Index: /trunk/src/VBox/Runtime/r0drv/linux/thread2-r0drv-linux.c
===================================================================
--- /trunk/src/VBox/Runtime/r0drv/linux/thread2-r0drv-linux.c	(revision 86547)
+++ /trunk/src/VBox/Runtime/r0drv/linux/thread2-r0drv-linux.c	(revision 86548)
@@ -55,42 +55,117 @@
 DECLHIDDEN(int) rtThreadNativeSetPriority(PRTTHREADINT pThread, RTTHREADTYPE enmType)
 {
-    int                 iSchedClass = SCHED_FIFO;
-    int                 rc = VINF_SUCCESS;
-    struct sched_param  Param = { 0 };
-
-    RT_NOREF_PV(pThread);
-#if RTLNX_VER_MIN(5,9,0)
-    RT_NOREF_PV(iSchedClass);
-    RT_NOREF_PV(Param);
-#endif
+#if RTLNX_VER_MIN(2,5,2)
+    /*
+     * Assignments are partially based on g_aTypesLinuxFree but
+     * scaled up in the high priority end.
+     *
+     * 5.9.0 - :
+     *      The sched_set_normal interfaces does not really check the input,
+     *      whereas sched_set_fifo & sched_set_fifo_low have fixed assignments.
+     * 2.6.11 - 5.9.0:
+     *      Use sched_setscheduler to try effect FIFO scheduling
+     *      for IO and TIMER threads, otherwise use set_user_nice.
+     * 2.5.2 - 5.9.0:
+     *      Use set_user_nice to renice the thread.
+     */
+    int                 iNice       = 0;
+# if RTLNX_VER_MAX(5,9,0)
+    int                 rc;
+#  if RTLNX_VER_MIN(2,6,11)
+    int                 iSchedClass = SCHED_NORMAL;
+    struct sched_param  Param       = { .sched_priority = 0 };
+#  endif
+# endif
     switch (enmType)
     {
+        case RTTHREADTYPE_INFREQUENT_POLLER:
+            iNice = +3;
+            break;
+
+        case RTTHREADTYPE_MAIN_HEAVY_WORKER:
+            iNice = +2;
+            break;
+
+        case RTTHREADTYPE_EMULATION:
+            iNice = +1;
+            break;
+
+        case RTTHREADTYPE_DEFAULT:
+        case RTTHREADTYPE_GUI:
+        case RTTHREADTYPE_MAIN_WORKER:
+            iNice =  0;
+            break;
+
+        case RTTHREADTYPE_VRDP_IO:
+        case RTTHREADTYPE_DEBUGGER:
+            iNice = -1;
+            break;
+
+        case RTTHREADTYPE_MSG_PUMP:
+            iNice = -2;
+            break;
+
         case RTTHREADTYPE_IO:
-#if RTLNX_VER_MAX(5,9,0)
-            /* Set max. priority to preempt all other threads on this CPU. */
-            Param.sched_priority = MAX_RT_PRIO - 1;
-#else 
-            /* Effectively changes prio to 50 */
+# if RTLNX_VER_MIN(5,9,0)
+            sched_set_fifo_low(current);
+            return VINF_SUCCESS;
+# else
+            iNice = -12;
+#  if RTLNX_VER_MIN(2,6,11)
+            iSchedClass = SCHED_FIFO;
+            Param.sched_priority = 1; /* => prio=98; */
+#  endif
+            break;
+# endif
+
+        case RTTHREADTYPE_TIMER:
+# if RTLNX_VER_MIN(5,9,0)
             sched_set_fifo(current);
-#endif
-            break;
-        case RTTHREADTYPE_TIMER:
-#if RTLNX_VER_MAX(5,9,0)
-            Param.sched_priority = 1; /* not 0 just in case */
-#else
-            /* Just one above SCHED_NORMAL class */
-            sched_set_fifo_low(current);
-#endif
-            break;
+            return VINF_SUCCESS;
+# else
+            iNice = -19;
+#  if RTLNX_VER_MIN(2,6,11)
+            iSchedClass = SCHED_FIFO;
+            Param.sched_priority = MAX_RT_PRIO / 2; /* => prio=49 */
+#  endif
+            break;
+# endif
         default:
-            /* pretend success instead of VERR_NOT_SUPPORTED */
-            return rc;
+            AssertMsgFailedReturn(("enmType=%d\n", enmType), VERR_INVALID_PARAMETER);
     }
-#if RTLNX_VER_MAX(5,9,0)
-    if ((sched_setscheduler(current, iSchedClass, &Param)) != 0) {
-        rc = VERR_GENERAL_FAILURE;
+
+# if RTLNX_VER_MIN(5,9,0)
+    /*
+     * We only get here for renice work.
+     */
+    sched_set_normal(current, iNice);
+
+# else  /* < 5.9.0 */
+#  if RTLNX_VER_MIN(2,6,11)
+    /*
+     * Try set scheduler parameters.
+     * Fall back on normal + nice if this fails for FIFO policy.*
+     */
+    rc = sched_setscheduler(current, iSchedClass, &Param);
+    if (rc)
+    {
+        Param.sched_priority = 0;
+        iSchedClass = SCHED_NORMAL;
+        rc = sched_setscheduler(current, iSchedClass, &Param);
     }
-#endif
-    return rc;
+
+    /*
+     * Renice if using normal scheduling class.
+     */
+    if (iSchedClass == SCHED_NORMAL)
+#  endif /* >= 2.6.11 */
+        set_user_nice(current, iNice);
+
+# endif /* < 5.9.0 */
+#else  /* < 2.5.2 */
+    RT_NOREF_PV(enmType);
+#endif /* < 2.5.2 */
+    RT_NOREF_PV(pThread);
+    return VINF_SUCCESS;
 }