Lower daemons priority to 124 (art).

To avoid jank.

Bug: 36631902
Test: boot marlin oc-dev and check priorities.
Test: test-art-host
Change-Id: I8f809fbf5ac9605f656884fcf68643c87f22d154
(cherry picked from commit 38a57ef702964b82836f89a1f996aff4d1453d94)
diff --git a/runtime/native/dalvik_system_VMRuntime.cc b/runtime/native/dalvik_system_VMRuntime.cc
index d81c13d..6ca951f 100644
--- a/runtime/native/dalvik_system_VMRuntime.cc
+++ b/runtime/native/dalvik_system_VMRuntime.cc
@@ -17,6 +17,8 @@
 #include "dalvik_system_VMRuntime.h"
 
 #ifdef ART_TARGET_ANDROID
+#include <sys/time.h>
+#include <sys/resource.h>
 extern "C" void android_set_application_target_sdk_version(uint32_t version);
 #endif
 #include <limits.h>
@@ -625,6 +627,23 @@
   return Runtime::Current()->GetPrunedDalvikCache() ? JNI_TRUE : JNI_FALSE;
 }
 
+static void VMRuntime_setSystemDaemonThreadPriority(JNIEnv* env ATTRIBUTE_UNUSED,
+                                                    jclass klass ATTRIBUTE_UNUSED) {
+#ifdef ART_TARGET_ANDROID
+  Thread* self = Thread::Current();
+  DCHECK(self != nullptr);
+  pid_t tid = self->GetTid();
+  // We use a priority lower than the default for the system daemon threads (eg HeapTaskDaemon) to
+  // avoid jank due to CPU contentions between GC and other UI-related threads. b/36631902.
+  // We may use a native priority that doesn't have a corresponding java.lang.Thread-level priority.
+  static constexpr int kSystemDaemonNiceValue = 4;  // priority 124
+  if (setpriority(PRIO_PROCESS, tid, kSystemDaemonNiceValue) != 0) {
+    PLOG(INFO) << *self << " setpriority(PRIO_PROCESS, " << tid << ", "
+               << kSystemDaemonNiceValue << ") failed";
+  }
+#endif
+}
+
 static JNINativeMethod gMethods[] = {
   FAST_NATIVE_METHOD(VMRuntime, addressOf, "(Ljava/lang/Object;)J"),
   NATIVE_METHOD(VMRuntime, bootClassPath, "()Ljava/lang/String;"),
@@ -662,6 +681,7 @@
   NATIVE_METHOD(VMRuntime, isBootClassPathOnDisk, "(Ljava/lang/String;)Z"),
   NATIVE_METHOD(VMRuntime, getCurrentInstructionSet, "()Ljava/lang/String;"),
   NATIVE_METHOD(VMRuntime, didPruneDalvikCache, "()Z"),
+  NATIVE_METHOD(VMRuntime, setSystemDaemonThreadPriority, "()V"),
 };
 
 void register_dalvik_system_VMRuntime(JNIEnv* env) {