Revert "Wake timer to create new SNT when needed for dedicated task (#16009)"

This reverts commit 7b3370a5579956404d742a2e104d72e7c89480e4.

3 files changed, 20 insertions, 73 deletions

diff --git a/test/ruby/test_ractor.rb b/test/ruby/test_ractor.rb
index d449f9f814..6ae511217a 100644
--- a/test/ruby/test_ractor.rb
+++ b/test/ruby/test_ractor.rb
@@ -201,45 +201,6 @@ class TestRactor < Test::Unit::TestCase
     RUBY
   end
 
-  def test_timer_thread_create_snt_for_dedicated_task
-    omit "timer thread works differently" if windows?
-    omit "test relies on this as a best-effort safety mechanism" unless defined?(Process::WNOHANG)
-    assert_separately([{ "RUBY_MAX_CPU" => "2" }], <<~'RUBY', timeout: 30)
-      $VERBOSE = nil
-      CHILD_PID = 0
-
-      rs = []
-      2.times do |i|
-        rs << Ractor.new(i) do |j|
-          if j == 0
-            pid = spawn("sleep 60", close_others: true)
-            Object.const_set(:CHILD_PID, pid)
-            Process.waitpid(pid) # block forever (dedicated task)
-          else
-            while CHILD_PID == 0
-              sleep 1 # make sure first ractor blocks forever first (this is what we're testing)
-            end
-            1_000.times do
-              [nil] * 100
-            end
-          end
-        end
-      end
-
-      rs.last.join
-      begin
-        result = Process.waitpid(CHILD_PID, Process::WNOHANG)
-      rescue Errno::ECHILD, Errno::ESRCH
-        # If it's somehow not a child (not running?), don't send it a signal
-      else
-        if result.nil?
-          Process.kill('KILL', CHILD_PID) rescue nil
-        end
-      end
-      rs.first.join # reap
-    RUBY
-  end
-
   def test_symbol_proc_is_shareable
     pr = :symbol.to_proc
     assert_make_shareable(pr)
diff --git a/thread_pthread.c b/thread_pthread.c
index f455ac828e..5b461efec8 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -90,11 +90,6 @@ static const void *const condattr_monotonic = NULL;
   #endif
 #endif
 
-#ifndef MINIMUM_SNT
-// make at least MINIMUM_SNT snts for debug.
-#define MINIMUM_SNT 0
-#endif
-
 #ifdef HAVE_SCHED_YIELD
 #define native_thread_yield() (void)sched_yield()
 #else
@@ -559,20 +554,6 @@ ractor_sched_timeslice_threads_contain_p(rb_vm_t *vm, rb_thread_t *th)
 
 static void ractor_sched_barrier_join_signal_locked(rb_vm_t *vm);
 
-static bool
-need_more_shared_native_threads_p(rb_vm_t *vm)
-{
-    ASSERT_ractor_sched_locked(vm, NULL);
-
-    unsigned int schedulable_ractor_cnt = vm->ractor.cnt;
-    unsigned int snt_cnt = vm->ractor.sched.snt_cnt;
-    RUBY_ASSERT(schedulable_ractor_cnt >= 1);
-    if (!vm->ractor.main_ractor->threads.sched.enable_mn_threads) {
-        schedulable_ractor_cnt--; // do not need snt for main ractor
-    }
-    return snt_cnt < MINIMUM_SNT || (snt_cnt < schedulable_ractor_cnt && snt_cnt < vm->ractor.sched.max_cpu);
-}
-
 // setup timeslice signals by the timer thread.
 static void
 thread_sched_setup_running_threads(struct rb_thread_sched *sched, rb_ractor_t *cr, rb_vm_t *vm,
@@ -583,7 +564,6 @@ thread_sched_setup_running_threads(struct rb_thread_sched *sched, rb_ractor_t *c
 #endif
 
     rb_thread_t *del_timeslice_th;
-    bool wakeup_timer_thread = false;
 
     if (del_th && sched->is_running_timeslice) {
         del_timeslice_th = del_th;
@@ -612,12 +592,6 @@ thread_sched_setup_running_threads(struct rb_thread_sched *sched, rb_ractor_t *c
                 ractor_sched_barrier_join_signal_locked(vm);
             }
             sched->is_running = false;
-
-            // If we need more SNTs, the timer thread should be awake and monitoring the situation so it can correct it.
-            if (!del_th->has_dedicated_nt && del_th->nt->dedicated > 0 && (sched->running != NULL || vm->ractor.sched.grq_cnt > 0) &&
-                need_more_shared_native_threads_p(vm)) {
-                wakeup_timer_thread = true;
-            }
         }
 
         if (add_th) {
@@ -642,7 +616,7 @@ thread_sched_setup_running_threads(struct rb_thread_sched *sched, rb_ractor_t *c
             ccan_list_add(&vm->ractor.sched.timeslice_threads, &add_timeslice_th->sched.node.timeslice_threads);
             sched->is_running_timeslice = true;
             if (was_empty) {
-                wakeup_timer_thread = true;
+                timer_thread_wakeup_locked(vm);
             }
         }
 
@@ -651,10 +625,6 @@ thread_sched_setup_running_threads(struct rb_thread_sched *sched, rb_ractor_t *c
             ccan_list_del_init(&del_timeslice_th->sched.node.timeslice_threads);
         }
 
-        if (wakeup_timer_thread) {
-            timer_thread_wakeup_locked(vm);
-        }
-
         VM_ASSERT(ractor_sched_running_threads_size(vm) == vm->ractor.sched.running_cnt);
         VM_ASSERT(ractor_sched_timeslice_threads_size(vm) <= vm->ractor.sched.running_cnt);
     }
@@ -1294,6 +1264,11 @@ ractor_sched_enq(rb_vm_t *vm, rb_ractor_t *r)
 #define SNT_KEEP_SECONDS 0
 #endif
 
+#ifndef MINIMUM_SNT
+// make at least MINIMUM_SNT snts for debug.
+#define MINIMUM_SNT 0
+#endif
+
 static rb_ractor_t *
 ractor_sched_deq(rb_vm_t *vm, rb_ractor_t *cr)
 {
diff --git a/thread_pthread_mn.c b/thread_pthread_mn.c
index d6602cea77..69e81e5fbc 100644
--- a/thread_pthread_mn.c
+++ b/thread_pthread_mn.c
@@ -394,14 +394,25 @@ nt_free_stack(void *mstack)
     rb_native_mutex_unlock(&nt_machine_stack_lock);
 }
 
+
 static int
 native_thread_check_and_create_shared(rb_vm_t *vm)
 {
     bool need_to_make = false;
 
-    ractor_sched_lock(vm, NULL);
+    rb_native_mutex_lock(&vm->ractor.sched.lock);
     {
-        if (need_more_shared_native_threads_p(vm)) {
+        unsigned int schedulable_ractor_cnt = vm->ractor.cnt;
+        RUBY_ASSERT(schedulable_ractor_cnt >= 1);
+
+        if (!vm->ractor.main_ractor->threads.sched.enable_mn_threads)
+            schedulable_ractor_cnt--; // do not need snt for main ractor
+
+        unsigned int snt_cnt = vm->ractor.sched.snt_cnt;
+        if (((int)snt_cnt < MINIMUM_SNT) ||
+            (snt_cnt < schedulable_ractor_cnt  &&
+             snt_cnt < vm->ractor.sched.max_cpu)) {
+
             RUBY_DEBUG_LOG("added snt:%u dnt:%u ractor_cnt:%u grq_cnt:%u",
                            vm->ractor.sched.snt_cnt,
                            vm->ractor.sched.dnt_cnt,
@@ -415,7 +426,7 @@ native_thread_check_and_create_shared(rb_vm_t *vm)
             RUBY_DEBUG_LOG("snt:%d ractor_cnt:%d", (int)vm->ractor.sched.snt_cnt, (int)vm->ractor.cnt);
         }
     }
-    ractor_sched_unlock(vm, NULL);
+    rb_native_mutex_unlock(&vm->ractor.sched.lock);
 
     if (need_to_make) {
         struct rb_native_thread *nt = native_thread_alloc();