register_callback", thread_register_callback, 0);
+    rb_define_singleton_method(klass, "unregister_callback", thread_unregister_callback, 0);
+    rb_define_singleton_method(klass, "register_and_unregister_callbacks", thread_register_and_unregister_callback, 0);
+}

include/ruby/thread.h

@@ -190,6 +190,44 @@ void *rb_nogvl(void *(*func)(void *), void *data1,
  */
 #define RUBY_CALL_WO_GVL_FLAG_SKIP_CHECK_INTS_
 
+#define RUBY_INTERNAL_THREAD_EVENT_READY      0x01 /** acquiring GVL */
+#define RUBY_INTERNAL_THREAD_EVENT_RESUMED    0x02 /** acquired GVL */
+#define RUBY_INTERNAL_THREAD_EVENT_SUSPENDED  0x04 /** released GVL */
+#define RUBY_INTERNAL_THREAD_EVENT_MASK       0x07 /** All Thread events */
+
+typedef void rb_internal_thread_event_data_t; // for future extension.
+
+typedef void (*rb_internal_thread_event_callback)(rb_event_flag_t event,
+              const rb_internal_thread_event_data_t *event_data,
+              void *user_data);
+typedef struct rb_internal_thread_event_hook rb_internal_thread_event_hook_t;
+
+/**
+ * Registers a thread event hook function.
+ *
+ * @param[in]  func    A callback.
+ * @param[in]  events  A set of events that `func` should run.
+ * @param[in]  data    Passed as-is to `func`.
+ * @return     An opaque pointer to the hook, to unregister it later.
+ * @note       This functionality is a noop on Windows.
+ * @warning    This function MUST not be called from a thread event callback.
+ */
+rb_internal_thread_event_hook_t *rb_internal_thread_add_event_hook(
+        rb_internal_thread_event_callback func, rb_event_flag_t events,
+        void *data);
+
+
+/**
+ * Unregister the passed hook.
+ *
+ * @param[in]  hook.  The hook to unregister.
+ * @return     Wether the hook was found and unregistered.
+ * @note       This functionality is a noop on Windows.
+ * @warning    This function MUST not be called from a thread event callback.
+*/
+bool rb_internal_thread_remove_event_hook(
+        rb_internal_thread_event_hook_t * hook);
+
 RBIMPL_SYMBOL_EXPORT_END()
 
 #endif /* RUBY_THREAD_H */

test/-ext-/thread/test_instrumentation_api.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: false
+class TestThreadInstrumentation < Test::Unit::TestCase
+  def setup
+    pend("TODO: No windows support yet") if /mswin|mingw|bccwin/ =~ RUBY_PLATFORM
+  end
+
+  def test_thread_instrumentation
+    require '-test-/thread/instrumentation'
+    Bug::ThreadInstrumentation.reset_counters
+    Bug::ThreadInstrumentation::register_callback
+
+    begin
+      threads = 5.times.map { Thread.new { sleep 0.05; 1 + 1; sleep 0.02 } }
+      threads.each(&:join)
+      Bug::ThreadInstrumentation.counters.each do |c|
+        assert_predicate c,:nonzero?
+      end
+    ensure
+      Bug::ThreadInstrumentation::unregister_callback
+    end
+  end
+
+  def test_thread_instrumentation_fork_safe
+    skip "No fork()" unless Process.respond_to?(:fork)
+
+    require '-test-/thread/instrumentation'
+    Bug::ThreadInstrumentation::register_callback
+
+    begin
+      pid = fork do
+        Bug::ThreadInstrumentation.reset_counters
+        threads = 5.times.map { Thread.new { sleep 0.05; 1 + 1; sleep 0.02 } }
+        threads.each(&:join)
+        Bug::ThreadInstrumentation.counters.each do |c|
+          assert_predicate c,:nonzero?
+        end
+      end
+      _, status = Process.wait2(pid)
+      assert_predicate status, :success?
+    ensure
+      Bug::ThreadInstrumentation::unregister_callback
+    end
+  end
+
+  def test_thread_instrumentation_unregister
+    require '-test-/thread/instrumentation'
+    assert Bug::ThreadInstrumentation::register_and_unregister_callbacks
+  end
+end
+

thread_pthread.c

@@ -98,6 +98,95 @@
 #  endif
 #endif
 
+struct rb_internal_thread_event_hook {
+    rb_internal_thread_event_callback callback;
+    rb_event_flag_t event;
+    void *user_data;
+
+    struct rb_internal_thread_event_hook *next;
+};
+
+static rb_internal_thread_event_hook_t *rb_internal_thread_event_hooks = NULL;
+static pthread_rwlock_t rb_internal_thread_event_hooks_rw_lock = PTHREAD_RWLOCK_INITIALIZER;
+
+rb_internal_thread_event_hook_t *
+rb_internal_thread_add_event_hook(rb_internal_thread_event_callback callback, rb_event_flag_t internal_event, void *user_data)
+{
+    rb_internal_thread_event_hook_t *hook = ALLOC_N(rb_internal_thread_event_hook_t, 1);
+    hook->callback = callback;
+    hook->user_data = user_data;
+    hook->event = internal_event;
+
+    int r;
+    if ((r = pthread_rwlock_wrlock(&rb_internal_thread_event_hooks_rw_lock))) {
+        rb_bug_errno("pthread_rwlock_wrlock", r);
+    }
+
+    hook->next = rb_internal_thread_event_hooks;
+    ATOMIC_PTR_EXCHANGE(rb_internal_thread_event_hooks, hook);
+
+    if ((r = pthread_rwlock_unlock(&rb_internal_thread_event_hooks_rw_lock))) {
+        rb_bug_errno("pthread_rwlock_unlock", r);
+    }
+    return hook;
+}
+
+bool
+rb_internal_thread_remove_event_hook(rb_internal_thread_event_hook_t * hook)
+{
+    int r;
+    if ((r = pthread_rwlock_wrlock(&rb_internal_thread_event_hooks_rw_lock))) {
+        rb_bug_errno("pthread_rwlock_wrlock", r);
+    }
+
+    bool success = FALSE;
+
+    if (rb_internal_thread_event_hooks == hook) {
+        ATOMIC_PTR_EXCHANGE(rb_internal_thread_event_hooks, hook->next);
+        success = TRUE;
+    } else {
+        rb_internal_thread_event_hook_t *h = rb_internal_thread_event_hooks;
+
+        do {
+            if (h->next == hook) {
+                h->next = hook->next;
+                success = TRUE;
+                break;
+            }
+        } while ((h = h->next));
+    }
+
+    if ((r = pthread_rwlock_unlock(&rb_internal_thread_event_hooks_rw_lock))) {
+        rb_bug_errno("pthread_rwlock_unlock", r);
+    }
+
+    if (success) {
+        ruby_xfree(hook);
+    }
+    return success;
+}
+
+static void
+rb_thread_execute_hooks(rb_event_flag_t event)
+{
+    int r;
+    if ((r = pthread_rwlock_rdlock(&rb_internal_thread_event_hooks_rw_lock))) {
+        rb_bug_errno("pthread_rwlock_rdlock", r);
+    }
+
+    if (rb_internal_thread_event_hooks) {
+        rb_internal_thread_event_hook_t *h = rb_internal_thread_event_hooks;
+        do {
+            if (h->event & event) {
+                (*h->callback)(event, NULL, h->user_data);
+            }
+        } while((h = h->next));
+    }
+    if ((r = pthread_rwlock_unlock(&rb_internal_thread_event_hooks_rw_lock))) {
+        rb_bug_errno("pthread_rwlock_unlock", r);
+    }
+}
+
 enum rtimer_state {
     /* alive, after timer_create: */
     RTIMER_DISARM,
@@ -295,6 +384,10 @@ thread_sched_to_running_common(struct rb_thread_sched *sched, rb_thread_t *th)
         // waiting -> ready
         thread_sched_to_ready_common(sched, th);
 
+        if (rb_internal_thread_event_hooks) {
+            rb_thread_execute_hooks(RUBY_INTERNAL_THREAD_EVENT_READY);
+        }
+
         // wait for running chance
         do {
             if (!sched->timer) {
@@ -319,6 +412,10 @@ thread_sched_to_running_common(struct rb_thread_sched *sched, rb_thread_t *th)
     // ready -> running
     sched->running = th;
 
+    if (rb_internal_thread_event_hooks) {
+        rb_thread_execute_hooks(RUBY_INTERNAL_THREAD_EVENT_RESUMED);
+    }
+
     if (!sched->timer) {
         if (!designate_timer_thread(sched) && !ubf_threads_empty()) {
             rb_thread_wakeup_timer_thread(-1);
@@ -337,6 +434,10 @@ thread_sched_to_running(struct rb_thread_sched *sched, rb_thread_t *th)
 static rb_thread_t *
 thread_sched_to_waiting_common(struct rb_thread_sched *sched)
 {
+    if (rb_internal_thread_event_hooks) {
+        rb_thread_execute_hooks(RUBY_INTERNAL_THREAD_EVENT_SUSPENDED);
+    }
+
     rb_thread_t *next;
     sched->running = NULL;
     next = ccan_list_top(&sched->readyq, rb_thread_t, sched.node.readyq);
@@ -687,9 +788,14 @@ native_thread_init(struct rb_native_thread *nt)
 void
 Init_native_thread(rb_thread_t *main_th)
 {
+    int r;
+    if ((r = pthread_rwlock_init(&rb_internal_thread_event_hooks_rw_lock, NULL))) {
+        rb_bug_errno("pthread_rwlock_init", r);
+    }
+
 #if defined(HAVE_PTHREAD_CONDATTR_SETCLOCK)
     if (condattr_monotonic) {
-        int r = pthread_condattr_init(condattr_monotonic);
+        r = pthread_condattr_init(condattr_monotonic);
         if (r == 0) {
             r = pthread_condattr_setclock(condattr_monotonic, CLOCK_MONOTONIC);
         }

thread_win32.c

@@ -29,6 +29,18 @@ static volatile DWORD ruby_native_thread_key = TLS_OUT_OF_INDEXES;
 
 static int w32_wait_events(HANDLE *events, int count, DWORD timeout, rb_thread_t *th);
 
+rb_internal_thread_event_hook_t *
+rb_internal_thread_add_event_hook(rb_internal_thread_event_callback callback, rb_event_flag_t internal_event, void *user_data)
+{
+    // not implemented
+}
+
+bool
+rb_internal_thread_remove_event_hook(rb_internal_thread_event_hook_t * hook)
+{
+    // not implemented
+}
+
 RBIMPL_ATTR_NORETURN()
 static void
 w32_error(const char *func)