rb_thread_lock_native_thread()

Introduce `rb_thread_lock_native_thread()` to allocate dedicated
native thread to the current Ruby thread for M:N threads.
This C API is similar to Go's `runtime.LockOSThread()`.

Accepted at https://github.com/ruby/dev-meeting-log/blob/master/2023/DevMeeting-2023-08-24.md
(and missed to implement on Ruby 3.3.0)

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

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

ext/-test-/thread/lock_native_thread/lock_native_thread.c

@@ -0,0 +1,49 @@
+
+#include "ruby/ruby.h"
+#include "ruby/thread.h"
+
+#ifdef HAVE_PTHREAD_H
+#include <pthread.h>
+
+static pthread_key_t tls_key;
+
+static VALUE
+get_tls(VALUE self)
+{
+    return (VALUE)pthread_getspecific(tls_key);
+}
+
+static VALUE
+set_tls(VALUE self, VALUE vn)
+{
+    pthread_setspecific(tls_key, (void *)vn);
+    return Qnil;
+}
+
+static VALUE
+lock_native_thread(VALUE self)
+{
+    return rb_thread_lock_native_thread() ? Qtrue : Qfalse;
+}
+
+void
+Init_lock_native_thread(void)
+{
+    int r;
+
+    if ((r = pthread_key_create(&tls_key, NULL)) != 0) {
+        rb_bug("pthread_key_create() returns %d", r);
+    }
+    pthread_setspecific(tls_key, NULL);
+
+    rb_define_method(rb_cThread, "lock_native_thread", lock_native_thread, 0);
+    rb_define_method(rb_cThread, "get_tls", get_tls, 0);
+    rb_define_method(rb_cThread, "set_tls", set_tls, 1);
+}
+
+#else // HAVE_PTHREAD_H
+Init_lock_native_thread(void)
+{
+    // do nothing
+}
+#endif // HAVE_PTHREAD_H

include/ruby/thread.h

@@ -190,6 +190,19 @@ void *rb_nogvl(void *(*func)(void *), void *data1,
  */
 #define RUBY_CALL_WO_GVL_FLAG_SKIP_CHECK_INTS_
 
+/**
+ * Declare the current Ruby thread should acquire a dedicated
+ * native thread on M:N thread scheduler.
+ *
+ * If a C extension (or a library which the extension relies on) should
+ * keep to run on a native thread (e.g. using thread-local-storage),
+ * this function allocates a dedicated native thread for the thread.
+ *
+ * @return `false` if the thread already running on a dedicated native
+ *         thread. Otherwise `true`.
+ */
+bool rb_thread_lock_native_thread(void);
+
 /**
  * Triggered when a new thread is started.
  *

test/-ext-/thread/test_lock_native_thread.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: false
+
+require 'envutil'
+
+mn_supported_p = -> do
+  out, *_ = EnvUtil.invoke_ruby([{'RUBY_MN_THREADS' => '1'}, '-v'], '', true)
+  return /\+MN/ =~ out
+end
+
+if mn_supported_p.call
+  # test only on MN threads
+else
+  return
+end
+
+class TestThreadLockNativeThread < Test::Unit::TestCase
+  def test_lock_native_thread
+    assert_separately([{'RUBY_MN_THREADS' => '1'}], <<-RUBY)
+      require '-test-/thread/lock_native_thread'
+
+      Thread.new{
+        assert_equal true, Thread.current.lock_native_thread
+      }.join
+
+      # main thread already has DNT
+      assert_equal false, Thread.current.lock_native_thread
+    RUBY
+  end
+
+  def test_lock_native_thread_tls
+    assert_separately([{'RUBY_MN_THREADS' => '1'}], <<-RUBY)
+      require '-test-/thread/lock_native_thread'
+      tn = 10
+      ln = 1_000
+
+      ts = tn.times.map{|i|
+        Thread.new(i){|i|
+          Thread.current.set_tls i
+          assert_equal true, Thread.current.lock_native_thread
+
+          ln.times{
+            assert_equal i, Thread.current.get_tls
+            Thread.pass
+          }
+        }
+      }
+      ts.each(&:join)
+    RUBY
+  end
+end

thread_none.c

@@ -318,4 +318,10 @@ rb_thread_sched_mark_zombies(rb_vm_t *vm)
     // do nothing
 }
 
+bool
+rb_thread_lock_native_thread(void)
+{
+    return false;
+}
+
 #endif /* THREAD_SYSTEM_DEPENDENT_IMPLEMENTATION */

thread_pthread.c

@@ -2304,6 +2304,11 @@ nt_start(void *ptr)
                 // timeout -> deleted.
                 break;
             }
+
+            if (nt->dedicated) {
+                // SNT becomes DNT while running
+                break;
+            }
         }
     }
 
@@ -3422,4 +3427,16 @@ rb_thread_execute_hooks(rb_event_flag_t event, rb_thread_t *th)
     }
 }
 
+// return true if the current thread acquires DNT.
+// return false if the current thread already acquires DNT.
+bool
+rb_thread_lock_native_thread(void)
+{
+    rb_thread_t *th = GET_THREAD();
+    bool is_snt = th->nt->dedicated == 0;
+    native_thread_dedicated_inc(th->vm, th->ractor, th->nt);
+
+    return is_snt;
+}
+
 #endif /* THREAD_SYSTEM_DEPENDENT_IMPLEMENTATION */

thread_pthread_mn.c

@@ -439,27 +439,35 @@ co_start(struct coroutine_context *from, struct coroutine_context *self)
 
     // Thread is terminated
 
-    VM_ASSERT(!th_has_dedicated_nt(th));
-
-    rb_vm_t *vm = th->vm;
-    bool has_ready_ractor = vm->ractor.sched.grq_cnt > 0; // at least this ractor is not queued
-
-    rb_thread_t *next_th = sched->running;
     struct rb_native_thread *nt = th->nt;
+    bool is_dnt = th_has_dedicated_nt(th);
     native_thread_assign(NULL, th);
     rb_ractor_set_current_ec(th->ractor, NULL);
 
-    if (!has_ready_ractor && next_th && !next_th->nt) {
-        // switch to the next thread
-        thread_sched_set_lock_owner(sched, NULL);
-        thread_sched_switch0(th->sched.context, next_th, nt, true);
-        th->sched.finished = true;
-    }
-    else {
-        // switch to the next Ractor
+    if (is_dnt) {
+        // SNT became DNT while running. Just return to the nt_context
+
         th->sched.finished = true;
         coroutine_transfer0(self, nt->nt_context, true);
     }
+    else {
+        rb_vm_t *vm = th->vm;
+        bool has_ready_ractor = vm->ractor.sched.grq_cnt > 0; // at least this ractor is not queued
+        rb_thread_t *next_th = sched->running;
+
+        if (!has_ready_ractor && next_th && !next_th->nt) {
+            // switch to the next thread
+            thread_sched_set_lock_owner(sched, NULL);
+            thread_sched_switch0(th->sched.context, next_th, nt, true);
+            th->sched.finished = true;
+        }
+        else {
+            // switch to the next Ractor
+            th->sched.finished = true;
+            coroutine_transfer0(self, nt->nt_context, true);
+        }
+    }
+
     rb_bug("unreachable");
 }
 

thread_win32.c

@@ -1003,4 +1003,10 @@ rb_ractor_sched_barrier_join(rb_vm_t *vm, rb_ractor_t *cr)
         vm->ractor.sync.lock_owner = NULL;
 }
 
+bool
+rb_thread_lock_native_thread(void)
+{
+    return false;
+}
+
 #endif /* THREAD_SYSTEM_DEPENDENT_IMPLEMENTATION */