From 92f2ab2e1b5a7c02ea6d3a3a07c7fbbfc725cdea Mon Sep 17 00:00:00 2001
From: William Kemper <wkemper@openjdk.org>
Date: Wed, 8 Oct 2025 18:14:26 +0000
Subject: [PATCH] 8264851: Shenandoah: Rework control loop mechanics to use
 timed waits

Reviewed-by: kdnilsen, shade
---
 .../gc/shenandoah/shenandoahControlThread.cpp | 27 ++++++++++++-------
 .../gc/shenandoah/shenandoahControlThread.hpp |  5 ++++
 2 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp b/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp
index b960255891f..590e7831f07 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp
@@ -43,7 +43,8 @@
 ShenandoahControlThread::ShenandoahControlThread() :
   ShenandoahController(),
   _requested_gc_cause(GCCause::_no_gc),
-  _degen_point(ShenandoahGC::_degenerated_outside_cycle) {
+  _degen_point(ShenandoahGC::_degenerated_outside_cycle),
+  _control_lock(Mutex::nosafepoint - 2, "ShenandoahGCRequest_lock", true) {
   set_name("Shenandoah Control Thread");
   create_and_start();
 }
@@ -228,7 +229,9 @@ void ShenandoahControlThread::run_service() {
       sleep = MIN2<int>(ShenandoahControlIntervalMax, MAX2(1, sleep * 2));
       last_sleep_adjust_time = current;
     }
-    os::naked_short_sleep(sleep);
+
+    MonitorLocker ml(&_control_lock, Mutex::_no_safepoint_check_flag);
+    ml.wait(sleep);
   }
 }
 
@@ -343,6 +346,16 @@ void ShenandoahControlThread::request_gc(GCCause::Cause cause) {
   }
 }
 
+void ShenandoahControlThread::notify_control_thread(GCCause::Cause cause) {
+  // Although setting gc request is under _controller_lock, the read side (run_service())
+  // does not take the lock. We need to enforce following order, so that read side sees
+  // latest requested gc cause when the flag is set.
+  MonitorLocker controller(&_control_lock, Mutex::_no_safepoint_check_flag);
+  _requested_gc_cause = cause;
+  _gc_requested.set();
+  controller.notify();
+}
+
 void ShenandoahControlThread::handle_requested_gc(GCCause::Cause cause) {
   if (should_terminate()) {
     log_info(gc)("Control thread is terminating, no more GCs");
@@ -354,8 +367,7 @@ void ShenandoahControlThread::handle_requested_gc(GCCause::Cause cause) {
   // The whitebox caller thread will arrange for itself to wait until the GC notifies
   // it that has reached the requested breakpoint (phase in the GC).
   if (cause == GCCause::_wb_breakpoint) {
-    _requested_gc_cause = cause;
-    _gc_requested.set();
+    notify_control_thread(cause);
     return;
   }
 
@@ -372,12 +384,7 @@ void ShenandoahControlThread::handle_requested_gc(GCCause::Cause cause) {
   size_t current_gc_id = get_gc_id();
   size_t required_gc_id = current_gc_id + 1;
   while (current_gc_id < required_gc_id && !should_terminate()) {
-    // Although setting gc request is under _gc_waiters_lock, but read side (run_service())
-    // does not take the lock. We need to enforce following order, so that read side sees
-    // latest requested gc cause when the flag is set.
-    _requested_gc_cause = cause;
-    _gc_requested.set();
-
+    notify_control_thread(cause);
     ml.wait();
     current_gc_id = get_gc_id();
   }
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahControlThread.hpp b/src/hotspot/share/gc/shenandoah/shenandoahControlThread.hpp
index 4975404bbaa..c9bb6419201 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahControlThread.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahControlThread.hpp
@@ -45,6 +45,9 @@ private:
   GCCause::Cause       _requested_gc_cause;
   ShenandoahGC::ShenandoahDegenPoint _degen_point;
 
+  // This lock is used to coordinate waking up the control thread
+  Monitor _control_lock;
+
 public:
   ShenandoahControlThread();
 
@@ -54,6 +57,8 @@ public:
   void request_gc(GCCause::Cause cause) override;
 
 private:
+  // Sets the requested cause and flag and notifies the control thread
+  void notify_control_thread(GCCause::Cause cause);
 
   bool check_cancellation_or_degen(ShenandoahGC::ShenandoahDegenPoint point);
   void service_concurrent_normal_cycle(GCCause::Cause cause);