Execution of system tasks in context of another process

A process requesting a system task to be executed in the context of
another process will be notified by a message when the task has
executed. This message will be on the form:
  {RequestType, RequestId, Pid, Result}.

A process requesting a system task to be executed can set priority
on the system task. The requester typically set the same priority
on the task as its own process priority, and by this avoiding
priority inversion. A request for execution of a system task is
made by calling the statically linked in NIF
erts_internal:request_system_task(Pid, Prio, Request). This is an
undocumented ERTS internal function that should remain so. It
should *only* be called from BIF implementations.

Currently defined system tasks are:
* garbage_collect
* check_process_code

Further system tasks can and will be implemented in the future.

The erlang:garbage_collect/[1,2] and erlang:check_process_code/[2,3]
BIFs are now implemented using system tasks. Both the
'garbage_collect' and the 'check_process_code' operations perform
or may perform garbage_collections. By doing these via the
system task functionality all garbage collect operations in the
system will be performed solely in the context of the process
being garbage collected. This makes it possible to later implement
functionality for disabling garbage collection of a process over
context switches.

Newly introduced BIFs:

* erlang:garbage_collect/2 - The new second argument is an option
  list. Introduced option:
  * {async, RequestId} - making it possible for users to issue
    asynchronous garbage collect requests.

* erlang:check_process_code/3 -  The new third argument is an
  option list. Introduced options:
  * {async, RequestId} - making it possible for users to issue
    asynchronous check process code requests.
  * {allow_gc, boolean()} - making it possible to issue requests
    that aren't allowed to garbage collect (operation will abort
    if gc should be needed).
  These options have been introduced as a preparation for
  parallelization of check_process_code operations when the
  code_server is about to purge a module.

1 files changed, 1100 insertions, 124 deletions

diff --git a/erts/emulator/beam/erl_process.c b/erts/emulator/beam/erl_process.c
index 434d5ca147..ad806da660 100644
--- a/erts/emulator/beam/erl_process.c
+++ b/erts/emulator/beam/erl_process.c
@@ -283,6 +283,40 @@ struct erts_system_profile_flags_t erts_system_profile_flags;
 #error "Need to store process_count in another type"
 #endif
 
+typedef enum {
+    ERTS_PSTT_GC,	/* Garbage Collect */
+    ERTS_PSTT_CPC	/* Check Process Code */
+} ErtsProcSysTaskType;
+
+#define ERTS_MAX_PROC_SYS_TASK_ARGS 2
+
+struct ErtsProcSysTask_ {
+    ErtsProcSysTask *next;
+    ErtsProcSysTask *prev;
+    ErtsProcSysTaskType type;
+    Eterm requester;
+    Eterm reply_tag;
+    Eterm req_id;
+    Uint req_id_sz;
+    Eterm arg[ERTS_MAX_PROC_SYS_TASK_ARGS];
+    ErlOffHeap off_heap;
+    Eterm heap[1];
+};
+
+#define ERTS_PROC_SYS_TASK_SIZE(HSz) \
+    (sizeof(ErtsProcSysTask) - sizeof(Eterm) + sizeof(Eterm)*(HSz))
+
+struct ErtsProcSysTaskQs_ {
+    int qmask;
+    int ncount;
+    ErtsProcSysTask *q[ERTS_NO_PROC_PRIO_LEVELS];
+};
+
+ERTS_SCHED_PREF_QUICK_ALLOC_IMPL(proc_sys_task_queues,
+				 ErtsProcSysTaskQs,
+				 50,
+				 ERTS_ALC_T_PROC_SYS_TSK_QS)
+
 ERTS_SCHED_PREF_QUICK_ALLOC_IMPL(misc_op_list,
 				 ErtsMiscOpList,
 				 10,
@@ -353,6 +387,14 @@ static void aux_work_timeout_early_init(int no_schedulers);
 static void aux_work_timeout_late_init(void);
 static void setup_aux_work_timer(void);
 
+static int execute_sys_tasks(Process *c_p,
+			     erts_aint32_t *statep,
+			     int in_reds);
+static int cleanup_sys_tasks(Process *c_p,
+			     erts_aint32_t in_state,
+			     int in_reds);
+
+
 #if defined(DEBUG) || 0
 #define ERTS_DBG_CHK_AUX_WORK_VAL(V) dbg_chk_aux_work_val((V))
 static void
@@ -2848,7 +2890,7 @@ dequeue_process(ErtsRunQueue *runq, int prio_q, erts_aint32_t *statep)
     if (statep)
 	*statep = state;
 
-    prio = (int) (ERTS_PSFLG_PRIO_MASK & state);
+    prio = (int) ERTS_PSFLGS_GET_PRQ_PRIO(state);
 
     rqi = &runq->procs.prio_info[prio];
 
@@ -2997,7 +3039,7 @@ immigrate(ErtsRunQueue *c_rq, ErtsMigrationPath *mp)
 		    erts_aint32_t state;
 		    state = erts_smp_atomic32_read_acqb(&proc->state);
 		    if (!(ERTS_PSFLG_BOUND & state)
-			&& (prio == (int) (ERTS_PSFLG_PRIO_MASK & state))) {
+			&& (prio == (int) ERTS_PSFLGS_GET_PRQ_PRIO(state))) {
 			ErtsRunQueueInfo *rqi = &rq->procs.prio_info[prio];
 			unqueue_process(rq, rpq, rqi, prio, prev_proc, proc);
 			erts_smp_runq_unlock(rq);
@@ -3079,7 +3121,7 @@ schedule_bound_processes(ErtsRunQueue *rq,
     while (proc) {
 	erts_aint32_t state = erts_smp_atomic32_read_acqb(&proc->state);
 	next = proc->next;
-	enqueue_process(rq, (int) (ERTS_PSFLG_PRIO_MASK & state), proc);
+	enqueue_process(rq, (int) ERTS_PSFLGS_GET_PRQ_PRIO(state), proc);
 	proc = next;
     }
 }
@@ -3184,7 +3226,7 @@ evacuate_run_queue(ErtsRunQueue *rq,
 		sbpp->last = proc;
 	    }
 	    else {
-		int prio = (int) (ERTS_PSFLG_PRIO_MASK & state);
+		int prio = (int) ERTS_PSFLGS_GET_PRQ_PRIO(state);
 		erts_smp_runq_unlock(rq);
 
 		to_rq = mp->prio[prio].runq;
@@ -3257,7 +3299,7 @@ try_steal_task_from_victim(ErtsRunQueue *rq, int *rq_lockedp, ErtsRunQueue *vrq,
 	    erts_aint32_t state = erts_smp_atomic32_read_acqb(&proc->state);
 	    if (!(ERTS_PSFLG_BOUND & state)) {
 		/* Steal process */
-		int prio = (int) (ERTS_PSFLG_PRIO_MASK & state);
+		int prio = (int) ERTS_PSFLGS_GET_PRQ_PRIO(state);
 		ErtsRunQueueInfo *rqi = &vrq->procs.prio_info[prio];
 		unqueue_process(vrq, rpq, rqi, prio, prev_proc, proc);
 		erts_smp_runq_unlock(vrq);
@@ -4575,6 +4617,7 @@ erts_init_scheduling(int no_schedulers, int no_schedulers_online)
 #endif
 
     init_misc_op_list_alloc();
+    init_proc_sys_task_queues_alloc();
 
 #ifdef ERTS_SMP
     set_wakeup_other_data();
@@ -4858,46 +4901,166 @@ erts_get_scheduler_data(void)
 
 #endif
 
+static Process *
+make_proxy_proc(Process *prev_proxy, Process *proc, erts_aint32_t prio)
+{
+    erts_aint32_t state;
+    Process *proxy;
+#ifdef ERTS_SMP
+    ErtsRunQueue *rq = RUNQ_READ_RQ(&proc->run_queue);
+#endif
+
+    state = (ERTS_PSFLG_PROXY
+	     | ERTS_PSFLG_IN_RUNQ
+	     | (((erts_aint32_t) 1) << (prio + ERTS_PSFLGS_IN_PRQ_MASK_OFFSET))
+	     | (prio << ERTS_PSFLGS_PRQ_PRIO_OFFSET)
+	     | (prio << ERTS_PSFLGS_USR_PRIO_OFFSET)
+	     | (prio << ERTS_PSFLGS_ACT_PRIO_OFFSET));
+
+    if (prev_proxy) {
+	proxy = prev_proxy;
+	ASSERT(erts_smp_atomic32_read_nob(&proxy->state) & ERTS_PSFLG_PROXY);
+	erts_smp_atomic32_set_nob(&proxy->state, state);
+#ifdef ERTS_SMP
+	RUNQ_SET_RQ(&proc->run_queue, rq);
+#endif
+    }
+    else {
+	proxy = erts_alloc(ERTS_ALC_T_PROC, sizeof(Process));
+#ifdef DEBUG
+	{
+	    int i;
+	    Uint32 *ui32 = (Uint32 *) (char *) proxy;
+	    for (i = 0; i < sizeof(Process)/sizeof(Uint32); i++)
+		ui32[i] = (Uint32) 0xdeadbeef;
+	}
+#endif
+	erts_smp_atomic32_init_nob(&proxy->state, state);
+#ifdef ERTS_SMP
+	erts_smp_atomic_init_nob(&proxy->run_queue,
+				 erts_smp_atomic_read_nob(&proc->run_queue));
+#endif
+    }
+
+    proxy->common.id = proc->common.id;
+
+    return proxy;
+}
+
+static ERTS_INLINE void
+free_proxy_proc(Process *proxy)
+{
+    ASSERT(erts_smp_atomic32_read_nob(&proxy->state) & ERTS_PSFLG_PROXY);
+    erts_free(ERTS_ALC_T_PROC, proxy);
+}
+
+
+static ERTS_INLINE int
+check_enqueue_in_prio_queue(erts_aint32_t *prq_prio_p,
+			    erts_aint32_t *newp,
+			    erts_aint32_t actual)
+{
+    erts_aint32_t aprio, qbit, max_qbit;
+
+    aprio = (actual >> ERTS_PSFLGS_ACT_PRIO_OFFSET) & ERTS_PSFLGS_PRIO_MASK;
+    qbit = 1 << aprio;
+
+    *prq_prio_p = aprio;
+
+    max_qbit = (actual >> ERTS_PSFLGS_IN_PRQ_MASK_OFFSET) & ERTS_PSFLGS_QMASK;
+    max_qbit |= 1 << ERTS_PSFLGS_QMASK_BITS;
+    max_qbit &= -max_qbit;
+    /*
+     * max_qbit now either contain bit set for highest prio queue or a bit
+     * out of range (which will have a value larger than valid range).
+     */
+
+    if (qbit >= max_qbit)
+	return 0; /* Already queued in higher or equal prio */
+
+    /* Need to enqueue (if already enqueued, it is in lower prio) */
+    *newp |= qbit << ERTS_PSFLGS_IN_PRQ_MASK_OFFSET;
+
+    if ((actual & (ERTS_PSFLG_IN_RUNQ|ERTS_PSFLGS_USR_PRIO_MASK))
+	!= (aprio << ERTS_PSFLGS_USR_PRIO_OFFSET)) {
+	/*
+	 * Process struct already enqueued, or actual prio not
+	 * equal to user prio, i.e., enqueue using proxy.
+	 */
+	return -1;
+    }
+
+    /*
+     * Enqueue using process struct.
+     */
+    *newp &= ~ERTS_PSFLGS_PRQ_PRIO_MASK;
+    *newp |= ERTS_PSFLG_IN_RUNQ | (aprio << ERTS_PSFLGS_PRQ_PRIO_OFFSET);
+    return 1;
+}
+
 /*
  * scheduler_out_process() return with c_rq locked.
  */
 static ERTS_INLINE int
-schedule_out_process(ErtsRunQueue *c_rq, erts_aint32_t state, Process *p)
+schedule_out_process(ErtsRunQueue *c_rq, erts_aint32_t state, Process *p, Process *proxy)
 {
-    erts_aint32_t a, e, n;
+    erts_aint32_t a, e, n, enq_prio = -1;
     int res = 0;
+    int enqueue; /* < 0 -> use proxy */
 
     a = state;
 
     while (1) {
 	n = e = a;
 
-	ASSERT(a & ERTS_PSFLG_RUNNING);
-	ASSERT(!(a & ERTS_PSFLG_IN_RUNQ));
+	ASSERT(a & (ERTS_PSFLG_RUNNING|ERTS_PSFLG_RUNNING_SYS));
 
-	n &= ~ERTS_PSFLG_RUNNING;
-	if ((a & (ERTS_PSFLG_ACTIVE|ERTS_PSFLG_SUSPENDED)) == ERTS_PSFLG_ACTIVE)
-	    n |= ERTS_PSFLG_IN_RUNQ;
+	enqueue = 0;
+
+	n &= ~(ERTS_PSFLG_RUNNING|ERTS_PSFLG_RUNNING_SYS);
+	if (a & ERTS_PSFLG_ACTIVE_SYS
+	    || (a & (ERTS_PSFLG_ACTIVE|ERTS_PSFLG_SUSPENDED)) == ERTS_PSFLG_ACTIVE) {
+	    enqueue = check_enqueue_in_prio_queue(&enq_prio, &n, a);
+	}
 	a = erts_smp_atomic32_cmpxchg_mb(&p->state, n, e);
 	if (a == e)
 	    break;
     }
 
-    if (!(n & ERTS_PSFLG_IN_RUNQ)) {
-	if (erts_system_profile_flags.runnable_procs)
-	    profile_runnable_proc(p, am_inactive);
+    if (!enqueue) {
+
+	if (erts_system_profile_flags.runnable_procs) {
+
+	    if (!(a & ERTS_PSFLG_ACTIVE_SYS)
+		&& (!(a & ERTS_PSFLG_ACTIVE)
+		    || (a & ERTS_PSFLG_SUSPENDED))) {
+		/* Process inactive */
+		profile_runnable_proc(p, am_inactive);
+	    }
+	}
+
+	if (proxy)
+	    free_proxy_proc(proxy);
     }
     else {
-	int prio = (int) (ERTS_PSFLG_PRIO_MASK & n);
+	Process *sched_p;
 	ErtsRunQueue *runq = erts_get_runq_proc(p);
 
-	ASSERT(!(n & ERTS_PSFLG_SUSPENDED));
+	ASSERT(!(n & ERTS_PSFLG_SUSPENDED) || (n & ERTS_PSFLG_ACTIVE_SYS));
+
+	if (enqueue < 0)
+	    sched_p = make_proxy_proc(proxy, p, enq_prio);
+	else {
+	    sched_p = p;
+	    if (proxy)
+		free_proxy_proc(proxy);
+	}
 
 #ifdef ERTS_SMP
 	if (!(ERTS_PSFLG_BOUND & n)) {
-	    ErtsRunQueue *new_runq = erts_check_emigration_need(runq, prio);
+	    ErtsRunQueue *new_runq = erts_check_emigration_need(runq, enq_prio);
 	    if (new_runq) {
-		RUNQ_SET_RQ(&p->run_queue, new_runq);
+		RUNQ_SET_RQ(&sched_p->run_queue, new_runq);
 		runq = new_runq;
 	    }
 	}
@@ -4908,7 +5071,7 @@ schedule_out_process(ErtsRunQueue *c_rq, erts_aint32_t state, Process *p)
 	erts_smp_runq_lock(runq);
 
 	/* Enqueue the process */
-	enqueue_process(runq, prio, p);
+	enqueue_process(runq, (int) enq_prio, sched_p);
 
 	if (runq == c_rq)
 	    return res;
@@ -4920,14 +5083,13 @@ schedule_out_process(ErtsRunQueue *c_rq, erts_aint32_t state, Process *p)
 }
 
 static ERTS_INLINE void
-add2runq(Process *p, erts_aint32_t state)
+add2runq(Process *p, erts_aint32_t state, erts_aint32_t prio)
 {
-    int prio = (int) (ERTS_PSFLG_PRIO_MASK & state);
     ErtsRunQueue *runq = erts_get_runq_proc(p);
 
 #ifdef ERTS_SMP
     if (!(ERTS_PSFLG_BOUND & state)) {
-	ErtsRunQueue *new_runq = erts_check_emigration_need(runq, prio);
+	ErtsRunQueue *new_runq = erts_check_emigration_need(runq, (int) prio);
 	if (new_runq) {
 	    RUNQ_SET_RQ(&p->run_queue, new_runq);
 	    runq = new_runq;
@@ -4939,101 +5101,236 @@ add2runq(Process *p, erts_aint32_t state)
     erts_smp_runq_lock(runq);
 
     /* Enqueue the process */
-    enqueue_process(runq, prio, p);
+    enqueue_process(runq, (int) prio, p);
 
     erts_smp_runq_unlock(runq);
     smp_notify_inc_runq(runq);
 
 }
 
-static ERTS_INLINE void
-schedule_process(Process *p, erts_aint32_t state, int active_enq)
+static ERTS_INLINE int
+change_proc_schedule_state(Process *p,
+			   erts_aint32_t clear_state_flags,
+			   erts_aint32_t set_state_flags,
+			   erts_aint32_t *statep,
+			   erts_aint32_t *enq_prio_p)
 {
-    erts_aint32_t a = state, n;
+    /*
+     * NOTE: ERTS_PSFLG_RUNNING, ERTS_PSFLG_RUNNING_SYS and
+     *       ERTS_PSFLG_ACTIVE_SYS are not allowed to be
+     *       altered by this function!
+     */
+    erts_aint32_t a = *statep, n;
+    int enqueue; /* < 0 -> use proxy */
+
+    ASSERT(!(a & ERTS_PSFLG_PROXY));
+    ASSERT((clear_state_flags & (ERTS_PSFLG_RUNNING
+				 | ERTS_PSFLG_RUNNING_SYS
+				 | ERTS_PSFLG_ACTIVE_SYS)) == 0);
+    ASSERT((set_state_flags & (ERTS_PSFLG_RUNNING
+			       | ERTS_PSFLG_RUNNING_SYS
+			       | ERTS_PSFLG_ACTIVE_SYS)) == 0);
 
     while (1) {
 	erts_aint32_t e;
 	n = e = a;
 
+	enqueue = 0;
+
 	if (a & ERTS_PSFLG_FREE)
-	    return; /* We don't want to schedule free processes... */
-	n |= ERTS_PSFLG_ACTIVE;
-	if (!(a & (ERTS_PSFLG_SUSPENDED|ERTS_PSFLG_RUNNING)))
-	    n |= ERTS_PSFLG_IN_RUNQ;
+	    break; /* We don't want to schedule free processes... */
+
+	if (clear_state_flags)
+	    n &= ~clear_state_flags;
+
+	if (set_state_flags)
+	    n |= set_state_flags;
+
+	if ((n & (ERTS_PSFLG_SUSPENDED
+		  | ERTS_PSFLG_RUNNING
+		  | ERTS_PSFLG_RUNNING_SYS
+		  | ERTS_PSFLG_IN_RUNQ
+		  | ERTS_PSFLG_ACTIVE)) == ERTS_PSFLG_ACTIVE) {
+	    /*
+	     * Active and seemingly need to be enqueued, but
+	     * process may be in a run queue via proxy, need
+	     * further inspection...
+	     */
+	    enqueue = check_enqueue_in_prio_queue(enq_prio_p, &n, a);
+	}
+
 	a = erts_smp_atomic32_cmpxchg_mb(&p->state, n, e);
 	if (a == e)
 	    break;
-	if (!active_enq && (a & ERTS_PSFLG_ACTIVE))
-	    return; /* Someone else activated process ... */
+	if (enqueue == 0 && n == a)
+	    break;
     }
 
-    if (erts_system_profile_flags.runnable_procs
-	&& !(a & (ERTS_PSFLG_ACTIVE|ERTS_PSFLG_SUSPENDED))) {
-    	profile_runnable_proc(p, am_active);
+    if (erts_system_profile_flags.runnable_procs) {
+
+	if (((n & (ERTS_PSFLG_SUSPENDED
+		   | ERTS_PSFLG_ACTIVE)) == ERTS_PSFLG_ACTIVE)
+	    && (!(a & (ERTS_PSFLG_ACTIVE_SYS
+		       | ERTS_PSFLG_RUNNING
+		       | ERTS_PSFLG_RUNNING_SYS)
+		  && (!(a & ERTS_PSFLG_ACTIVE)
+		      || (a & ERTS_PSFLG_SUSPENDED))))) {
+	    /* We activated a prevously inactive process */
+	    profile_runnable_proc(p, am_active);
+	}
+
     }
 
-    if ((n & ERTS_PSFLG_IN_RUNQ) && !(a & ERTS_PSFLG_IN_RUNQ))
-	add2runq(p, n);
+    *statep = a;
+
+    return enqueue;
+}
+
+static ERTS_INLINE void
+schedule_process(Process *p, erts_aint32_t in_state)
+{
+    erts_aint32_t enq_prio  = -1;
+    erts_aint32_t state = in_state;
+    int enqueue = change_proc_schedule_state(p,
+					     0,
+					     ERTS_PSFLG_ACTIVE,
+					     &state,
+					     &enq_prio);
+    if (enqueue)
+	add2runq(enqueue > 0 ? p : make_proxy_proc(NULL, p, enq_prio),
+		 state,
+		 enq_prio);
 }
 
 void
 erts_schedule_process(Process *p, erts_aint32_t state)
 {
-    schedule_process(p, state, 0);
+    schedule_process(p, state);
+}
+
+static void
+schedule_process_sys_task(Process *p, erts_aint32_t state, Process *proxy)
+{
+    erts_aint32_t a = state, n, enq_prio = -1;
+    int enqueue; /* < 0 -> use proxy */
+
+    ASSERT(!(state & ERTS_PSFLG_PROXY));
+
+    while (1) {
+	erts_aint32_t e;
+	n = e = a;
+
+	if (a & ERTS_PSFLG_FREE)
+	    return; /* We don't want to schedule free processes... */
+
+	enqueue = 0;
+	n |= ERTS_PSFLG_ACTIVE_SYS;
+	if (!(a & (ERTS_PSFLG_RUNNING|ERTS_PSFLG_RUNNING_SYS)))
+	    enqueue = check_enqueue_in_prio_queue(&enq_prio, &n, a);
+	a = erts_smp_atomic32_cmpxchg_mb(&p->state, n, e);
+	if (a == e)
+	    break;
+	if (a == n && !enqueue)
+	    goto cleanup;
+    }
+
+    if (erts_system_profile_flags.runnable_procs) {
+
+	if (!(a & (ERTS_PSFLG_ACTIVE_SYS
+		   | ERTS_PSFLG_RUNNING
+		   | ERTS_PSFLG_RUNNING_SYS))
+	    && (!(a & ERTS_PSFLG_ACTIVE) || (a & ERTS_PSFLG_SUSPENDED))) {
+	    /* We activated a prevously inactive process */
+	    profile_runnable_proc(p, am_active);
+	}
+
+    }
+
+    if (enqueue) {
+	Process *sched_p;
+	if (enqueue > 0)
+	    sched_p = p;
+	else {
+	    sched_p = make_proxy_proc(proxy, p, enq_prio);
+	    proxy = NULL;
+	}
+	add2runq(sched_p, n, enq_prio);
+    }
+
+cleanup:
+    if (proxy)
+	free_proxy_proc(proxy);
 }
 
 static ERTS_INLINE int
 suspend_process(Process *c_p, Process *p)
 {
-    erts_aint32_t state = erts_smp_atomic32_read_acqb(&p->state);
+    erts_aint32_t state;
     int suspended = 0;
     ERTS_SMP_LC_ASSERT(ERTS_PROC_LOCK_STATUS & erts_proc_lc_my_proc_locks(p));
 
+    state = erts_smp_atomic32_read_acqb(&p->state);
+
     if ((state & ERTS_PSFLG_SUSPENDED))
 	suspended = -1;
     else {
 	if (c_p == p) {
 	    state = erts_smp_atomic32_read_bor_relb(&p->state,
 						    ERTS_PSFLG_SUSPENDED);
-	    state |= ERTS_PSFLG_SUSPENDED;
 	    ASSERT(state & ERTS_PSFLG_RUNNING);
-	    suspended = 1;
+	    suspended = (state & ERTS_PSFLG_SUSPENDED) ? -1: 1;
 	}
 	else {
 	    while (!(state & (ERTS_PSFLG_RUNNING|ERTS_PSFLG_EXITING))) {
-		erts_aint32_t e, n;
+		erts_aint32_t n, e;
+
 		n = e = state;
 		n |= ERTS_PSFLG_SUSPENDED;
 		state = erts_smp_atomic32_cmpxchg_relb(&p->state, n, e);
 		if (state == e) {
-		    state = n;
 		    suspended = 1;
 		    break;
 		}
+		if (state & ERTS_PSFLG_SUSPENDED) {
+		    suspended = -1;
+		    break;
+		}
 	    }
 	}
     }
 
-    if (state & ERTS_PSFLG_SUSPENDED) {
+    if (suspended) {
 
 	ASSERT(!(ERTS_PSFLG_RUNNING & state)
 	       || p == erts_get_current_process());
 
-	if (erts_system_profile_flags.runnable_procs
-	    && (p->rcount == 0)
-	    && (state & ERTS_PSFLG_ACTIVE)) {
-	    profile_runnable_proc(p, am_inactive);
+	if (suspended > 0 && erts_system_profile_flags.runnable_procs) {
+
+	    /* 'state' is before our change... */
+
+	    if ((state & (ERTS_PSFLG_ACTIVE
+			  | ERTS_PSFLG_ACTIVE_SYS
+			  | ERTS_PSFLG_RUNNING
+			  | ERTS_PSFLG_RUNNING_SYS
+			  | ERTS_PSFLG_SUSPENDED)) == ERTS_PSFLG_ACTIVE) {
+		/* We made process inactive */
+		profile_runnable_proc(p, am_inactive);
+	    }
+
 	}
 
 	p->rcount++;  /* count number of suspend */
     }
+
     return suspended;
 }
 
 static ERTS_INLINE void
 resume_process(Process *p)
 {
-    erts_aint32_t state;
+    erts_aint32_t state, enq_prio = -1;
+    int enqueue;
+
     ERTS_SMP_LC_ASSERT(ERTS_PROC_LOCK_STATUS & erts_proc_lc_my_proc_locks(p));
 
     ASSERT(p->rcount > 0);
@@ -5041,14 +5338,16 @@ resume_process(Process *p)
     if (--p->rcount > 0)  /* multiple suspend */
 	return;
 
-    state = erts_smp_atomic32_read_band_mb(&p->state, ~ERTS_PSFLG_SUSPENDED);
-    state &= ~ERTS_PSFLG_SUSPENDED;
-    if ((state & (ERTS_PSFLG_EXITING
-		  | ERTS_PSFLG_ACTIVE
-		  | ERTS_PSFLG_IN_RUNQ
-		  | ERTS_PSFLG_RUNNING)) == ERTS_PSFLG_ACTIVE) {
-	schedule_process(p, state, 1);
-    }
+    state = erts_smp_atomic32_read_nob(&p->state);
+    enqueue = change_proc_schedule_state(p,
+					 ERTS_PSFLG_SUSPENDED,
+					 0,
+					 &state,
+					 &enq_prio);
+    if (enqueue)
+	add2runq(enqueue > 0 ? p : make_proxy_proc(NULL, p, enq_prio),
+		 state,
+		 enq_prio);
 }
 
 int
@@ -6032,7 +6331,8 @@ pid2proc_not_running(Process *c_p, ErtsProcLocks c_p_locks,
 		goto done;
 	}
 	else {
-	    if (!(ERTS_PSFLG_RUNNING & erts_smp_atomic32_read_acqb(&rp->state)))
+	    if (!((ERTS_PSFLG_RUNNING|ERTS_PSFLG_RUNNING_SYS)
+		  & erts_smp_atomic32_read_acqb(&rp->state)))
 		goto done;
 
 	}
@@ -6695,7 +6995,7 @@ Eterm
 erts_get_process_priority(Process *p)
 {
     erts_aint32_t state = erts_smp_atomic32_read_nob(&p->state);
-    switch (state & ERTS_PSFLG_PRIO_MASK) {
+    switch (ERTS_PSFLGS_GET_USR_PRIO(state)) {
     case PRIORITY_MAX:		return am_max;
     case PRIORITY_HIGH:		return am_high;
     case PRIORITY_NORMAL:	return am_normal;
@@ -6718,18 +7018,60 @@ erts_set_process_priority(Process *p, Eterm value)
     }
 
     a = erts_smp_atomic32_read_nob(&p->state);
-    if (nprio == (a & ERTS_PSFLG_PRIO_MASK))
+    if (nprio == ERTS_PSFLGS_GET_USR_PRIO(a))
 	oprio = nprio;
     else {
-	erts_aint32_t e, n;
+	int slocked = 0;
+	erts_aint32_t e, n, aprio;
+
+	if (a & ERTS_PSFLG_ACTIVE_SYS) {
+	    erts_smp_proc_lock(p, ERTS_PROC_LOCK_STATUS);
+	    slocked = 1;
+	}
+
 	do {
-	    oprio = a & ERTS_PSFLG_PRIO_MASK;
+	    oprio = ERTS_PSFLGS_GET_USR_PRIO(a);
 	    n = e = a;
 
-	    ASSERT(!(a & ERTS_PSFLG_IN_RUNQ));
+	    if (!(a & ERTS_PSFLG_ACTIVE_SYS))
+		aprio = nprio;
+	    else {
+		int max_qbit;
+
+		if (!slocked) {
+		    erts_smp_proc_lock(p, ERTS_PROC_LOCK_STATUS);
+		    slocked = 1;
+		}
+
+		max_qbit = p->sys_task_qs->qmask;
+		max_qbit &= -max_qbit;
+		switch (max_qbit) {
+		case MAX_BIT:
+		    aprio = PRIORITY_MAX;
+		    break;
+		case HIGH_BIT:
+		    aprio = PRIORITY_HIGH;
+		    break;
+		case NORMAL_BIT:
+		    aprio = PRIORITY_NORMAL;
+		    break;
+		case LOW_BIT:
+		    aprio = PRIORITY_LOW;
+		    break;
+		default:
+		    ERTS_INTERNAL_ERROR("Invalid qmask");
+		    aprio = -1;
+		}
+
+		if (aprio > nprio) /* low value -> high prio */
+		    aprio = nprio;
+	    }
+
+	    n &= ~(ERTS_PSFLGS_USR_PRIO_MASK
+		   | ERTS_PSFLGS_ACT_PRIO_MASK);
+	    n |= ((nprio << ERTS_PSFLGS_USR_PRIO_OFFSET)
+		  | (aprio << ERTS_PSFLGS_ACT_PRIO_OFFSET));
 
-	    n &= ~ERTS_PSFLG_PRIO_MASK;
-	    n |= nprio;
 	    a = erts_smp_atomic32_cmpxchg_mb(&p->state, n, e);
 	} while (a != e);
     }
@@ -6763,6 +7105,7 @@ erts_set_process_priority(Process *p, Eterm value)
 
 Process *schedule(Process *p, int calls)
 {
+    Process *proxy_p = NULL;
     ErtsRunQueue *rq;
     erts_aint_t dt;
     ErtsSchedulerData *esdp;
@@ -6805,6 +7148,8 @@ Process *schedule(Process *p, int calls)
 	actual_reds = reds = 0;
 	erts_smp_runq_lock(rq);
     } else {
+    sched_out_proc:
+
 #ifdef ERTS_SMP
 	ERTS_SMP_CHK_HAVE_ONLY_MAIN_PROC_LOCK(p);
 	esdp = p->scheduler_data;
@@ -6858,10 +7203,11 @@ Process *schedule(Process *p, int calls)
 
 	esdp->reductions += reds;
 
-	schedule_out_process(rq, state, p); /* Returns with rq locked! */
+	schedule_out_process(rq, state, p, proxy_p); /* Returns with rq locked! */
+	proxy_p = NULL;
 
 	ERTS_PROC_REDUCTIONS_EXECUTED(rq,
-				      (int) (state & ERTS_PSFLG_PRIO_MASK),
+				      (int) ERTS_PSFLGS_GET_USR_PRIO(state),
 				      reds,
 				      actual_reds);
 
@@ -6870,18 +7216,19 @@ Process *schedule(Process *p, int calls)
 	p->scheduler_data = NULL;
 #endif
 
+	erts_smp_proc_unlock(p, ERTS_PROC_LOCK_MAIN|ERTS_PROC_LOCK_STATUS);
 
 	if (state & ERTS_PSFLG_FREE) {
 #ifdef ERTS_SMP
 	    ASSERT(esdp->free_process == p);
 	    esdp->free_process = NULL;
-#else	    
-	    erts_free_proc(p);
+#else
+	    state = erts_smp_atomic32_read_nob(&p->state);
+	    if (!(state & ERTS_PSFLG_IN_RUNQ))
+		erts_free_proc(p);
 #endif
 	}
 
-	erts_smp_proc_unlock(p, ERTS_PROC_LOCK_MAIN|ERTS_PROC_LOCK_STATUS);
-
 #ifdef ERTS_SMP
 	ASSERT(!esdp->free_process);
 #endif
@@ -7088,6 +7435,8 @@ Process *schedule(Process *p, int calls)
 	 * Find a new process to run.
 	 */
     pick_next_process: {
+	    erts_aint32_t psflg_band_mask;
+	    erts_aint32_t running_flag;
 	    int prio_q;
 	    int qmask;
 
@@ -7121,18 +7470,62 @@ Process *schedule(Process *p, int calls)
 
 	    ASSERT(p); /* Wrong qmask in rq->flags? */
 
+	    psflg_band_mask = ~(((erts_aint32_t) 1) << (ERTS_PSFLGS_GET_PRQ_PRIO(state)
+							+ ERTS_PSFLGS_IN_PRQ_MASK_OFFSET));
+
+	    if (!(state & ERTS_PSFLG_PROXY))
+		psflg_band_mask &= ~ERTS_PSFLG_IN_RUNQ;
+	    else {
+		proxy_p = p;
+		p = erts_proc_lookup_raw(proxy_p->common.id);
+		if (!p) {
+		    free_proxy_proc(proxy_p);
+		    proxy_p = NULL;
+		    goto pick_next_process;
+		}
+		state = erts_smp_atomic32_read_nob(&p->state);
+	    }
+
+
+	    if (state & ERTS_PSFLG_ACTIVE_SYS)
+		running_flag = ERTS_PSFLG_RUNNING_SYS;
+	    else
+		running_flag = ERTS_PSFLG_RUNNING;
+
 	    while (1) {
 		erts_aint32_t exp, new, tmp;
 		tmp = new = exp = state;
-		new &= ~ERTS_PSFLG_IN_RUNQ;
-		tmp = state & (ERTS_PSFLG_SUSPENDED|ERTS_PSFLG_PENDING_EXIT);
-		if (tmp != ERTS_PSFLG_SUSPENDED)
-		    new |= ERTS_PSFLG_RUNNING;
+		new &= psflg_band_mask;
+		if (!(state & (ERTS_PSFLG_RUNNING
+			       | ERTS_PSFLG_RUNNING_SYS))) {
+		    tmp = state & (ERTS_PSFLG_SUSPENDED
+				   | ERTS_PSFLG_PENDING_EXIT
+				   | ERTS_PSFLG_ACTIVE_SYS);
+		    if (tmp != ERTS_PSFLG_SUSPENDED)
+			new |= running_flag;
+		}
 		state = erts_smp_atomic32_cmpxchg_relb(&p->state, new, exp);
 		if (state == exp) {
-		    tmp = state & (ERTS_PSFLG_SUSPENDED|ERTS_PSFLG_PENDING_EXIT);
-		    if (tmp == ERTS_PSFLG_SUSPENDED)
+		    if ((state & (ERTS_PSFLG_RUNNING
+				  | ERTS_PSFLG_RUNNING_SYS
+				  | ERTS_PSFLG_FREE))
+			|| ((state & (ERTS_PSFLG_SUSPENDED
+				      | ERTS_PSFLG_PENDING_EXIT
+				      | ERTS_PSFLG_ACTIVE_SYS))
+			    == ERTS_PSFLG_SUSPENDED)) {
+			if (state & ERTS_PSFLG_FREE) {
+#ifdef ERTS_SMP
+			    erts_smp_proc_dec_refc(p);
+#else
+			    erts_free_proc(p);
+#endif
+			}
+			if (proxy_p) {
+			    free_proxy_proc(proxy_p);
+			    proxy_p = NULL;
+			}
 			goto pick_next_process;
+		    }
 		    state = new;
 		    break;
 		}
@@ -7162,7 +7555,7 @@ Process *schedule(Process *p, int calls)
 					  (UWord) esdp->no);
 	    int migrated = old && old != esdp->no;
 
-	    prio = (int) (state & ERTS_PSFLG_PRIO_MASK);
+	    prio = (int) ERTS_PSFLGS_GET_USR_PRIO(state);
 
 	    erts_smp_spin_lock(&erts_sched_stat.lock);
 	    erts_sched_stat.prio[prio].total_executed++;
@@ -7182,9 +7575,6 @@ Process *schedule(Process *p, int calls)
 	ASSERT(!p->scheduler_data);
 	p->scheduler_data = esdp;
 #endif
-	/* Never run a suspended process */
-	ASSERT(!(ERTS_PSFLG_SUSPENDED & erts_smp_atomic32_read_nob(&p->state)));
-
 	reds = context_reds;
 
 	if (IS_TRACED(p)) {
@@ -7210,21 +7600,574 @@ Process *schedule(Process *p, int calls)
 	    erts_check_my_tracer_proc(p);
 #endif
 
+	if (state & ERTS_PSFLG_RUNNING_SYS) {
+	    reds -= execute_sys_tasks(p, &state, reds);
+	    if (reds <= 0) {
+		p->fcalls = reds;
+		goto sched_out_proc;
+	    }
+
+	    ASSERT(state & ERTS_PSFLG_RUNNING_SYS);
+	    ASSERT(!(state & ERTS_PSFLG_RUNNING));
+
+	    while (1) {
+		erts_aint32_t n, e;
+
+		if (((state & (ERTS_PSFLG_SUSPENDED
+			       | ERTS_PSFLG_ACTIVE)) != ERTS_PSFLG_ACTIVE)
+		    && !(state & ERTS_PSFLG_EXITING))
+		    goto sched_out_proc;
+
+		n = e = state;
+		n &= ~ERTS_PSFLG_RUNNING_SYS;
+		n |= ERTS_PSFLG_RUNNING;
+
+		state = erts_smp_atomic32_cmpxchg_mb(&p->state, n, e);
+		if (state == e) {
+		    state = n;
+		    break;
+		}
+
+		ASSERT(state & ERTS_PSFLG_RUNNING_SYS);
+		ASSERT(!(state & ERTS_PSFLG_RUNNING));
+	    }
+	}
+
 	if (!(state & ERTS_PSFLG_EXITING)
 	    && ((FLAGS(p) & F_FORCE_GC)
 		|| (MSO(p).overhead > BIN_VHEAP_SZ(p)))) {
 	    reds -= erts_garbage_collect(p, 0, p->arg_reg, p->arity);
-	    if (reds < 0) {
-		reds = 1;
+	    if (reds <= 0) {
+		p->fcalls = reds;
+		goto sched_out_proc;
 	    }
 	}
+
+	if (proxy_p) {
+	    free_proxy_proc(proxy_p);
+	    proxy_p = NULL;
+	}
 	    
 	p->fcalls = reds;
 	ERTS_SMP_CHK_HAVE_ONLY_MAIN_PROC_LOCK(p);
+
+	/* Never run a suspended process */
+	ASSERT(!(ERTS_PSFLG_SUSPENDED & erts_smp_atomic32_read_nob(&p->state)));
+
 	return p;
     }
 }
 
+static int
+notify_sys_task_executed(Process *c_p, ErtsProcSysTask *st, Eterm st_result)
+{
+    Process *rp = erts_proc_lookup(st->requester);
+    if (rp) {
+	ErtsProcLocks rp_locks;
+	ErlOffHeap *ohp;
+	ErlHeapFragment* bp;
+	Eterm *hp, msg, req_id, result;
+	Uint st_result_sz, hsz;
+#ifdef DEBUG
+	Eterm *hp_start;
+#endif
+
+	rp_locks = (c_p == rp) ? ERTS_PROC_LOCK_MAIN : 0;
+
+	st_result_sz = is_immed(st_result) ? 0 : size_object(st_result);
+	hsz = st->req_id_sz + st_result_sz + 4 /* 3-tuple */;
+
+	hp = erts_alloc_message_heap(hsz,
+				     &bp,
+				     &ohp,
+				     rp,
+				     &rp_locks);
+
+#ifdef DEBUG
+	hp_start = hp;
+#endif
+
+	req_id = st->req_id_sz == 0 ? st->req_id : copy_struct(st->req_id,
+							       st->req_id_sz,
+							       &hp,
+							       ohp);
+
+	result = st_result_sz == 0 ? st_result :  copy_struct(st_result,
+							      st_result_sz,
+							      &hp,
+							      ohp);
+
+	ASSERT(is_immed(st->reply_tag));
+
+	msg = TUPLE3(hp, st->reply_tag, req_id, result);
+
+#ifdef DEBUG
+	hp += 4;
+	ASSERT(hp_start + hsz == hp);
+#endif
+
+	erts_queue_message(rp,
+			   &rp_locks,
+			   bp,
+			   msg,
+			   NIL
+#ifdef USE_VM_PROBES
+			   , NIL
+#endif
+	    );
+
+	if (c_p == rp)
+	    rp_locks &= ~ERTS_PROC_LOCK_MAIN;
+
+	if (rp_locks)
+	    erts_smp_proc_unlock(rp, rp_locks);
+    }
+
+    erts_cleanup_offheap(&st->off_heap);
+
+    erts_free(ERTS_ALC_T_PROC_SYS_TSK, st);
+
+    return rp ? 1 : 0;
+}
+
+static ERTS_INLINE ErtsProcSysTask *
+fetch_sys_task(Process *c_p, erts_aint32_t state, int *qmaskp)
+{
+    ErtsProcSysTaskQs *unused_qs = NULL;
+    int qbit, qmask;
+    ErtsProcSysTask *st, **qp;
+
+    erts_smp_proc_lock(c_p, ERTS_PROC_LOCK_STATUS);
+
+    if (!c_p->sys_task_qs) {
+	qmask = 0;
+	st = NULL;
+	goto update_state;
+    }
+
+    qmask = c_p->sys_task_qs->qmask;
+
+    if ((state & (ERTS_PSFLG_ACTIVE
+		  | ERTS_PSFLG_EXITING
+		  | ERTS_PSFLG_SUSPENDED)) == ERTS_PSFLG_ACTIVE) {
+	/* No sys tasks if we got exclusively higher prio user work to do */
+	st = NULL;
+	switch (ERTS_PSFLGS_GET_USR_PRIO(state)) {
+	case PRIORITY_MAX:
+	    if (!(qmask & MAX_BIT))
+		goto done;
+	    break;
+	case PRIORITY_HIGH:
+	    if (!(qmask & (MAX_BIT|HIGH_BIT)))
+		goto done;
+	    break;
+	default:
+	    break;
+	}
+    }
+
+    qbit = qmask & -qmask;
+    switch (qbit) {
+    case MAX_BIT:
+	qp = &c_p->sys_task_qs->q[PRIORITY_MAX];
+	break;
+    case HIGH_BIT:
+	qp = &c_p->sys_task_qs->q[PRIORITY_HIGH];
+	break;
+    case NORMAL_BIT:
+	if (!(qmask & PRIORITY_LOW)
+	    || ++c_p->sys_task_qs->ncount <= RESCHEDULE_LOW) {
+	    qp = &c_p->sys_task_qs->q[PRIORITY_NORMAL];
+	    break;
+	}
+	c_p->sys_task_qs->ncount = 0;
+	/* Fall through */
+    case LOW_BIT:
+	qp = &c_p->sys_task_qs->q[PRIORITY_LOW];
+	break;
+    default:
+	ERTS_INTERNAL_ERROR("Invalid qmask");
+    }
+
+    st = *qp;
+    ASSERT(st);
+    if (st->next != st) {
+	*qp = st->next;
+	st->next->prev = st->prev;
+	st->prev->next = st->next;
+    }
+    else {
+	erts_aint32_t a, e, n, st_prio, qmask2;
+
+	*qp = NULL;
+	qmask &= ~qbit;
+	c_p->sys_task_qs->qmask = qmask;
+
+    update_state:
+
+	qmask2 = qmask;
+
+	switch (qmask2 & -qmask2) {
+	case MAX_BIT:
+	    st_prio = PRIORITY_MAX;
+	    break;
+	case HIGH_BIT:
+	    st_prio = PRIORITY_HIGH;
+	    break;
+	case NORMAL_BIT:
+	    st_prio = PRIORITY_NORMAL;
+	    break;
+	case LOW_BIT:
+	    st_prio = PRIORITY_LOW;
+	    break;
+	case 0:
+	    st_prio = PRIORITY_LOW;
+	    unused_qs = c_p->sys_task_qs;
+	    c_p->sys_task_qs = NULL;
+	    break;
+	default:
+	    ERTS_INTERNAL_ERROR("Invalid qmask");
+	}
+
+	a = state;
+	do {
+	    erts_aint32_t prio = ERTS_PSFLGS_GET_USR_PRIO(a);
+
+	    if (prio > st_prio)
+		prio = st_prio;
+
+	    n = e = a;
+
+	    n &= ~ERTS_PSFLGS_ACT_PRIO_MASK;
+	    n |= (prio << ERTS_PSFLGS_ACT_PRIO_OFFSET);
+
+	    if (!qmask)
+		n &= ~ERTS_PSFLG_ACTIVE_SYS;
+
+	    if (a == n)
+		break;
+	    a = erts_smp_atomic32_cmpxchg_nob(&c_p->state, n, e);
+	} while (a != e);
+    }
+
+done:
+
+    erts_smp_proc_unlock(c_p, ERTS_PROC_LOCK_STATUS);
+
+    if (unused_qs)
+	proc_sys_task_queues_free(unused_qs);
+
+    *qmaskp = qmask;
+
+    return st;
+}
+
+static int
+execute_sys_tasks(Process *c_p, erts_aint32_t *statep, int in_reds)
+{
+    int garbage_collected = 0;
+    erts_aint32_t state = *statep;
+    int max_reds = in_reds;
+    int reds = 0;
+    int qmask = 0;
+
+    ERTS_SMP_LC_ASSERT(erts_proc_lc_my_proc_locks(c_p) == ERTS_PROC_LOCK_MAIN);
+
+    do {
+	ErtsProcSysTask *st;
+	Eterm st_res;
+
+	if (state & (ERTS_PSFLG_EXITING|ERTS_PSFLG_PENDING_EXIT)) {
+#ifdef ERTS_SMP
+	    if (state & ERTS_PSFLG_PENDING_EXIT)
+		erts_handle_pending_exit(c_p, ERTS_PROC_LOCK_MAIN);
+#endif
+	    ASSERT(ERTS_PROC_IS_EXITING(c_p));
+	    break;
+	}
+
+	st = fetch_sys_task(c_p, state, &qmask);
+	if (!st)
+	    break;
+
+	switch (st->type) {
+	case ERTS_PSTT_GC:
+	    if (!garbage_collected) {
+		FLAGS(c_p) |= F_NEED_FULLSWEEP;
+		reds += erts_garbage_collect(c_p, 0, c_p->arg_reg, c_p->arity);
+		garbage_collected = 1;
+	    }
+	    st_res = am_true;
+	    break;
+	case ERTS_PSTT_CPC:
+	    st_res = erts_check_process_code(c_p,
+					     st->arg[0],
+					     st->arg[1] == am_true,
+					     &reds);
+	    break;
+	default:
+	    ERTS_INTERNAL_ERROR("Invalid process sys task type");
+	    st_res = am_false;
+	}
+
+	if (st)
+	    reds += notify_sys_task_executed(c_p, st, st_res);
+
+	state = erts_smp_atomic32_read_acqb(&c_p->state);
+    } while (qmask && reds < max_reds);
+
+    *statep = state;
+
+    return reds;
+}
+
+static int
+cleanup_sys_tasks(Process *c_p, erts_aint32_t in_state, int in_reds)
+{
+    erts_aint32_t state = in_state;
+    int max_reds = in_reds;
+    int reds = 0;
+    int qmask = 0;
+
+    ERTS_SMP_LC_ASSERT(erts_proc_lc_my_proc_locks(c_p) == ERTS_PROC_LOCK_MAIN);
+
+    do {
+	ErtsProcSysTask *st;
+	Eterm st_res;
+
+	st = fetch_sys_task(c_p, state, &qmask);
+	if (!st)
+	    break;
+
+	switch (st->type) {
+	case ERTS_PSTT_GC:
+	    st_res = am_false;
+	    break;
+	case ERTS_PSTT_CPC:
+	    st_res = am_false;
+	    break;
+	default:
+	    ERTS_INTERNAL_ERROR("Invalid process sys task type");
+	    st_res = am_false;
+	    break;
+	}
+
+	reds += notify_sys_task_executed(c_p, st, st_res);
+
+	state = erts_smp_atomic32_read_acqb(&c_p->state);
+    } while (qmask && reds < max_reds);
+
+    return reds;
+}
+
+BIF_RETTYPE
+erts_internal_request_system_task_3(BIF_ALIST_3)
+{
+    Process *rp = erts_proc_lookup(BIF_ARG_1);
+    ErtsProcSysTaskQs *stqs, *free_stqs = NULL;
+    ErtsProcSysTask *st = NULL;
+    erts_aint32_t prio, rp_state;
+    int rp_locked;
+    Eterm noproc_res, req_type;
+
+    if (!rp && !is_internal_pid(BIF_ARG_1)) {
+	if (!is_external_pid(BIF_ARG_1))
+	    goto badarg;
+	if (external_pid_dist_entry(BIF_ARG_1) != erts_this_dist_entry)
+	    goto badarg;
+    }
+
+    switch (BIF_ARG_2) {
+    case am_max:	prio = PRIORITY_MAX;	break;
+    case am_high:	prio = PRIORITY_HIGH;	break;
+    case am_normal:	prio = PRIORITY_NORMAL;	break;
+    case am_low:	prio = PRIORITY_LOW;	break;
+    default: goto badarg;
+    }
+
+    if (is_not_tuple(BIF_ARG_3))
+	goto badarg;
+    else {
+	int i;
+	Eterm *tp = tuple_val(BIF_ARG_3);
+	Uint arity = arityval(*tp);
+	Eterm req_id;
+	Uint req_id_sz;
+	Eterm arg[ERTS_MAX_PROC_SYS_TASK_ARGS];
+	Uint arg_sz[ERTS_MAX_PROC_SYS_TASK_ARGS];
+	Uint tot_sz;
+	Eterm *hp;
+
+	if (arity < 2)
+	    goto badarg;
+	if (arity > 2 + ERTS_MAX_PROC_SYS_TASK_ARGS)
+	    goto badarg;
+	req_type = tp[1];
+	req_id = tp[2];
+	req_id_sz = is_immed(req_id) ? req_id : size_object(req_id);
+	tot_sz = req_id_sz;
+	for (i = 0; i < ERTS_MAX_PROC_SYS_TASK_ARGS; i++) {
+	    int tix = 3 + i;
+	    if (tix > arity) {
+		arg[i] = THE_NON_VALUE;
+		arg_sz[i] = 0;
+	    }
+	    else {
+		arg[i] = tp[tix];
+		if (is_immed(arg[i]))
+		    arg_sz[i] = 0;
+		else {
+		    arg_sz[i] = size_object(arg[i]);
+		    tot_sz += arg_sz[i];
+		}
+	    }
+	}
+	st = erts_alloc(ERTS_ALC_T_PROC_SYS_TSK,
+			ERTS_PROC_SYS_TASK_SIZE(tot_sz));
+	st->next = st->prev = st; /* Prep for empty prio queue */
+	ERTS_INIT_OFF_HEAP(&st->off_heap);
+	hp = &st->heap[0];
+
+	st->requester = BIF_P->common.id;
+	st->reply_tag = req_type;
+	st->req_id_sz = req_id_sz;
+	st->req_id = req_id_sz == 0 ? req_id : copy_struct(req_id,
+							   req_id_sz,
+							   &hp,
+							   &st->off_heap);
+
+	for (i = 0; i < ERTS_MAX_PROC_SYS_TASK_ARGS; i++)
+	    st->arg[i] = arg_sz[i] == 0 ? arg[i] : copy_struct(arg[i],
+							       arg_sz[i],
+							       &hp,
+							       &st->off_heap);
+	ASSERT(&st->heap[0] + tot_sz == hp);
+    }
+
+    switch (req_type) {
+
+    case am_garbage_collect:
+	st->type = ERTS_PSTT_GC;
+	noproc_res = am_false;
+	if (!rp)
+	    goto noproc;
+	break;
+
+    case am_check_process_code:
+	if (is_not_atom(st->arg[0]))
+	    goto badarg;
+	if (st->arg[1] != am_true && st->arg[1] != am_false)
+	    goto badarg;
+	noproc_res = am_false;
+	st->type = ERTS_PSTT_CPC;
+	if (!rp)
+	    goto noproc;
+	break;
+
+    default:
+	goto badarg;
+    }
+
+    rp_state = erts_smp_atomic32_read_nob(&rp->state);
+
+    rp_locked = 0;
+
+    free_stqs = NULL;
+    if (rp_state & ERTS_PSFLG_ACTIVE_SYS)
+	stqs = NULL;
+    else {
+    alloc_qs:
+	stqs = proc_sys_task_queues_alloc();
+	stqs->qmask = 1 << prio;
+	stqs->ncount = 0;
+	stqs->q[PRIORITY_MAX] = NULL;
+	stqs->q[PRIORITY_HIGH] = NULL;
+	stqs->q[PRIORITY_NORMAL] = NULL;
+	stqs->q[PRIORITY_LOW] = NULL;
+	stqs->q[prio] = st;
+    }
+
+    if (!rp_locked) {
+	rp_locked = 1;
+	erts_smp_proc_lock(rp, ERTS_PROC_LOCK_STATUS);
+
+	rp_state = erts_smp_atomic32_read_nob(&rp->state);
+	if (rp_state & ERTS_PSFLG_EXITING) {
+	    erts_smp_proc_unlock(rp, ERTS_PROC_LOCK_STATUS);
+	    rp = NULL;
+	    free_stqs = stqs;
+	    goto noproc;
+	}
+    }
+
+    if (!rp->sys_task_qs) {
+	if (stqs)
+	    rp->sys_task_qs = stqs;
+	else
+	    goto alloc_qs;
+    }
+    else {
+	if (stqs)
+	    free_stqs = stqs;
+	stqs = rp->sys_task_qs;
+	if (!stqs->q[prio]) {
+	    stqs->q[prio] = st;
+	    stqs->qmask |= 1 << prio;
+	}
+	else {
+	    st->next = stqs->q[prio];
+	    st->prev = stqs->q[prio]->prev;
+	    st->next->prev = st;
+	    st->prev->next = st;
+	    ASSERT(stqs->qmask & (1 << prio));
+	}
+    }
+
+    if (ERTS_PSFLGS_GET_ACT_PRIO(rp_state) > prio) {
+	erts_aint32_t n, a, e;
+	/* Need to elevate actual prio */
+
+	a = rp_state;
+	do {
+	    if (ERTS_PSFLGS_GET_ACT_PRIO(a) <= prio) {
+		n = a;
+		break;
+	    }
+	    n = e = a;
+	    n &= ~ERTS_PSFLGS_ACT_PRIO_MASK;
+	    n |= (prio << ERTS_PSFLGS_ACT_PRIO_OFFSET);
+	    a = erts_smp_atomic32_cmpxchg_nob(&rp->state, n, e);    
+	} while (a != e);
+	rp_state = n;
+    }
+
+    erts_smp_proc_unlock(rp, ERTS_PROC_LOCK_STATUS);
+
+    schedule_process_sys_task(rp, rp_state, NULL);
+
+    if (free_stqs)
+	proc_sys_task_queues_free(free_stqs);
+
+    BIF_RET(am_ok);
+
+noproc:
+
+    notify_sys_task_executed(BIF_P, st, noproc_res);
+    if (free_stqs)
+	proc_sys_task_queues_free(free_stqs);
+    BIF_RET(am_ok);
+
+badarg:
+
+    if (st) {
+	erts_cleanup_offheap(&st->off_heap);
+	erts_free(ERTS_ALC_T_PROC_SYS_TSK, st);
+    }
+    if (free_stqs)
+	proc_sys_task_queues_free(free_stqs);
+    BIF_ERROR(BIF_P, BADARG);
+}
+
 void
 erts_sched_stat_modify(int what)
 {
@@ -7520,7 +8463,8 @@ erl_create_process(Process* parent, /* Parent of process (default group leader).
 	prio = (erts_aint32_t) so->priority;
     }
 
-    state |= (prio & ERTS_PSFLG_PRIO_MASK);
+    state |= (((prio & ERTS_PSFLGS_PRIO_MASK) << ERTS_PSFLGS_ACT_PRIO_OFFSET)
+	      | ((prio & ERTS_PSFLGS_PRIO_MASK) << ERTS_PSFLGS_USR_PRIO_OFFSET));
 
     if (!rq)
 	rq = erts_get_runq_proc(parent);
@@ -7599,6 +8543,8 @@ erl_create_process(Process* parent, /* Parent of process (default group leader).
     p->bin_old_vheap    = 0;
     p->bin_vheap_mature = 0;
 
+    p->sys_task_qs = NULL;
+
     /* No need to initialize p->fcalls. */
 
     p->current = p->initial+INITIAL_MOD;
@@ -7764,7 +8710,7 @@ erl_create_process(Process* parent, /* Parent of process (default group leader).
      * Schedule process for execution.
      */
 
-    schedule_process(p, state, 0);
+    schedule_process(p, state);
 
     VERBOSE(DEBUG_PROCESSES, ("Created a new process: %T\n",p->common.id));
 
@@ -7815,6 +8761,7 @@ void erts_init_empty_process(Process *p)
     p->bin_vheap_sz = BIN_VH_MIN_SIZE;
     p->bin_old_vheap_sz = BIN_VH_MIN_SIZE;
     p->bin_old_vheap = 0;
+    p->sys_task_qs = NULL;
     p->bin_vheap_mature = 0;
 #ifdef ERTS_SMP
     p->common.u.alive.ptimer = NULL;
@@ -8070,33 +9017,21 @@ delete_process(Process* p)
     p->fvalue = NIL;
 }
 
-static ERTS_INLINE erts_aint32_t
-set_proc_exiting_state(Process *p, erts_aint32_t state)
-{
-    erts_aint32_t a, n, e;
-    a = state;
-    while (1) {
-	n = e = a;
-	n &= ~(ERTS_PSFLG_SUSPENDED|ERTS_PSFLG_PENDING_EXIT);
-	n |= ERTS_PSFLG_EXITING|ERTS_PSFLG_ACTIVE;
-	if (!(a & (ERTS_PSFLG_IN_RUNQ|ERTS_PSFLG_RUNNING)))
-	    n |= ERTS_PSFLG_IN_RUNQ;
-	a = erts_smp_atomic32_cmpxchg_relb(&p->state, n, e);
-	if (a == e)
-	    break;
-    }
-    return a;
-}
-
 static ERTS_INLINE void
 set_proc_exiting(Process *p,
-		 erts_aint32_t state,
+		 erts_aint32_t in_state,
 		 Eterm reason,
 		 ErlHeapFragment *bp)
 {
+    erts_aint32_t state = in_state, enq_prio = -1;
+    int enqueue;
     ERTS_SMP_LC_ASSERT(erts_proc_lc_my_proc_locks(p) == ERTS_PROC_LOCKS_ALL);
 
-    state = set_proc_exiting_state(p, state);
+    enqueue = change_proc_schedule_state(p,
+					 ERTS_PSFLG_SUSPENDED|ERTS_PSFLG_PENDING_EXIT,
+					 ERTS_PSFLG_EXITING|ERTS_PSFLG_ACTIVE,
+					 &state,
+					 &enq_prio);
 
     p->fvalue = reason;
     if (bp)
@@ -8111,15 +9046,37 @@ set_proc_exiting(Process *p,
     cancel_timer(p);
     p->i = (BeamInstr *) beam_exit;
 
-    if (erts_system_profile_flags.runnable_procs
-	&& !(state & (ERTS_PSFLG_ACTIVE|ERTS_PSFLG_SUSPENDED))) {
-    	profile_runnable_proc(p, am_active);
-    }
-
-    if (!(state & (ERTS_PSFLG_IN_RUNQ|ERTS_PSFLG_RUNNING)))
-	add2runq(p, state);
+    if (enqueue)
+	add2runq(enqueue > 0 ? p : make_proxy_proc(NULL, p, enq_prio),
+		 state,
+		 enq_prio);
 }
 
+static ERTS_INLINE erts_aint32_t
+set_proc_self_exiting(Process *c_p)
+{
+#ifdef DEBUG
+    int enqueue;
+#endif
+    erts_aint32_t state, enq_prio = -1;
+
+    ERTS_SMP_LC_ASSERT(erts_proc_lc_my_proc_locks(c_p) == ERTS_PROC_LOCKS_ALL);
+
+    state = erts_smp_atomic32_read_nob(&c_p->state);
+    ASSERT(state & (ERTS_PSFLG_RUNNING|ERTS_PSFLG_RUNNING_SYS));
+
+#ifdef DEBUG
+    enqueue =
+#endif
+	change_proc_schedule_state(c_p,
+				   ERTS_PSFLG_SUSPENDED|ERTS_PSFLG_PENDING_EXIT,
+				   ERTS_PSFLG_EXITING|ERTS_PSFLG_ACTIVE,
+				   &state,
+				   &enq_prio);
+
+    ASSERT(!enqueue);
+    return state;
+}
 
 #ifdef ERTS_SMP
 
@@ -8167,7 +9124,7 @@ handle_pending_exiters(ErtsProcList *pnd_xtrs)
 	if (p) {
 	    if (erts_proclist_same(plp, p)) {
 		erts_aint32_t state = erts_smp_atomic32_read_acqb(&p->state);
-		if (!(state & ERTS_PSFLG_RUNNING)) {
+		if (!(state & (ERTS_PSFLG_RUNNING|ERTS_PSFLG_RUNNING_SYS))) {
 		    ASSERT(state & ERTS_PSFLG_PENDING_EXIT);
 		    erts_handle_pending_exit(p, ERTS_PROC_LOCKS_ALL);
 		}
@@ -8388,7 +9345,7 @@ send_exit_signal(Process *c_p,		/* current process if and only
 		}
 		set_proc_exiting(c_p, state, rsn, NULL);
 	    }
-	    else if (!(state & ERTS_PSFLG_RUNNING)) {
+	    else if (!(state & (ERTS_PSFLG_RUNNING|ERTS_PSFLG_RUNNING_SYS))) {
 		/* Process not running ... */
 		ErtsProcLocks need_locks = ~(*rp_locks) & ERTS_PROC_LOCKS_ALL;
 		if (need_locks
@@ -8765,9 +9722,6 @@ resume_suspend_monitor(ErtsSuspendMonitor *smon, void *vc_p)
 void 
 erts_do_exit_process(Process* p, Eterm reason)
 {
-#ifdef ERTS_SMP
-    erts_aint32_t state;
-#endif
     p->arity = 0;		/* No live registers */
     p->fvalue = reason;
 
@@ -8792,10 +9746,9 @@ erts_do_exit_process(Process* p, Eterm reason)
 #endif
 
 #ifndef ERTS_SMP
-    set_proc_exiting_state(p, erts_smp_atomic32_read_nob(&p->state));
+    set_proc_self_exiting(p);
 #else
-    state = set_proc_exiting_state(p, erts_smp_atomic32_read_nob(&p->state));
-    if (state & ERTS_PSFLG_PENDING_EXIT) {
+    if (ERTS_PSFLG_PENDING_EXIT & set_proc_self_exiting(p)) {
 	/* Process exited before pending exit was received... */
 	p->pending_exit.reason = THE_NON_VALUE;
 	if (p->pending_exit.bp) {
@@ -8849,6 +9802,7 @@ erts_continue_exit_process(Process *p)
     DistEntry *dep;
     struct saved_calls *scb;
     process_breakpoint_time_t *pbt;
+    erts_aint32_t state;
 
 #ifdef DEBUG
     int yield_allowed = 1;
@@ -8885,6 +9839,12 @@ erts_continue_exit_process(Process *p)
 	p->flags &= ~F_USING_DB;
     }
 
+    state = erts_smp_atomic32_read_acqb(&p->state);
+    if (state & ERTS_PSFLG_ACTIVE_SYS) {
+	if (cleanup_sys_tasks(p, state, CONTEXT_REDS) >= CONTEXT_REDS/2)
+	    goto yield;
+    }
+
     if (p->flags & F_USING_DDLL) {
 	erts_ddll_proc_dead(p, ERTS_PROC_LOCK_MAIN);
 	p->flags &= ~F_USING_DDLL;
@@ -8962,17 +9922,31 @@ erts_continue_exit_process(Process *p)
     {
 	/* Inactivate and notify free */
 	erts_aint32_t n, e, a = erts_smp_atomic32_read_nob(&p->state);
+#ifdef ERTS_SMP
+	int refc_inced = 0;
+#endif
 	while (1) {
 	    n = e = a;
 	    ASSERT(a & ERTS_PSFLG_EXITING);
 	    n |= ERTS_PSFLG_FREE;
 	    n &= ~ERTS_PSFLG_ACTIVE;
+#ifdef ERTS_SMP
+	    if ((n & ERTS_PSFLG_IN_RUNQ) && !refc_inced) {
+		erts_smp_proc_inc_refc(p);
+		refc_inced = 1;
+	    }
+#endif
 	    a = erts_smp_atomic32_cmpxchg_mb(&p->state, n, e);
 	    if (a == e)
 		break;
 	}
-    }
 
+#ifdef ERTS_SMP
+	if (refc_inced && !(n & ERTS_PSFLG_IN_RUNQ))
+	    erts_smp_proc_dec_refc(p);
+#endif
+    }
+    
     dep = ((p->flags & F_DISTRIBUTION)
 	   ? ERTS_PROC_SET_DIST_ENTRY(p, ERTS_PROC_LOCKS_ALL, NULL)
 	   : NULL);
@@ -9075,7 +10049,7 @@ timeout_proc(Process* p)
 
     state = erts_smp_atomic32_read_acqb(&p->state);
     if (!(state & ERTS_PSFLG_ACTIVE))
-	schedule_process(p, state, 0);
+	schedule_process(p, state);
 }
 
 
@@ -9153,7 +10127,9 @@ erts_program_counter_info(int to, void *to_arg, Process *p)
     print_function_from_pc(to, to_arg, p->cp);
     erts_print(to, to_arg, ")\n");
     state = erts_smp_atomic32_read_acqb(&p->state);
-    if (!(state & (ERTS_PSFLG_RUNNING|ERTS_PSFLG_GC))) {
+    if (!(state & (ERTS_PSFLG_RUNNING
+		   | ERTS_PSFLG_RUNNING_SYS
+		   | ERTS_PSFLG_GC))) {
         erts_print(to, to_arg, "arity = %d\n",p->arity);
 	if (!ERTS_IS_CRASH_DUMPING) {
 	    /*