[v4,14/16] timer: Implement the hierarchical pull model

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index f61447913db9..95e89495251a 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -244,6 +244,7 @@  enum cpuhp_state {
 	CPUHP_AP_PERF_POWERPC_HV_24x7_ONLINE,
 	CPUHP_AP_PERF_POWERPC_HV_GPCI_ONLINE,
 	CPUHP_AP_PERF_CSKY_ONLINE,
+	CPUHP_AP_TMIGR_ONLINE,
 	CPUHP_AP_WATCHDOG_ONLINE,
 	CPUHP_AP_WORKQUEUE_ONLINE,
 	CPUHP_AP_RANDOM_ONLINE,
diff --git a/kernel/time/Makefile b/kernel/time/Makefile
index 7e875e63ff3b..4af2a264a160 100644
--- a/kernel/time/Makefile
+++ b/kernel/time/Makefile
@@ -17,6 +17,9 @@  endif
 obj-$(CONFIG_GENERIC_SCHED_CLOCK)		+= sched_clock.o
 obj-$(CONFIG_TICK_ONESHOT)			+= tick-oneshot.o tick-sched.o
 obj-$(CONFIG_LEGACY_TIMER_TICK)			+= tick-legacy.o
+ifeq ($(CONFIG_SMP),y)
+ obj-$(CONFIG_NO_HZ_COMMON)			+= timer_migration.o
+endif
 obj-$(CONFIG_HAVE_GENERIC_VDSO)			+= vsyscall.o
 obj-$(CONFIG_DEBUG_FS)				+= timekeeping_debug.o
 obj-$(CONFIG_TEST_UDELAY)			+= test_udelay.o
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index f8b2065df79b..214397d84747 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -53,6 +53,7 @@ 
 #include <asm/io.h>
 
 #include "tick-internal.h"
+#include "timer_migration.h"
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/timer.h>
@@ -592,10 +593,13 @@  trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
 
 	/*
 	 * We might have to IPI the remote CPU if the base is idle and the
-	 * timer is not deferrable. If the other CPU is on the way to idle
-	 * then it can't set base->is_idle as we hold the base lock:
+	 * timer is pinned. If it is a non pinned timer, it is only queued
+	 * on the remote CPU, when timer was running during queueing. Then
+	 * everything is handled by remote CPU anyway. If the other CPU is
+	 * on the way to idle then it can't set base->is_idle as we hold
+	 * the base lock:
 	 */
-	if (base->is_idle)
+	if (base->is_idle && timer->flags & TIMER_PINNED)
 		wake_up_nohz_cpu(base->cpu);
 }
 
@@ -1859,6 +1863,36 @@  void forward_and_idle_timer_bases(unsigned long basej, u64 basem,
 	 */
 	is_idle = time_after(nextevt, basej + 1);
 
+	if (is_idle) {
+		u64 next_tmigr;
+
+		next_tmigr = tmigr_cpu_deactivate(tevt->global);
+
+		tevt->global = KTIME_MAX;
+
+		/*
+		 * If CPU is the last going idle in timer migration
+		 * hierarchy, make sure CPU will wake up in time to handle
+		 * remote timers. next_tmigr == KTIME_MAX if other CPUs are
+		 * still active.
+		 */
+		if (next_tmigr < tevt->local) {
+			u64 tmp;
+
+			/* If we missed a tick already, force 0 delta */
+			if (next_tmigr < basem)
+				next_tmigr = basem;
+
+			tmp = div_u64(next_tmigr - basem, TICK_NSEC);
+
+			nextevt = basej + (unsigned long)tmp;
+			tevt->local = next_tmigr;
+			is_idle = time_after(nextevt, basej + 1);
+		} else {
+			nextevt = nextevt_local;
+		}
+	}
+
 	/* We need to mark both bases in sync */
 	base_local->is_idle = base_global->is_idle = is_idle;
 
@@ -1898,6 +1932,9 @@  void timer_clear_idle(void)
 	 */
 	__this_cpu_write(timer_bases[BASE_LOCAL].is_idle, false);
 	__this_cpu_write(timer_bases[BASE_GLOBAL].is_idle, false);
+
+	/* Activate without holding the timer_base->lock */
+	tmigr_cpu_activate();
 }
 #endif
 
@@ -1954,6 +1991,15 @@  static void run_timer_base(int index)
 	__run_timer_base(base);
 }
 
+#ifdef CONFIG_SMP
+void timer_expire_remote(unsigned int cpu)
+{
+       struct timer_base *base = per_cpu_ptr(&timer_bases[BASE_GLOBAL], cpu);
+
+       __run_timer_base(base);
+}
+#endif
+
 /*
  * This function runs timers and the timer-tq in bottom half context.
  */
@@ -1963,6 +2009,9 @@  static __latent_entropy void run_timer_softirq(struct softirq_action *h)
 	if (IS_ENABLED(CONFIG_NO_HZ_COMMON)) {
 		run_timer_base(BASE_GLOBAL);
 		run_timer_base(BASE_DEF);
+
+		if (is_timers_nohz_active())
+			tmigr_handle_remote();
 	}
 }
 
@@ -1977,7 +2026,8 @@  static void run_local_timers(void)
 
 	for (int i = 0; i < NR_BASES; i++, base++) {
 		/* Raise the softirq only if required. */
-		if (time_after_eq(jiffies, base->next_expiry)) {
+		if (time_after_eq(jiffies, base->next_expiry) ||
+		    (i == BASE_DEF && tmigr_requires_handle_remote())) {
 			raise_softirq(TIMER_SOFTIRQ);
 			return;
 		}
diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
new file mode 100644
index 000000000000..bb0eecae3390
--- /dev/null
+++ b/kernel/time/timer_migration.c
@@ -0,0 +1,1239 @@ 
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Infrastructure for migrateable timers
+ *
+ * Copyright(C) 2022 linutronix GmbH
+ */
+#include <linux/cpuhotplug.h>
+#include <linux/slab.h>
+#include <linux/smp.h>
+#include <linux/spinlock.h>
+#include <linux/timerqueue.h>
+
+#include "timer_migration.h"
+#include "tick-internal.h"
+
+/*
+ * The timer migration mechanism is built on a hierarchy of groups. The
+ * lowest level group contains CPUs, the next level groups of CPU groups
+ * and so forth. The CPU groups are kept per node so for the normal case
+ * lock contention won't happen accross nodes. Depending on the number of
+ * CPUs per node even the next level might be kept as groups of CPU groups
+ * per node and only the levels above cross the node topology.
+ *
+ * Example topology for a two node system with 24 CPUs each.
+ *
+ * LVL 2			[GRP2:0]
+ *			      GRP1:0 = GRP1:M
+ *
+ * LVL 1            [GRP1:0]		         [GRP1:1]
+ *	         GRP0:0 - GRP0:2	      GRP0:3 - GRP0:5
+ *
+ * LVL 0  [GRP0:0]  [GRP0:1]  [GRP0:2]  [GRP0:3]  [GRP0:4]  [GRP0:5]
+ * CPUS     0-7       8-15     16-23     24-31	   32-39     40-47
+ *
+ * The groups hold a timer queue of events sorted by expiry time. These
+ * queues are updated when CPUs go in idle. When they come out of idle
+ * ignore bit of events is set.
+ *
+ * Each group has a designated migrator CPU/group as long as a CPU/group is
+ * active in the group. This designated role is necessary to avoid that all
+ * active CPUs in a group try to migrate expired timers from other cpus,
+ * which would result in massive lock bouncing.
+ *
+ * When a CPU is awake, it checks in it's own timer tick the group
+ * hierarchy up to the point where it is assigned the migrator role or if
+ * no CPU is active, it also checks the groups where no migrator is set
+ * (TMIGR_NONE).
+ *
+ * If it finds expired timers in one of the group queues it pulls them over
+ * from the idle CPU and runs the timer function. After that it updates the
+ * group and the parent groups if required.
+ *
+ * CPUs which go idle arm their CPU local timer hardware for the next local
+ * (pinned) timer event. If the next migrateable timer expires after the
+ * next local timer or the CPU has no migrateable timer pending then the
+ * CPU does not queue an event in the LVL0 group. If the next migrateable
+ * timer expires before the next local timer then the CPU queues that timer
+ * in the LVL0 group. In both cases the CPU marks itself idle in the LVL0
+ * group.
+ *
+ * If the CPU is the migrator of the group then it delegates that role to
+ * the next active CPU in the group or sets migrator to TMIGR_NONE when
+ * there is no active CPU in the group. This delegation needs to be
+ * propagated up the hierarchy so hand over from other leaves can happen at
+ * all hierarchy levels w/o doing a search.
+ *
+ * When the last CPU in the system goes idle, then it drops all migrator
+ * duties up to the top level of the hierarchy (LVL2 in the example). It
+ * then has to make sure, that it arms it's own local hardware timer for
+ * the earliest event in the system.
+ *
+ * Lifetime rules:
+ *
+ * The groups are built up at init time or when CPUs come online. They are
+ * not destroyed when a group becomes empty due to offlining. The group
+ * just won't participate in the hierachary management anymore. Destroying
+ * groups would result in interesting race conditions which would just make
+ * the whole mechanism slow and complex.
+ *
+ * Locking rules:
+ *
+ * For setting up new groups and handling events it's required to lock both
+ * child and parent group. The lock odering is always bottom up. This also
+ * includes the per CPU locks in struct tmigr_cpu. For updating migrator
+ * and active CPU/group information atomic_cmpxchg() is used instead and
+ * only per CPU tmigr_cpu->lock is held.
+ *
+ * During setup of groups tmigr_level_list is required. It is protected by
+ * tmigr_mutex.
+ */
+
+#ifdef DEBUG
+# define DBG_BUG_ON(x) BUG_ON(x)
+#else
+# define DBG_BUG_ON(x)
+#endif
+
+static DEFINE_MUTEX(tmigr_mutex);
+static struct list_head *tmigr_level_list __read_mostly;
+
+static unsigned int tmigr_cores_per_group __read_mostly;
+static unsigned int tmigr_hierarchy_levels __read_mostly;
+static unsigned int tmigr_crossnode_level __read_mostly;
+
+static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
+
+#define TMIGR_NONE	0xFF
+#define BIT_CNT		8
+
+static DEFINE_STATIC_KEY_FALSE(tmigr_enabled);
+
+static inline bool is_tmigr_enabled(void)
+{
+	return static_branch_unlikely(&tmigr_enabled);
+}
+
+/*
+ * Returns true, when @childmask corresponds to group migrator or when group
+ * is not active - so no migrator is set.
+ */
+static bool tmigr_check_migrator(struct tmigr_group *group, u32 childmask)
+{
+	union tmigr_state s;
+
+	s.state = atomic_read(group->migr_state);
+
+	if ((s.migrator != (u8)childmask) && (s.migrator != TMIGR_NONE))
+		return false;
+
+	return true;
+}
+
+typedef bool (*up_f)(struct tmigr_group *, struct tmigr_group *, void *);
+
+static void __walk_groups(up_f up, void *data,
+			  struct tmigr_cpu *tmc)
+{
+	struct tmigr_group *child = NULL, *group = tmc->tmgroup;
+
+	do {
+		DBG_BUG_ON(group->level >= tmigr_hierarchy_levels);
+
+		if (up(group, child, data))
+			break;
+
+		child = group;
+		group = group->parent;
+	} while (group);
+}
+
+static void walk_groups(up_f up, void *data, struct tmigr_cpu *tmc)
+{
+	lockdep_assert_held(&tmc->lock);
+
+	__walk_groups(up, data, tmc);
+}
+
+/**
+ * struct tmigr_walk - data required for walking the hierarchy
+ * @evt:		Pointer to tmigr_event which needs to be queued (of idle
+ *			child group)
+ * @childmask:		childmask of child group
+ * @nextexp:		Next CPU event expiry information which is handed
+ *			into tmigr code by timer code
+ *			(forward_and_idle_timer_bases()); it is furthermore
+ *			used for next event which is queued if timer
+ *			migration hierarchy is completely idle
+ * @childstate:		tmigr_group->migr_state of child - will be only reread
+ *			when cmpxchg in group fails (is required for deactive
+ *			path and new timer path)
+ * @groupstate:		tmigr_group->migr_state of group - will be only reread
+ *			when cmpxchg in group fails (is required for active,
+ *			deactive and new timer path)
+ */
+struct tmigr_walk {
+	struct tmigr_event	*evt;
+	u32			childmask;
+	u64			nextexp;
+	union tmigr_state	childstate;
+	union tmigr_state	groupstate;
+};
+
+/**
+ * struct tmigr_remote_data -	data required for (check) remote expiry
+ *				hierarchy walk
+ * @basej:	timer base in jiffies
+ * @now:	timer base monotonic
+ * @childmask:	childmask of child group
+ * @check:	is set to 1 if there is the need to handle remote timers;
+ *		required in tmigr_check_handle_remote() only
+ * @wakeup:	returns expiry of first timer in idle timer migration hierarchy
+ *		to make sure timer is handled in time; it is stored in per CPU
+ *		tmigr_cpu struct of CPU which expires remote timers
+ */
+struct tmigr_remote_data {
+	unsigned long	basej;
+	u64		now;
+	u32		childmask;
+	int		check;
+	u64		wakeup;
+};
+
+/*
+ * Returns next event of timerqueue @group->events
+ *
+ * Removes timers with ignore bits and update next_expiry and event cpu
+ * value in group. Expiry value of group event is updated in
+ * tmigr_update_events() only.
+ */
+static struct tmigr_event *tmigr_next_groupevt(struct tmigr_group *group)
+{
+	struct timerqueue_node *node = NULL;
+	struct tmigr_event *evt = NULL;
+
+	lockdep_assert_held(&group->lock);
+
+	group->next_expiry = KTIME_MAX;
+
+	while ((node = timerqueue_getnext(&group->events))) {
+		evt = container_of(node, struct tmigr_event, nextevt);
+
+		if (!test_bit(0, &evt->ignore)) {
+			group->next_expiry = evt->nextevt.expires;
+			return evt;
+		}
+
+		/*
+		 * Remove next timers with ignore bits, because group lock
+		 * is held anyway
+		 */
+		if (!timerqueue_del(&group->events, node))
+			break;
+	}
+
+	return NULL;
+}
+
+/*
+ * Return next event which is already expired of group timerqueue
+ *
+ * Event is also removed from queue.
+ */
+static struct tmigr_event *tmigr_next_expired_groupevt(struct tmigr_group *group,
+						     u64 now)
+{
+	struct tmigr_event *evt = tmigr_next_groupevt(group);
+
+	if (!evt || now < evt->nextevt.expires)
+		return NULL;
+
+	/*
+	 * Event is already expired. Remove it. If it's not the last event,
+	 * then update all group event related information.
+	 */
+	if (timerqueue_del(&group->events, &evt->nextevt))
+		tmigr_next_groupevt(group);
+	else
+		group->next_expiry = KTIME_MAX;
+
+	return evt;
+}
+
+static u64 tmigr_next_groupevt_expires(struct tmigr_group *group)
+{
+	struct tmigr_event *evt;
+
+	evt = tmigr_next_groupevt(group);
+
+	if (!evt)
+		return KTIME_MAX;
+	else
+		return evt->nextevt.expires;
+}
+
+static bool tmigr_active_up(struct tmigr_group *group,
+			    struct tmigr_group *child,
+			    void *ptr)
+{
+	union tmigr_state curstate, newstate;
+	struct tmigr_walk *data = ptr;
+	bool walk_done;
+	u32 childmask;
+
+	childmask = data->childmask;
+	newstate = curstate = data->groupstate;
+
+retry:
+	walk_done = true;
+
+	if (newstate.migrator == TMIGR_NONE) {
+		newstate.migrator = (u8)childmask;
+
+		/* Changes need to be propagated */
+		walk_done = false;
+	}
+
+	newstate.active |= (u8)childmask;
+
+	newstate.seq++;
+
+	if (atomic_cmpxchg(group->migr_state, curstate.state, newstate.state) != curstate.state) {
+		newstate.state = curstate.state = atomic_read(group->migr_state);
+		goto retry;
+	}
+
+	if (group->parent && (walk_done == false)) {
+		data->groupstate.state = atomic_read(group->parent->migr_state);
+		data->childmask = group->childmask;
+	}
+
+	/*
+	 * Group is active, event will be ignored - bit is updated without
+	 * holding the lock. In case bit is set while another CPU already
+	 * handles remote events, nothing happens, because it is clear that
+	 * CPU became active just in this moment, or in worst case event is
+	 * handled remote. Nothing to worry about.
+	 */
+	set_bit(0, &group->groupevt.ignore);
+
+	return walk_done;
+}
+
+static void __tmigr_cpu_activate(struct tmigr_cpu *tmc)
+{
+	struct tmigr_walk data;
+	data.childmask = tmc->childmask;
+	data.groupstate.state = atomic_read(tmc->tmgroup->migr_state);
+
+	set_bit(0, &tmc->cpuevt.ignore);
+
+	walk_groups(&tmigr_active_up, &data, tmc);
+}
+
+void tmigr_cpu_activate(void)
+{
+	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+
+	if (!is_tmigr_enabled() || !tmc->tmgroup || !tmc->online || !tmc->idle)
+		return;
+
+	raw_spin_lock(&tmc->lock);
+	tmc->idle = 0;
+	tmc->wakeup = KTIME_MAX;
+	__tmigr_cpu_activate(tmc);
+	raw_spin_unlock(&tmc->lock);
+}
+
+/*
+ * Returns true, if there is nothing to be propagated to the next level
+ *
+ * @data->nextexp is reset to KTIME_MAX; it is reused for first global
+ * event which needs to be handled by migrator (in toplevel group)
+ *
+ * This is the only place where group event expiry value is set.
+ */
+static bool tmigr_update_events(struct tmigr_group *group,
+				struct tmigr_group *child,
+				struct tmigr_walk *data)
+{
+	struct tmigr_event *evt, *first_childevt;
+	bool walk_done;
+	u64 nextexp;
+
+	if (child) {
+		if (data->childstate.active)
+			return true;
+
+		raw_spin_lock(&child->lock);
+		raw_spin_lock_nested(&group->lock, SINGLE_DEPTH_NESTING);
+
+		first_childevt = tmigr_next_groupevt(child);
+		nextexp = child->next_expiry;
+		evt = &child->groupevt;
+	} else {
+		nextexp = data->nextexp;
+
+		/*
+		 * Set @data->nextexp to KTIME_MAX; it is reused for first
+		 * global event which needs to be handled by migrator (in
+		 * toplevel group)
+		 */
+		data->nextexp = KTIME_MAX;
+
+		first_childevt = evt = data->evt;
+		if (test_bit(0, &evt->ignore))
+			return true;
+
+		raw_spin_lock(&group->lock);
+	}
+
+	if (nextexp == KTIME_MAX) {
+		set_bit(0, &evt->ignore);
+		walk_done = true;
+		goto unlock;
+	}
+
+	walk_done = !group->parent;
+
+	/*
+	 * Update of event cpu and ignore bit is required only when @child
+	 * is set (child is equal or higher than lvl0), but it doesn't
+	 * matter if it is written once more to per cpu event; make the
+	 * update unconditional.
+	 */
+	evt->cpu = first_childevt->cpu;
+	clear_bit(0, &evt->ignore);
+
+	/*
+	 * If child event is already queued in group, remove it from queue
+	 * when expiry time changed only
+	 */
+	if (timerqueue_node_queued(&evt->nextevt)) {
+		if (evt->nextevt.expires == nextexp)
+			goto check_toplvl;
+
+		if (evt->nextevt.expires != nextexp &&
+		    !timerqueue_del(&group->events, &evt->nextevt))
+			group->next_expiry = KTIME_MAX;
+	}
+
+	evt->nextevt.expires = nextexp;
+
+	if (timerqueue_add(&group->events, &evt->nextevt))
+		group->next_expiry = nextexp;
+
+check_toplvl:
+	if (walk_done && (data->groupstate.migrator == TMIGR_NONE)) {
+		/*
+		 * Toplevel group is idle and it has to be ensured global
+		 * timers are handled in time. (This could be optimized by
+		 * keeping track of the last global scheduled event and
+		 * only arming it on CPU if the new event is earlier. Not
+		 * sure if its worth the complexity.)
+		 */
+		data->nextexp = tmigr_next_groupevt_expires(group);
+	}
+
+unlock:
+	raw_spin_unlock(&group->lock);
+
+	if (child)
+		raw_spin_unlock(&child->lock);
+
+	return walk_done;
+}
+
+static bool tmigr_new_timer_up(struct tmigr_group *group,
+			       struct tmigr_group *child,
+			       void *ptr)
+{
+	struct tmigr_walk *data = ptr;
+	bool walk_done;
+
+	walk_done = tmigr_update_events(group, child, data);
+
+	if (!walk_done) {
+		/* Update state information for next iteration */
+		data->childstate.state = atomic_read(group->migr_state);
+		if (group->parent)
+			data->groupstate.state = atomic_read(group->parent->migr_state);
+	}
+
+	return walk_done;
+}
+
+/*
+ * Returns expiry of next timer that needs to be handled. KTIME_MAX is
+ * returned, when an active CPU will handle all timer migration hierarchy
+ * timers.
+ */
+static u64 tmigr_new_timer(struct tmigr_cpu *tmc, u64 nextexp)
+{
+	struct tmigr_walk data = { .evt = &tmc->cpuevt,
+				   .nextexp = nextexp };
+
+	lockdep_assert_held(&tmc->lock);
+
+	if (tmc->remote)
+		return KTIME_MAX;
+
+	clear_bit(0, &tmc->cpuevt.ignore);
+
+	data.groupstate.state = atomic_read(tmc->tmgroup->migr_state);
+
+	walk_groups(&tmigr_new_timer_up, &data, tmc);
+
+	/* If there is a new first global event, make sure it is handled */
+	return data.nextexp;
+}
+
+static bool tmigr_inactive_up(struct tmigr_group *group,
+			      struct tmigr_group *child,
+			      void *ptr)
+{
+	union tmigr_state curstate, newstate;
+	struct tmigr_walk *data = ptr;
+	bool walk_done;
+	u32 childmask;
+
+	childmask = data->childmask;
+	newstate = curstate = data->groupstate;
+
+retry:
+	walk_done = true;
+
+	/* Reset active bit when child is no longer active */
+	if (!data->childstate.active)
+		newstate.active &= ~(u8)childmask;
+
+	if (newstate.migrator == (u8)childmask) {
+		/*
+		 * Find a new migrator for the group, because child group
+		 * is idle!
+		 */
+		if (!data->childstate.active) {
+			unsigned long new_migr_bit, active = newstate.active;
+
+			new_migr_bit = find_first_bit(&active, BIT_CNT);
+
+			/* Changes need to be propagated */
+			walk_done = false;
+			data->childmask = group->childmask;
+
+			if (new_migr_bit != BIT_CNT)
+				newstate.migrator = BIT(new_migr_bit);
+			else
+				newstate.migrator = TMIGR_NONE;
+		}
+	}
+
+	newstate.seq++;
+
+	DBG_BUG_ON((newstate.migrator != TMIGR_NONE) && !(newstate.active));
+
+	if (atomic_cmpxchg(group->migr_state, curstate.state, newstate.state) != curstate.state) {
+		/*
+		 * Something changed in child/parent group in the meantime,
+		 * reread the state of child and parent; Update of
+		 * data->childstate is required for event handling;
+		 */
+		if (child)
+			data->childstate.state = atomic_read(child->migr_state);
+		newstate.state = curstate.state = atomic_read(group->migr_state);
+
+		goto retry;
+	}
+
+	data->groupstate = newstate;
+
+	/* Event Handling */
+	tmigr_update_events(group, child, data);
+
+	if (group->parent && (walk_done == false)) {
+		data->childstate = newstate;
+		data->groupstate.state = atomic_read(group->parent->migr_state);
+	}
+
+	/*
+	 * data->nextexp was set by tmigr_update_events() and contains the
+	 * expiry of first global event which needs to be handled
+	 */
+	if (data->nextexp != KTIME_MAX) {
+		DBG_BUG_ON(group->parent);
+		/*
+		 * Toplevel path - If this cpu is about going offline wake
+		 * up some random other cpu so it will take over the
+		 * migrator duty and program its timer properly. Ideally
+		 * wake the cpu with the closest expiry time, but that's
+		 * overkill to figure out.
+		 */
+		if (!(this_cpu_ptr(&tmigr_cpu)->online)) {
+			unsigned int cpu = smp_processor_id();
+
+			cpu = cpumask_any_but(cpu_online_mask, cpu);
+			smp_send_reschedule(cpu);
+		}
+	}
+
+	return walk_done;
+}
+
+static u64 __tmigr_cpu_deactivate(struct tmigr_cpu *tmc, u64 nextexp)
+{
+	struct tmigr_walk data = { .childmask = tmc->childmask,
+				   .evt = &tmc->cpuevt,
+				   .nextexp = nextexp,
+				   .childstate.state = 0 };
+
+	data.groupstate.state = atomic_read(tmc->tmgroup->migr_state);
+
+	/*
+	 * If nextexp is KTIME_MAX, CPU event will be ignored because,
+	 * local timer expires before global timer, no global timer is set
+	 * or CPU goes offline.
+	 */
+	if (nextexp != KTIME_MAX)
+		clear_bit(0, &tmc->cpuevt.ignore);
+
+	walk_groups(&tmigr_inactive_up, &data, tmc);
+	return data.nextexp;
+}
+
+/**
+ * tmigr_cpu_deactivate - Put current CPU into inactive state
+ * @nextexp:	The next timer event expiry set in the current CPU
+ *
+ * Must be called with interrupts disabled.
+ *
+ * Return: next event of the current CPU or next event from the hierarchy
+ * if this CPU is the top level migrator or hierarchy is completely idle.
+ */
+u64 tmigr_cpu_deactivate(u64 nextexp)
+{
+	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+	u64 ret = KTIME_MAX;
+
+	if (!is_tmigr_enabled() || !tmc->tmgroup || !tmc->online)
+		return ret;
+
+	raw_spin_lock(&tmc->lock);
+
+	/*
+	 * CPU is already deactivated in timer migration
+	 * hierarchy. tick_nohz_get_sleep_length() calls
+	 * tick_nohz_next_event() and thereby timer idle path is
+	 * executed once more. tmc->wakeup holds the first timer, when
+	 * timer migration hierarchy is completely idle and remote
+	 * expiry was done. If there is no new next expiry value
+	 * handed in which should be inserted into the timer migration
+	 * hierarchy, wakeup value is returned.
+	 */
+	if (tmc->idle) {
+		ret = tmc->wakeup;
+
+		tmc->wakeup = KTIME_MAX;
+
+		if (nextexp != KTIME_MAX) {
+			if (nextexp != tmc->cpuevt.nextevt.expires ||
+			    test_bit(0, &tmc->cpuevt.ignore))
+				ret = tmigr_new_timer(tmc, nextexp);
+		}
+
+		goto unlock;
+	}
+
+	/*
+	 * When tmigr_remote is active, set cpu inactive path and queuing of
+	 * nextexp is done by handle remote path.
+	 */
+	ret = __tmigr_cpu_deactivate(tmc, nextexp);
+
+	tmc->idle = 1;
+
+unlock:
+	raw_spin_unlock(&tmc->lock);
+	return ret;
+}
+
+static u64 tmigr_handle_remote_cpu(unsigned int cpu, u64 now,
+				   unsigned long jif)
+{
+	struct timer_events tevt;
+	struct tmigr_walk data;
+	struct tmigr_cpu *tmc;
+	u64 next = KTIME_MAX;
+	unsigned long flags;
+
+	tmc = per_cpu_ptr(&tmigr_cpu, cpu);
+
+	raw_spin_lock_irqsave(&tmc->lock, flags);
+	/*
+	 * Remote CPU is offline or no longer idle or other cpu handles cpu
+	 * timers already or next event was already expired - return!
+	 */
+	if (!tmc->online || tmc->remote || test_bit(0, &tmc->cpuevt.ignore) ||
+	    now < tmc->cpuevt.nextevt.expires) {
+		raw_spin_unlock_irqrestore(&tmc->lock, flags);
+		return next;
+	}
+
+	tmc->remote = 1;
+
+	/* Drop the lock to allow the remote CPU to exit idle */
+	raw_spin_unlock_irqrestore(&tmc->lock, flags);
+
+	if (cpu != smp_processor_id())
+		timer_expire_remote(cpu);
+
+	/* next event of cpu */
+	get_next_timer_interrupt_remote(jif, now, &tevt, cpu);
+
+	raw_spin_lock_irqsave(&tmc->lock, flags);
+	/*
+	 * Nothing more to do when CPU came out of idle in the meantime - needs
+	 * to be checked when holding the base lock to prevent race.
+	 */
+	if (!tmc->idle)
+		goto unlock;
+
+	data.evt = &tmc->cpuevt;
+	data.nextexp = tevt.global;
+	data.groupstate.state = atomic_read(tmc->tmgroup->migr_state);
+	clear_bit(0, &tmc->cpuevt.ignore);
+
+	walk_groups(&tmigr_new_timer_up, &data, tmc);
+
+	next = data.nextexp;
+
+unlock:
+	tmc->remote = 0;
+	raw_spin_unlock_irqrestore(&tmc->lock, flags);
+
+	return next;
+}
+
+static bool tmigr_handle_remote_up(struct tmigr_group *group,
+				   struct tmigr_group *child,
+				   void *ptr)
+{
+	struct tmigr_remote_data *data = ptr;
+	u64 now, next = KTIME_MAX;
+	unsigned long flags, jif;
+	struct tmigr_event *evt;
+	u32 childmask;
+
+	jif = data->basej;
+	now = data->now;
+
+	childmask = data->childmask;
+
+again:
+	/*
+	 * Handle the group only if @childmask is the migrator or if the
+	 * group has no migrator. Otherwise the group is active and is
+	 * handled by its own migrator.
+	 */
+	if (!tmigr_check_migrator(group, childmask))
+		return true;
+
+	raw_spin_lock_irqsave(&group->lock, flags);
+
+	evt = tmigr_next_expired_groupevt(group, now);
+
+	if (evt) {
+		unsigned int remote_cpu;
+
+		remote_cpu = READ_ONCE(evt->cpu);
+
+		raw_spin_unlock_irqrestore(&group->lock, flags);
+
+		next = tmigr_handle_remote_cpu(remote_cpu, now, jif);
+
+		/* check if there is another event, that needs to be handled */
+		goto again;
+	} else {
+		raw_spin_unlock_irqrestore(&group->lock, flags);
+	}
+
+	/* Update of childmask for next level */
+	data->childmask = group->childmask;
+	data->wakeup = next;
+
+	return false;
+}
+
+/**
+ * tmigr_handle_remote - Handle migratable timers on remote idle CPUs
+ *
+ * Called from the timer soft interrupt with interrupts enabled.
+ */
+void tmigr_handle_remote(void)
+{
+	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+	struct tmigr_remote_data data;
+	unsigned long flags;
+
+	if (!is_tmigr_enabled() || !tmc->tmgroup || !tmc->online)
+		return;
+
+	/*
+	 * NOTE: This is a doubled check because migrator test will be done
+	 * in tmigr_handle_remote_up() anyway. Keep this check to fasten
+	 * the return when nothing has to be done.
+	 */
+	if (!tmigr_check_migrator(tmc->tmgroup, tmc->childmask))
+		return;
+
+	data.now = get_jiffies_update(&data.basej);
+	data.childmask = tmc->childmask;
+	data.wakeup = KTIME_MAX;
+
+	__walk_groups(&tmigr_handle_remote_up, &data, tmc);
+
+	raw_spin_lock_irqsave(&tmc->lock, flags);
+	if (tmc->idle)
+		tmc->wakeup = data.wakeup;
+
+	raw_spin_unlock_irqrestore(&tmc->lock, flags);
+
+	return;
+}
+
+static bool tmigr_requires_handle_remote_up(struct tmigr_group *group,
+					 struct tmigr_group *child,
+					 void *ptr)
+{
+	struct tmigr_remote_data *data = ptr;
+	u32 childmask;
+
+	childmask = data->childmask;
+
+	/*
+	 * Handle the group only if child is the migrator or if the group
+	 * has no migrator. Otherwise the group is active and is handled by
+	 * its own migrator.
+	 */
+	if (!tmigr_check_migrator(group, childmask))
+		return true;
+
+	/*
+	 * Racy lockless check for next_expiry
+	 */
+	if (data->now >= group->next_expiry) {
+		data->check = 1;
+		return true;
+	}
+
+	/* Update of childmask for next level */
+	data->childmask = group->childmask;
+	return false;
+}
+
+int tmigr_requires_handle_remote(void)
+{
+	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+	struct tmigr_remote_data data;
+
+	if (!is_tmigr_enabled() || !tmc->tmgroup || !tmc->online)
+		return 0;
+
+	if (!tmigr_check_migrator(tmc->tmgroup, tmc->childmask))
+		return 0;
+
+	data.now = get_jiffies_update(&data.basej);
+	data.childmask = tmc->childmask;
+
+	__walk_groups(&tmigr_requires_handle_remote_up, &data, tmc);
+
+	return data.check;
+}
+
+static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,
+			     unsigned int node, atomic_t *migr_state)
+{
+	union tmigr_state s;
+
+	raw_spin_lock_init(&group->lock);
+
+	group->level = lvl;
+	group->numa_node = lvl < tmigr_crossnode_level ? node : NUMA_NO_NODE;
+
+	group->num_childs = 0;
+
+	/*
+	 * num_cores is required for level=0 groups only during setup and
+	 * when siblings exists but it doesn't matter if this value is set
+	 * in other groups as well
+	 */
+	group->num_cores = 1;
+
+	s.migrator = TMIGR_NONE;
+	s.active = 0;
+	s.seq = 0;
+	atomic_set(migr_state, s.state);
+
+	group->migr_state = migr_state;
+
+	timerqueue_init_head(&group->events);
+	timerqueue_init(&group->groupevt.nextevt);
+	group->groupevt.nextevt.expires = KTIME_MAX;
+	group->next_expiry = KTIME_MAX;
+	set_bit(0, &group->groupevt.ignore);
+}
+
+static bool sibling_in_group(int newcpu, struct tmigr_group *group)
+{
+	int i, cpu;
+
+	/* Find a sibling of newcpu in group members */
+	for (i = 0; i < group->num_childs; i++) {
+		cpu = group->cpus[i];
+
+		if (cpumask_test_cpu(newcpu, topology_sibling_cpumask(cpu)))
+			return true;
+	}
+	return false;
+}
+
+static struct tmigr_group *tmigr_get_group(unsigned int cpu, unsigned int node,
+					   unsigned int lvl)
+{
+	struct tmigr_group *tmp, *group = NULL;
+	bool first_loop = true;
+	atomic_t *migr_state;
+
+reloop:
+	/* Try to attach to an exisiting group first */
+	list_for_each_entry(tmp, &tmigr_level_list[lvl], list) {
+		/*
+		 * If @lvl is below the cross numa node level, check whether
+		 * this group belongs to the same numa node.
+		 */
+		if (lvl < tmigr_crossnode_level && tmp->numa_node != node)
+			continue;
+
+		/* Capacity left? */
+		if (tmp->num_childs >= TMIGR_CHILDS_PER_GROUP)
+			continue;
+
+		/*
+		 * If this is the lowest level of the hierarchy, make sure
+		 * that thread siblings share a group. It is only executed
+		 * when siblings exist. ALL groups of lowest level needs to
+		 * be checked for thread sibling, before thread cpu is
+		 * added to a random group with capacity. When all groups
+		 * are checked and no thread sibling was found, reloop of
+		 * level zero groups is required to get a group with
+		 * capacity.
+		 */
+		if (!lvl && (tmigr_cores_per_group != TMIGR_CHILDS_PER_GROUP)) {
+			if (first_loop == true && !sibling_in_group(cpu, tmp)) {
+				continue;
+			} else if (first_loop == false) {
+				if (tmp->num_cores >= tmigr_cores_per_group)
+					continue;
+				else
+					tmp->num_cores++;
+			}
+		}
+
+		group = tmp;
+		break;
+	}
+
+	if (group) {
+		return group;
+	} else if (first_loop == true) {
+		first_loop = false;
+		goto reloop;
+	}
+
+	/* Allocate and	set up a new group with corresponding migr_state */
+	group = kzalloc_node(sizeof(*group), GFP_KERNEL, node);
+	if (!group)
+		return ERR_PTR(-ENOMEM);
+
+	migr_state = kzalloc_node(sizeof(atomic_t), GFP_KERNEL, node);
+	if (!migr_state) {
+		kfree(group);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	tmigr_init_group(group, lvl, node, migr_state);
+	/* Setup successful. Add it to the hierarchy */
+	list_add(&group->list, &tmigr_level_list[lvl]);
+	return group;
+}
+
+static void tmigr_connect_child_parent(struct tmigr_group *child,
+				       struct tmigr_group *parent)
+{
+	union tmigr_state childstate;
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&child->lock, flags);
+	raw_spin_lock_nested(&parent->lock, SINGLE_DEPTH_NESTING);
+
+	child->parent = parent;
+	child->childmask = BIT(parent->num_childs++);
+
+	raw_spin_unlock(&parent->lock);
+	raw_spin_unlock_irqrestore(&child->lock, flags);
+
+	/*
+	 * To prevent inconsistent states, active childs needs to be active
+	 * in new parent as well. Inactive childs are already marked
+	 * inactive in parent group.
+	 */
+	childstate.state = atomic_read(child->migr_state);
+	if (childstate.migrator != TMIGR_NONE) {
+		struct tmigr_walk data;
+
+		data.childmask = child->childmask;
+		data.groupstate.state = atomic_read(parent->migr_state);
+
+		/*
+		 * There is only one new level per time. When connecting
+		 * child and parent and set child active when parent is
+		 * inactive, parent needs to be the upperst
+		 * level. Otherwise there went something wrong!
+		 */
+		WARN_ON(!tmigr_active_up(parent, child, &data) && parent->parent);
+	}
+}
+
+static int tmigr_setup_groups(unsigned int cpu, unsigned int node)
+{
+	struct tmigr_group *group, *child, **stack;
+	int top = 0, err = 0, i = 0;
+	struct list_head *lvllist;
+	size_t sz;
+
+	sz = sizeof(struct tmigr_group *) * tmigr_hierarchy_levels;
+	stack = kzalloc(sz, GFP_KERNEL);
+	if (!stack)
+		return -ENOMEM;
+
+	do {
+		group = tmigr_get_group(cpu, node, i);
+		if (IS_ERR(group)) {
+			err = IS_ERR(group);
+			break;
+		}
+
+		top = i;
+		stack[i++] = group;
+
+		/*
+		 * When booting only less CPUs of a system than CPUs are
+		 * available, not all calculated hierarchy levels are required.
+		 *
+		 * The loop is aborted as soon as the highest level, which might
+		 * be different from tmigr_hierarchy_levels, contains only a
+		 * single group.
+		 */
+		if (group->parent || i == tmigr_hierarchy_levels ||
+		    (list_empty(&tmigr_level_list[i]) &&
+		     list_is_singular(&tmigr_level_list[i - 1])))
+			break;
+
+	} while (i < tmigr_hierarchy_levels);
+
+	do {
+		group = stack[--i];
+
+		if (err < 0) {
+			list_del(&group->list);
+			kfree(group);
+			continue;
+		}
+
+		DBG_BUG_ON(i != group->level);
+
+		/*
+		 * Update tmc -> group / child -> group connection
+		 */
+		if (i == 0) {
+			struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+			unsigned long flags;
+
+			raw_spin_lock_irqsave(&group->lock, flags);
+
+			tmc->tmgroup = group;
+			tmc->childmask = BIT(group->num_childs);
+
+			group->cpus[group->num_childs++] = cpu;
+
+			raw_spin_unlock_irqrestore(&group->lock, flags);
+
+			/* There are no childs that needs to be connected */
+			continue;
+		} else {
+			child = stack[i - 1];
+			tmigr_connect_child_parent(child, group);
+		}
+
+		/* check if upperst level was newly created */
+		if (top != i)
+			continue;
+
+		DBG_BUG_ON(top == 0);
+
+		lvllist = &tmigr_level_list[top];
+		if (group->num_childs == 1 && list_is_singular(lvllist)) {
+			lvllist = &tmigr_level_list[top - 1];
+			list_for_each_entry(child, lvllist, list) {
+				if (child->parent)
+					continue;
+
+				tmigr_connect_child_parent(child, group);
+			}
+		}
+	} while (i > 0);
+
+	kfree(stack);
+
+	return err;
+}
+
+static int tmigr_add_cpu(unsigned int cpu)
+{
+	unsigned int node = cpu_to_node(cpu);
+	int ret;
+	mutex_lock(&tmigr_mutex);
+	ret = tmigr_setup_groups(cpu, node);
+	mutex_unlock(&tmigr_mutex);
+
+	return ret;
+}
+
+static int tmigr_cpu_online(unsigned int cpu)
+{
+	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+	unsigned long flags;
+	unsigned int ret;
+
+	/* First online attempt? Initialize CPU data */
+	if (!tmc->tmgroup) {
+		raw_spin_lock_init(&tmc->lock);
+
+		ret = tmigr_add_cpu(cpu);
+		if (ret < 0)
+			return ret;
+
+		if (tmc->childmask == 0)
+			return -EINVAL;
+
+		timerqueue_init(&tmc->cpuevt.nextevt);
+		tmc->cpuevt.nextevt.expires = KTIME_MAX;
+		set_bit(0, &tmc->cpuevt.ignore);
+		tmc->cpuevt.cpu = cpu;
+
+		tmc->remote = 0;
+		tmc->idle = 0;
+		tmc->wakeup = KTIME_MAX;
+	}
+	raw_spin_lock_irqsave(&tmc->lock, flags);
+	__tmigr_cpu_activate(tmc);
+	tmc->online = 1;
+	raw_spin_unlock_irqrestore(&tmc->lock, flags);
+	return 0;
+}
+
+static int tmigr_cpu_offline(unsigned int cpu)
+{
+	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+
+	raw_spin_lock_irq(&tmc->lock);
+	tmc->online = 0;
+	__tmigr_cpu_deactivate(tmc, KTIME_MAX);
+	raw_spin_unlock_irq(&tmc->lock);
+
+	return 0;
+}
+
+static int __init tmigr_init(void)
+{
+	unsigned int cpulvl, nodelvl, cpus_per_node, i, ns;
+	unsigned int nnodes = num_possible_nodes();
+	unsigned int ncpus = num_possible_cpus();
+	int ret = -ENOMEM;
+	size_t sz;
+
+	/* Nothing to do if running on UP */
+	if (ncpus == 1)
+		return 0;
+
+	/*
+	 * Unfortunately there is no reliable way to determine the number of SMT
+	 * siblings in a generic way. tmigr_init() is called after SMP bringup,
+	 * so for the normal boot case it can be assumed that all siblings have
+	 * been brought up and the number of siblings of the current cpu can be
+	 * used. If someone booted with 'maxcpus=N/2' on the kernel command line
+	 * and (at least x86) bring up the siblings later then the siblings will
+	 * end up in different groups. Bad luck.
+	 */
+	ns = cpumask_weight(topology_sibling_cpumask(raw_smp_processor_id()));
+	tmigr_cores_per_group = TMIGR_CHILDS_PER_GROUP;
+	if (ns >= 2 && ns < TMIGR_CHILDS_PER_GROUP)
+		tmigr_cores_per_group /= ns;
+
+	/*
+	 * Calculate the required hierarchy levels. Unfortunately there is no
+	 * reliable information available, unless all possible CPUs have been
+	 * brought up and all numa nodes are populated.
+	 *
+	 * Estimate the number of levels with the number of possible nodes and
+	 * the number of possible cpus. Assume CPUs are spread evenly accross
+	 * nodes. We cannot rely on cpumask_of_node() because there only already
+	 * online CPUs are considered.
+	 */
+	cpus_per_node = DIV_ROUND_UP(ncpus, nnodes);
+
+	/* Calc the hierarchy levels required to hold the CPUs of a node */
+	cpulvl = DIV_ROUND_UP(order_base_2(cpus_per_node),
+			      ilog2(TMIGR_CHILDS_PER_GROUP));
+
+	/* Calculate the extra levels to connect all nodes */
+	nodelvl = DIV_ROUND_UP(order_base_2(nnodes),
+			       ilog2(TMIGR_CHILDS_PER_GROUP));
+
+	tmigr_hierarchy_levels = cpulvl + nodelvl;
+
+	/*
+	 * If a numa node spawns more than one CPU level group then the next
+	 * level(s) of the hierarchy contains groups which handle all CPU groups
+	 * of the same numa node. The level above goes accross numa nodes. Store
+	 * this information for the setup code to decide when node matching is
+	 * not longer required.
+	 */
+	tmigr_crossnode_level = cpulvl;
+
+	sz = sizeof(struct list_head) * tmigr_hierarchy_levels;
+	tmigr_level_list = kzalloc(sz, GFP_KERNEL);
+	if (!tmigr_level_list)
+		goto err;
+
+	for (i = 0; i < tmigr_hierarchy_levels; i++)
+		INIT_LIST_HEAD(&tmigr_level_list[i]);
+
+	pr_info("Timer migration: %d hierarchy levels; %d childs per group;"
+		" %d cores_per_group; %d crossnode level\n",
+		tmigr_hierarchy_levels, TMIGR_CHILDS_PER_GROUP,
+		tmigr_cores_per_group, tmigr_crossnode_level);
+
+	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
+				tmigr_cpu_online, tmigr_cpu_offline);
+	if (ret)
+		goto err;
+
+	static_branch_enable(&tmigr_enabled);
+
+	return 0;
+
+err:
+	pr_err("Timer migration setup failed\n");
+	return ret;
+}
+late_initcall(tmigr_init);
diff --git a/kernel/time/timer_migration.h b/kernel/time/timer_migration.h
new file mode 100644
index 000000000000..ab2953c89ef2
--- /dev/null
+++ b/kernel/time/timer_migration.h
@@ -0,0 +1,123 @@ 
+#ifndef _KERNEL_TIME_MIGRATION_H
+#define _KERNEL_TIME_MIGRATION_H
+
+/* Per group capacity. Must be a power of 2! */
+#define TMIGR_CHILDS_PER_GROUP 8
+
+/**
+ * struct tmigr_event - a timer event associated to a CPU
+ * @nextevt:	The node to enqueue an event in the parent group queue
+ * @cpu:	The CPU to which this event belongs
+ * @ignore:	Hint whether the event could be ignored; it is set when
+ *		CPU or group is active;
+ */
+struct tmigr_event {
+	struct timerqueue_node	nextevt;
+	unsigned int		cpu;
+	unsigned long		ignore;
+};
+
+/**
+ * struct tmigr_group - timer migration hierarchy group
+ * @lock:		Lock protecting the event information
+ * @cpus:		Array with CPUs which are member of group; required for
+ *			sibling CPUs; used only when level == 0
+ * @parent:		Pointer to parent group
+ * @list:		List head that is added to per level tmigr_level_list
+ * @level:		Hierarchy level of group
+ * @numa_node:		Is set to numa node when level < tmigr_crossnode_level;
+ *			otherwise it is set to NUMA_NO_NODE; Required for setup
+ *			only
+ * @num_childs:		Counter of group childs; Required for setup only
+ * @num_cores:		Counter of cores per group; Required for setup only when
+ *			level == 0 and siblings exist
+ * @migr_state:		State of group (see struct tmigr_state)
+ * @childmask:		childmask of group in parent group; is set during setup
+ *			never changed; could be read lockless
+ * @events:		Timer queue for child events queued in the group
+ * @groupevt:		Next event of group; it is only reliable when group is
+ *			!active (ignore bit is set when group is active)
+ * @next_expiry:	Base monotonic expiry time of next event of group;
+ *			Used for racy lockless check whether remote expiry is
+ *			required; it is always reliable
+ */
+struct tmigr_group {
+	raw_spinlock_t		lock;
+	unsigned int		cpus[TMIGR_CHILDS_PER_GROUP];
+	struct tmigr_group	*parent;
+	struct list_head	list;
+	unsigned int		level;
+	unsigned int		numa_node;
+	unsigned int		num_childs;
+	unsigned int		num_cores;
+	atomic_t		*migr_state;
+	u32			childmask;
+	struct timerqueue_head	events;
+	struct tmigr_event	groupevt;
+	u64			next_expiry;
+};
+
+/**
+ * struct tmigr_cpu - timer migration per CPU group
+ * @lock:	Lock protecting tmigr_cpu group information
+ * @online:	Indicates wheter CPU is online
+ * @idle:	Indicates wheter CPU is idle in timer migration hierarchy
+ * @remote:	Is set when timers of CPU are expired remote
+ * @tmgroup:	Pointer to parent group
+ * @childmask:	childmask of tmigr_cpu in parent group
+ * @cpuevt:	CPU event which could be queued into parent group
+ * @wakeup:	Stores the first timer when the timer migration hierarchy is
+ *		completely idle and remote expiry was done; is returned to
+ *		timer code when tmigr_cpu_deactive() is called and group is
+ *		idle; afterwards a reset to KTIME_MAX is required;
+ */
+struct tmigr_cpu {
+	raw_spinlock_t		lock;
+	int			online;
+	int			idle;
+	int			remote;
+	struct tmigr_group	*tmgroup;
+	u32			childmask;
+	struct tmigr_event	cpuevt;
+	u64			wakeup;
+};
+
+/**
+ * union tmigr_state - state of tmigr_group
+ * @state:	Combined version of the state - only used for atomic
+ * 		read/cmpxchg function
+ * @struct:	Splitted version of the state - only use the struct members to
+ *		update information to stay independant of endianess
+ */
+union tmigr_state {
+	u32 state;
+	/**
+	 * struct - splitted state of tmigr_group
+	 * @active:	Contains each childmask bit of active childs
+	 * @migrator:	Contains childmask of child which is migrator
+	 * @seq:	Seqence number to prevent race when update in child
+	 *		group are propagated in wrong order (especially when
+	 *		migrator changes are involved)
+	 */
+	struct {
+		u8	active;
+		u8	migrator;
+		u16	seq;
+	} __packed;
+};
+
+#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
+extern void tmigr_handle_remote(void);
+extern int tmigr_requires_handle_remote(void);
+extern void tmigr_cpu_activate(void);
+extern u64 tmigr_cpu_deactivate(u64 nextevt);
+extern void timer_expire_remote(unsigned int cpu);
+#else
+static inline void tmigr_handle_remote(void) { }
+extern inline int tmigr_requires_handle_remote(void) { return 0; }
+static inline void tmigr_cpu_activate(void) { }
+static inline u64 tmigr_cpu_deactivate(u64 nextevt) { return KTIME_MAX; }
+extern inline void timer_expire_remote(unsigned int cpu) { }
+#endif
+
+#endif