M:N thread scheduler for Ractors

This patch introduce M:N thread scheduler for Ractor system.

In general, M:N thread scheduler employs N native threads (OS threads)
to manage M user-level threads (Ruby threads in this case).
On the Ruby interpreter, 1 native thread is provided for 1 Ractor
and all Ruby threads are managed by the native thread.

From Ruby 1.9, the interpreter uses 1:1 thread scheduler which means
1 Ruby thread has 1 native thread. M:N scheduler change this strategy.

Because of compatibility issue (and stableness issue of the implementation)
main Ractor doesn't use M:N scheduler on default. On the other words,
threads on the main Ractor will be managed with 1:1 thread scheduler.

There are additional settings by environment variables:

`RUBY_MN_THREADS=1` enables M:N thread scheduler on the main ractor.
Note that non-main ractors use the M:N scheduler without this
configuration. With this configuration, single ractor applications
run threads on M:1 thread scheduler (green threads, user-level threads).

`RUBY_MAX_CPU=n` specifies maximum number of native threads for
M:N scheduler (default: 8).

This patch will be reverted soon if non-easy issues are found.

[Bug #19842]

1 files changed, 18 insertions, 0 deletions

diff --git a/eval.c b/eval.c
index 844c537cc4..be450a02f5 100644
--- a/eval.c
+++ b/eval.c
@@ -2110,3 +2110,21 @@ Init_eval(void)
     id_signo = rb_intern_const("signo");
     id_status = rb_intern_const("status");
 }
+
+int
+rb_errno(void)
+{
+    return *rb_orig_errno_ptr();
+}
+
+void
+rb_errno_set(int e)
+{
+    *rb_orig_errno_ptr() = e;
+}
+
+int *
+rb_errno_ptr(void)
+{
+    return rb_orig_errno_ptr();
+}