@@ -231,6 +231,7 @@ 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,
@@ -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
@@ -166,6 +166,7 @@ extern void fetch_next_timer_interrupt_remote(unsigned long basej, u64 basem,
extern void timer_lock_remote_bases(unsigned int cpu);
extern void timer_unlock_remote_bases(unsigned int cpu);
extern bool timer_base_is_idle(void);
+extern void timer_expire_remote(unsigned int cpu);
# endif
#else /* CONFIG_NO_HZ_COMMON */
static inline void timers_update_nohz(void) { }
@@ -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>
@@ -2169,6 +2170,64 @@ bool timer_base_is_idle(void)
{
return __this_cpu_read(timer_bases[BASE_LOCAL].is_idle);
}
+
+static void __run_timer_base(struct timer_base *base);
+
+/**
+ * timer_expire_remote() - expire global timers of cpu
+ * @cpu: Remote CPU
+ *
+ * Expire timers of global base of remote CPU.
+ */
+void timer_expire_remote(unsigned int cpu)
+{
+ struct timer_base *base = per_cpu_ptr(&timer_bases[BASE_GLOBAL], cpu);
+
+ __run_timer_base(base);
+}
+
+static void timer_use_tmigr(unsigned long basej, u64 basem,
+ unsigned long *nextevt, bool *tick_stop_path,
+ bool timer_base_idle, struct timer_events *tevt)
+{
+ u64 next_tmigr;
+
+ if (timer_base_idle)
+ next_tmigr = tmigr_cpu_new_timer(tevt->global);
+ else if (tick_stop_path)
+ next_tmigr = tmigr_cpu_deactivate(tevt->global);
+ else
+ next_tmigr = tmigr_quick_check(tevt->global);
+
+ /*
+ * If the CPU is the last going idle in timer migration hierarchy, make
+ * sure the 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;
+ }
+}
+# else
+static void timer_use_tmigr(unsigned long basej, u64 basem,
+ unsigned long *nextevt, bool *tick_stop_path,
+ bool timer_base_idle, struct timer_events *tevt)
+{
+ /*
+ * Make sure first event is written into tevt->local to not miss a
+ * timer on !SMP systems.
+ */
+ tevt->local = min_t(u64, tevt->local, tevt->global);
+}
# endif /* CONFIG_SMP */
static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
@@ -2177,7 +2236,7 @@ static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
struct timer_events tevt = { .local = KTIME_MAX, .global = KTIME_MAX };
struct timer_base *base_local, *base_global;
unsigned long nextevt;
- u64 expires;
+ bool idle_is_possible;
/*
* Pretend that there is no timer pending if the cpu is offline.
@@ -2199,6 +2258,22 @@ static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
base_global, &tevt);
/*
+ * If the next event is only one jiffie ahead there is no need to call
+ * timer migration hierarchy related functions. The value for the next
+ * global timer in @tevt struct equals then KTIME_MAX. This is also
+ * true, when the timer base is idle.
+ *
+ * The proper timer migration hierarchy function depends on the callsite
+ * and whether timer base is idle or not. @nextevt will be updated when
+ * this CPU needs to handle the first timer migration hierarchy
+ * event. See timer_use_tmigr() for detailed information.
+ */
+ idle_is_possible = time_after(nextevt, basej + 1);
+ if (idle_is_possible)
+ timer_use_tmigr(basej, basem, &nextevt, idle,
+ base_local->is_idle, &tevt);
+
+ /*
* We have a fresh next event. Check whether we can forward the
* base.
*/
@@ -2210,7 +2285,10 @@ static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
*/
if (idle) {
/*
- * Bases are idle if the next event is more than a tick away.
+ * Bases are idle if the next event is more than a tick
+ * away. Caution: @nextevt could have changed by enqueueing a
+ * global timer into timer migration hierarchy. Therefore a new
+ * check is required here.
*
* If the base is marked idle then any timer add operation must
* forward the base clk itself to keep granularity small. This
@@ -2223,14 +2301,23 @@ static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
trace_timer_base_idle(true, base_local->cpu);
}
*idle = base_local->is_idle;
+
+ /*
+ * When timer base is not set idle, undo the effect of
+ * tmigr_cpu_deactivate() to prevent inconsitent states - active
+ * timer base but inactive timer migration hierarchy.
+ *
+ * When timer base was already marked idle, nothing will be
+ * changed here.
+ */
+ if (!base_local->is_idle && idle_is_possible)
+ tmigr_cpu_activate();
}
raw_spin_unlock(&base_global->lock);
raw_spin_unlock(&base_local->lock);
- expires = min_t(u64, tevt.local, tevt.global);
-
- return cmp_next_hrtimer_event(basem, expires);
+ return cmp_next_hrtimer_event(basem, tevt.local);
}
/**
@@ -2238,8 +2325,11 @@ static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
* @basej: base time jiffies
* @basem: base time clock monotonic
*
- * Returns the tick aligned clock monotonic time of the next pending
- * timer or KTIME_MAX if no timer is pending.
+ * Returns the tick aligned clock monotonic time of the next pending timer or
+ * KTIME_MAX if no timer is pending. If timer of global base was queued into
+ * timer migration hierarchy, first global timer is not taken into account. If
+ * it was the last CPU of timer migration hierarchy going idle, first global
+ * event is taken into account.
*/
u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
{
@@ -2281,6 +2371,9 @@ 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);
trace_timer_base_idle(false, smp_processor_id());
+
+ /* Activate without holding the timer_base->lock */
+ tmigr_cpu_activate();
}
#endif
@@ -2350,6 +2443,9 @@ 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();
}
}
@@ -2364,7 +2460,8 @@ 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;
}
new file mode 100644
@@ -0,0 +1,1761 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Infrastructure for migratable 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 <trace/events/ipi.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 across 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 flag 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 migratable timer expires after the
+ * next local timer or the CPU has no migratable timer pending then the
+ * CPU does not queue an event in the LVL0 group. If the next migratable
+ * 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.
+ *
+ * When CPU comes out of idle and when a group has at least a single active
+ * child, the ignore flag of the tmigr_event is set. This indicates, that
+ * the event is ignored even if it is still enqueued in the parent groups
+ * timer queue. It will be removed when touching the timer queue the next
+ * time. This spares locking in active path as the lock protects (after
+ * setup) only event information. For more information about locking,
+ * please read the section "Locking rules".
+ *
+ * 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 hierarchy 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 ordering is always bottom up. This also
+ * includes the per CPU locks in struct tmigr_cpu. For updating the migrator and
+ * active CPU/group information atomic_try_cmpxchg() is used instead and only
+ * the per CPU tmigr_cpu->lock is held.
+ *
+ * During the setup of groups tmigr_level_list is required. It is protected by
+ * @tmigr_mutex.
+ *
+ * When @timer_base->lock as well as tmigr related locks are required, the lock
+ * ordering is: first @timer_base->lock, afterwards tmigr related locks.
+ *
+ *
+ * Protection of the tmigr group state information:
+ * ------------------------------------------------
+ *
+ * The state information with the list of active children and migrator needs to
+ * be protected by a sequence counter. It prevents a race when updates in child
+ * groups are propagated in changed order. The state update is performed
+ * lockless and group wise. The following scenario describes what happens
+ * without updating the sequence counter:
+ *
+ * Therefore, let's take three groups and four CPUs (CPU2 and CPU3 as well
+ * as GRP0:1 will not change during the scenario):
+ *
+ * LVL 1 [GRP1:0]
+ * migrator = GRP0:1
+ * active = GRP0:0, GRP0:1
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = CPU0 migrator = CPU2
+ * active = CPU0 active = CPU2
+ * / \ / \
+ * CPUs 0 1 2 3
+ * active idle active idle
+ *
+ *
+ * 1. CPU0 goes idle. As the update is performed group wise, in the first step
+ * only GRP0:0 is updated. The update of GRP1:0 is pending as CPU0 has to
+ * walk the hierarchy.
+ *
+ * LVL 1 [GRP1:0]
+ * migrator = GRP0:1
+ * active = GRP0:0, GRP0:1
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * --> migrator = TMIGR_NONE migrator = CPU2
+ * --> active = active = CPU2
+ * / \ / \
+ * CPUs 0 1 2 3
+ * --> idle idle active idle
+ *
+ * 2. While CPU0 goes idle and continues to update the state, CPU1 comes out of
+ * idle. CPU1 updates GRP0:0. The update for GRP1:0 is pending as CPU1 also
+ * has to walk the hierarchy. Both CPUs (CPU0 and CPU1) now walk the
+ * hierarchy to perform the needed update from their point of view. The
+ * currently visible state looks the following:
+ *
+ * LVL 1 [GRP1:0]
+ * migrator = GRP0:1
+ * active = GRP0:0, GRP0:1
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * --> migrator = CPU1 migrator = CPU2
+ * --> active = CPU1 active = CPU2
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle --> active active idle
+ *
+ * 3. Here is the race condition: CPU1 managed to propagate its changes (from
+ * step 2) through the hierarchy to GRP1:0 before CPU0 (step 1) did. The
+ * active members of GRP1:0 remain unchanged after the update since it is
+ * still valid from CPU1 current point of view:
+ *
+ * LVL 1 [GRP1:0]
+ * --> migrator = GRP0:1
+ * --> active = GRP0:0, GRP0:1
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = CPU1 migrator = CPU2
+ * active = CPU1 active = CPU2
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle active active idle
+ *
+ * 4. Now CPU0 finally propagates its changes (from step 1) to GRP1:0.
+ *
+ * LVL 1 [GRP1:0]
+ * --> migrator = GRP0:1
+ * --> active = GRP0:1
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = CPU1 migrator = CPU2
+ * active = CPU1 active = CPU2
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle active active idle
+ *
+ *
+ * The race of CPU0 vs. CPU1 led to an inconsistent state in GRP1:0. CPU1 is
+ * active and is correctly listed as active in GRP0:0. However GRP1:0 does not
+ * have GRP0:0 listed as active, which is wrong. The sequence counter has been
+ * added to avoid inconsistent states during updates. The state is updated
+ * atomically only if all members, including the sequence counter, match the
+ * expected value (compare-and-exchange).
+ *
+ * Looking back at the previous example with the addition of the sequence
+ * counter: The update as performed by CPU0 in step 4 will fail. CPU1 changed
+ * the sequence number during the update in step 3 so the expected old value (as
+ * seen by CPU0 before starting the walk) does not match.
+ *
+ * Prevent race between new event and last CPU going inactive
+ * ----------------------------------------------------------
+ *
+ * When the last CPU is going idle and there is a concurrent update of a new
+ * first global timer of an idle CPU, the group and child states have to be read
+ * while holding the lock in tmigr_update_events(). The following scenario shows
+ * what happens, when this is not done.
+ *
+ * 1. Only CPU2 is active:
+ *
+ * LVL 1 [GRP1:0]
+ * migrator = GRP0:1
+ * active = GRP0:1
+ * next_expiry = KTIME_MAX
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = TMIGR_NONE migrator = CPU2
+ * active = active = CPU2
+ * next_expiry = KTIME_MAX next_expiry = KTIME_MAX
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle idle active idle
+ *
+ * 2. Now CPU 2 goes idle (and has no global timer, that has to be handled) and
+ * propagates that to GRP0:1:
+ *
+ * LVL 1 [GRP1:0]
+ * migrator = GRP0:1
+ * active = GRP0:1
+ * next_expiry = KTIME_MAX
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = TMIGR_NONE --> migrator = TMIGR_NONE
+ * active = --> active =
+ * next_expiry = KTIME_MAX next_expiry = KTIME_MAX
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle idle --> idle idle
+ *
+ * 3. Now the idle state is propagated up to GRP1:0. As this is now the last
+ * child going idle in top level group, the expiry of the next group event
+ * has to be handed back to make sure no event is lost. As there is no event
+ * enqueued, KTIME_MAX is handed back to CPU2.
+ *
+ * LVL 1 [GRP1:0]
+ * --> migrator = TMIGR_NONE
+ * --> active =
+ * next_expiry = KTIME_MAX
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = TMIGR_NONE migrator = TMIGR_NONE
+ * active = active =
+ * next_expiry = KTIME_MAX next_expiry = KTIME_MAX
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle idle --> idle idle
+ *
+ * 4. CPU 0 has a new timer queued from idle and it expires at TIMER0. CPU0
+ * propagates that to GRP0:0:
+ *
+ * LVL 1 [GRP1:0]
+ * migrator = TMIGR_NONE
+ * active =
+ * next_expiry = KTIME_MAX
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = TMIGR_NONE migrator = TMIGR_NONE
+ * active = active =
+ * --> next_expiry = TIMER0 next_expiry = KTIME_MAX
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle idle idle idle
+ *
+ * 5. GRP0:0 is not active, so the new timer has to be propagated to
+ * GRP1:0. Therefore the GRP1:0 state has to be read. When the stalled value
+ * (from step 2) is read, the timer is enqueued into GRP1:0, but nothing is
+ * handed back to CPU0, as it seems that there is still an active child in
+ * top level group.
+ *
+ * LVL 1 [GRP1:0]
+ * migrator = TMIGR_NONE
+ * active =
+ * --> next_expiry = TIMER0
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = TMIGR_NONE migrator = TMIGR_NONE
+ * active = active =
+ * next_expiry = TIMER0 next_expiry = KTIME_MAX
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle idle idle idle
+ *
+ * This is prevented by reading the state when holding the lock (when a new
+ * timer has to be propagated from idle path)::
+ *
+ * CPU2 (tmigr_inactive_up()) CPU0 (tmigr_new_timer_up())
+ * -------------------------- ---------------------------
+ * // step 3:
+ * cmpxchg(&GRP1:0->state);
+ * tmigr_update_events() {
+ * spin_lock(&GRP1:0->lock);
+ * // ... update events ...
+ * // hand back first expiry when GRP1:0 is idle
+ * spin_unlock(&GRP1:0->lock);
+ * // ^^^ release state modification
+ * }
+ * tmigr_update_events() {
+ * spin_lock(&GRP1:0->lock)
+ * // ^^^ acquire state modification
+ * group_state = atomic_read(&GRP1:0->state)
+ * // .... update events ...
+ * // hand back first expiry when GRP1:0 is idle
+ * spin_unlock(&GRP1:0->lock) <3>
+ * // ^^^ makes state visible for other
+ * // callers of tmigr_new_timer_up()
+ * }
+ *
+ * When CPU0 grabs the lock directly after cmpxchg, the first timer is reported
+ * back to CPU0 and also later on to CPU2. So no timer is missed. A concurrent
+ * update of the group state from active path is no problem, as the upcoming CPU
+ * will take care of the group events.
+ *
+ * Required event and timerqueue update after a remote expiry:
+ * -----------------------------------------------------------
+ *
+ * After expiring timers of a remote CPU, a walk through the hierarchy and
+ * update of events and timerqueues is required. It is obviously needed if there
+ * is a 'new' global timer but also if there is no new global timer but the
+ * remote CPU is still idle.
+ *
+ * 1. CPU0 and CPU1 are idle and have both a global timer expiring at the same
+ * time. So both have an event enqueued in the timerqueue of GRP0:0. CPU3 is
+ * also idle and has no global timer pending. CPU2 is the only active CPU and
+ * thus also the migrator:
+ *
+ * LVL 1 [GRP1:0]
+ * migrator = GRP0:1
+ * active = GRP0:1
+ * --> timerqueue = evt-GRP0:0
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = TMIGR_NONE migrator = CPU2
+ * active = active = CPU2
+ * groupevt.ignore = false groupevt.ignore = true
+ * groupevt.cpu = CPU0 groupevt.cpu =
+ * timerqueue = evt-CPU0, timerqueue =
+ * evt-CPU1
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle idle active idle
+ *
+ * 2. CPU2 starts to expire remote timers. It starts with LVL0 group
+ * GRP0:1. There is no event queued in the timerqueue, so CPU2 continues with
+ * the parent of GRP0:1: GRP1:0. In GRP1:0 it dequeues the first event. It
+ * looks at tmigr_event::cpu struct member and expires the pending timer(s)
+ * of CPU0.
+ *
+ * LVL 1 [GRP1:0]
+ * migrator = GRP0:1
+ * active = GRP0:1
+ * --> timerqueue =
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = TMIGR_NONE migrator = CPU2
+ * active = active = CPU2
+ * groupevt.ignore = false groupevt.ignore = true
+ * --> groupevt.cpu = CPU0 groupevt.cpu =
+ * timerqueue = evt-CPU0, timerqueue =
+ * evt-CPU1
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle idle active idle
+ *
+ * 3. Some work has to be done after expiring the timers of CPU0. If we stop
+ * here, then CPU1's pending global timer(s) will not expire in time and the
+ * timerqueue of GRP0:0 has still an event for CPU0 enqueued which has just
+ * been processed. So it is required to walk the hierarchy from CPU0's point
+ * of view and update it accordingly. CPU0's event will be removed from the
+ * timerqueue because it has no pending timer. If CPU0 would have a timer
+ * pending then it has to expire after CPU1's first timer because all timers
+ * from this period were just expired. Either way CPU1's event will be first
+ * in GRP0:0's timerqueue and therefore set in the CPU field of the group
+ * event which is then enqueued in GRP1:0's timerqueue as GRP0:0 is still not
+ * active:
+ *
+ * LVL 1 [GRP1:0]
+ * migrator = GRP0:1
+ * active = GRP0:1
+ * --> timerqueue = evt-GRP0:0
+ * / \
+ * LVL 0 [GRP0:0] [GRP0:1]
+ * migrator = TMIGR_NONE migrator = CPU2
+ * active = active = CPU2
+ * groupevt.ignore = false groupevt.ignore = true
+ * --> groupevt.cpu = CPU1 groupevt.cpu =
+ * --> timerqueue = evt-CPU1 timerqueue =
+ * / \ / \
+ * CPUs 0 1 2 3
+ * idle idle active idle
+ *
+ * Now CPU2 (migrator) will continue step 2 at GRP1:0 and will expire the
+ * timer(s) of CPU1.
+ *
+ * The hierarchy walk in step 3 can be skipped if the migrator notices that a
+ * CPU of GRP0:0 is active again. The CPU will mark GRP0:0 active and take care
+ * of the group as migrator and any needed updates within the hierarchy.
+ */
+
+static DEFINE_MUTEX(tmigr_mutex);
+static struct list_head *tmigr_level_list __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 inline bool tmigr_is_not_available(struct tmigr_cpu *tmc)
+{
+ return !(tmc->tmgroup && tmc->online);
+}
+
+/*
+ * Returns true, when @childmask corresponds to the group migrator or when the
+ * group is not active - so no migrator is set.
+ */
+static bool tmigr_check_migrator(struct tmigr_group *group, u8 childmask)
+{
+ union tmigr_state s;
+
+ s.state = atomic_read(&group->migr_state);
+
+ if ((s.migrator == childmask) || (s.migrator == TMIGR_NONE))
+ return true;
+
+ return false;
+}
+
+static bool tmigr_check_migrator_and_lonely(struct tmigr_group *group, u8 childmask)
+{
+ bool lonely, migrator = false;
+ unsigned long active;
+ union tmigr_state s;
+
+ s.state = atomic_read(&group->migr_state);
+
+ if ((s.migrator == childmask) || (s.migrator == TMIGR_NONE))
+ migrator = true;
+
+ active = s.active;
+ lonely = bitmap_weight(&active, BIT_CNT) <= 1;
+
+ return (migrator && lonely);
+}
+
+static bool tmigr_check_lonely(struct tmigr_group *group)
+{
+ unsigned long active;
+ union tmigr_state s;
+
+ s.state = atomic_read(&group->migr_state);
+
+ active = s.active;
+
+ return bitmap_weight(&active, BIT_CNT) <= 1;
+}
+
+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 {
+ WARN_ON_ONCE(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
+ * @nextexp: Next CPU event expiry information which is handed into
+ * the timer migration code by the timer code
+ * (get_next_timer_interrupt())
+ * @firstexp: Contains the first event expiry information when last
+ * active CPU of hierarchy is on the way to idle to make
+ * sure CPU will be back in time.
+ * @evt: Pointer to tmigr_event which needs to be queued (of idle
+ * child group)
+ * @childmask: childmask of child group
+ * @remote: Is set, when the new timer path is executed in
+ * tmigr_handle_remote_cpu()
+ */
+struct tmigr_walk {
+ u64 nextexp;
+ u64 firstexp;
+ struct tmigr_event *evt;
+ u8 childmask;
+ bool remote;
+};
+
+/**
+ * struct tmigr_remote_data - data required for remote expiry hierarchy walk
+ * @basej: timer base in jiffies
+ * @now: timer base monotonic
+ * @firstexp: returns expiry of the first timer in the idle timer
+ * migration hierarchy to make sure the timer is handled in
+ * time; it is stored in the per CPU tmigr_cpu struct of
+ * CPU which expires remote timers
+ * @childmask: childmask of child group
+ * @check: is set if there is the need to handle remote timers;
+ * required in tmigr_requires_handle_remote() only
+ * @tmc_active: this flag indicates, whether the CPU which triggers
+ * the hierarchy walk is !idle in the timer migration
+ * hierarchy. When the CPU is idle and the whole hierarchy is
+ * idle, only the first event of the top level has to be
+ * considered.
+ */
+struct tmigr_remote_data {
+ unsigned long basej;
+ u64 now;
+ u64 firstexp;
+ u8 childmask;
+ bool check;
+ bool tmc_active;
+};
+
+/*
+ * Returns the next event of the timerqueue @group->events
+ *
+ * Removes timers with ignore flag and update next_expiry of the group. Values
+ * of the group event are 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);
+
+ WRITE_ONCE(group->next_expiry, KTIME_MAX);
+
+ while ((node = timerqueue_getnext(&group->events))) {
+ evt = container_of(node, struct tmigr_event, nextevt);
+
+ if (!evt->ignore) {
+ WRITE_ONCE(group->next_expiry, evt->nextevt.expires);
+ return evt;
+ }
+
+ /*
+ * Remove next timers with ignore flag, because the group lock
+ * is held anyway
+ */
+ if (!timerqueue_del(&group->events, node))
+ break;
+ }
+
+ return NULL;
+}
+
+/*
+ * Return the next event (with the expiry equal or before @now)
+ *
+ * Event, which is returned, is also removed from the 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;
+
+ /*
+ * The event is ready to expire. Remove it and update next group event.
+ */
+ timerqueue_del(&group->events, &evt->nextevt);
+ tmigr_next_groupevt(group);
+
+ 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;
+ u8 childmask;
+
+ childmask = data->childmask;
+ /*
+ * No memory barrier is required here in contrast to
+ * tmigr_inactive_up(), as the group state change does not depend on the
+ * child state.
+ */
+ curstate.state = atomic_read(&group->migr_state);
+
+ do {
+ newstate = curstate;
+ walk_done = true;
+
+ if (newstate.migrator == TMIGR_NONE) {
+ newstate.migrator = childmask;
+
+ /* Changes need to be propagated */
+ walk_done = false;
+ }
+
+ newstate.active |= childmask;
+ newstate.seq++;
+
+ } while (!atomic_try_cmpxchg(&group->migr_state, &curstate.state, newstate.state));
+
+ if ((walk_done == false) && group->parent)
+ data->childmask = group->childmask;
+
+ /*
+ * The group is active (again). The group event might be still queued
+ * into the parent group's timerqueue but can now be handled by the
+ * migrator of this group. Therefore the ignore flag for the group event
+ * is updated to reflect this.
+ *
+ * The update of the ignore flag in the active path is done lockless. In
+ * worst case the migrator of the parent group observes the change too
+ * late and expires remotely all events belonging to this group. The
+ * lock is held while updating the ignore flag in idle path. So this
+ * state change will not be lost.
+ */
+ group->groupevt.ignore = true;
+
+ return walk_done;
+}
+
+static void __tmigr_cpu_activate(struct tmigr_cpu *tmc)
+{
+ struct tmigr_walk data;
+
+ data.childmask = tmc->childmask;
+
+ tmc->cpuevt.ignore = true;
+ WRITE_ONCE(tmc->wakeup, KTIME_MAX);
+
+ walk_groups(&tmigr_active_up, &data, tmc);
+}
+
+/**
+ * tmigr_cpu_activate() - set this CPU active in timer migration hierarchy
+ *
+ * Call site timer_clear_idle() is called with interrupts disabled.
+ */
+void tmigr_cpu_activate(void)
+{
+ struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+
+ if (tmigr_is_not_available(tmc))
+ return;
+
+ if (WARN_ON_ONCE(!tmc->idle))
+ return;
+
+ raw_spin_lock(&tmc->lock);
+ tmc->idle = false;
+ __tmigr_cpu_activate(tmc);
+ raw_spin_unlock(&tmc->lock);
+}
+
+/*
+ * Returns true, if there is nothing to be propagated to the next level
+ *
+ * @data->firstexp is set to expiry of first gobal event of the (top level of
+ * the) hierarchy, but only when hierarchy is completely idle.
+ *
+ * The child and group states need to be read under the lock, to prevent a race
+ * against a concurrent tmigr_inactive_up() run when the last CPU goes idle. See
+ * also section "Prevent race between new event and last CPU going inactive" in
+ * the documentation at the top.
+ *
+ * This is the only place where the 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;
+ union tmigr_state childstate, groupstate;
+ bool remote = data->remote;
+ bool walk_done = false;
+ u64 nextexp;
+
+ if (child) {
+ raw_spin_lock(&child->lock);
+ raw_spin_lock_nested(&group->lock, SINGLE_DEPTH_NESTING);
+
+ childstate.state = atomic_read(&child->migr_state);
+ groupstate.state = atomic_read(&group->migr_state);
+
+ if (childstate.active) {
+ walk_done = true;
+ goto unlock;
+ }
+
+ first_childevt = tmigr_next_groupevt(child);
+ nextexp = child->next_expiry;
+ evt = &child->groupevt;
+
+ evt->ignore = (nextexp == KTIME_MAX) ? true : false;
+ } else {
+ nextexp = data->nextexp;
+
+ first_childevt = evt = data->evt;
+
+ /*
+ * Walking the hierarchy is required in any case when a
+ * remote expiry was done before. This ensures to not lose
+ * already queued events in non active groups (see section
+ * "Required event and timerqueue update after a remote
+ * expiry" in the documentation at the top).
+ *
+ * The two call sites which are executed without a remote expiry
+ * before, are not prevented from propagating changes through
+ * the hierarchy by the return:
+ * - When entering this path by tmigr_new_timer(), @evt->ignore
+ * is never set.
+ * - tmigr_inactive_up() takes care of the propagation by
+ * itself and ignores the return value. But an immediate
+ * return is required because nothing has to be done in this
+ * level as the event could be ignored.
+ */
+ if (evt->ignore && !remote)
+ return true;
+
+ raw_spin_lock(&group->lock);
+
+ childstate.state = 0;
+ groupstate.state = atomic_read(&group->migr_state);
+ }
+
+ /*
+ * If the child event is already queued in the group, remove it from the
+ * queue when the expiry time changed only or when it could be ignored.
+ */
+ if (timerqueue_node_queued(&evt->nextevt)) {
+ if ((evt->nextevt.expires == nextexp) && !evt->ignore)
+ goto check_toplvl;
+
+ if (!timerqueue_del(&group->events, &evt->nextevt))
+ WRITE_ONCE(group->next_expiry, KTIME_MAX);
+ }
+
+ if (evt->ignore) {
+ /*
+ * When the next child event could be ignored (nextexp is
+ * KTIME_MAX) and there was no remote timer handling before or
+ * the group is already active, there is no need to walk the
+ * hierarchy even if there is a parent group.
+ *
+ * The other way round: even if the event could be ignored, but
+ * if a remote timer handling was executed before and the group
+ * is not active, walking the hierarchy is required to not miss
+ * an enqueued timer in the non active group. The enqueued timer
+ * of the group needs to be propagated to a higher level to
+ * ensure it is handled.
+ */
+ if (!remote || groupstate.active)
+ walk_done = true;
+ } else {
+ evt->nextevt.expires = nextexp;
+ evt->cpu = first_childevt->cpu;
+
+ if (timerqueue_add(&group->events, &evt->nextevt))
+ WRITE_ONCE(group->next_expiry, nextexp);
+ }
+
+check_toplvl:
+ if (!group->parent && (groupstate.migrator == TMIGR_NONE)) {
+ walk_done = true;
+
+ /*
+ * Nothing to do when update was done during remote timer
+ * handling. First timer in top level group which needs to be
+ * handled when top level group is not active, is calculated
+ * directly in tmigr_handle_remote_up().
+ */
+ if (remote)
+ goto unlock;
+
+ /*
+ * The top level group is idle and it has to be ensured the
+ * global timers are handled in time. (This could be optimized
+ * by keeping track of the last global scheduled event and only
+ * arming it on the CPU if the new event is earlier. Not sure if
+ * its worth the complexity.)
+ */
+ data->firstexp = 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;
+
+ return tmigr_update_events(group, child, data);
+}
+
+/*
+ * Returns the expiry of the next timer that needs to be handled. KTIME_MAX is
+ * returned, if an active CPU will handle all the timer migration hierarchy
+ * timers.
+ */
+static u64 tmigr_new_timer(struct tmigr_cpu *tmc, u64 nextexp)
+{
+ struct tmigr_walk data = { .nextexp = nextexp,
+ .firstexp = KTIME_MAX,
+ .evt = &tmc->cpuevt };
+
+ lockdep_assert_held(&tmc->lock);
+
+ if (tmc->remote)
+ return KTIME_MAX;
+
+ tmc->cpuevt.ignore = false;
+ data.remote = false;
+
+ walk_groups(&tmigr_new_timer_up, &data, tmc);
+
+ /* If there is a new first global event, make sure it is handled */
+ return data.firstexp;
+}
+
+static void tmigr_handle_remote_cpu(unsigned int cpu, u64 now,
+ unsigned long jif)
+{
+ struct timer_events tevt;
+ struct tmigr_walk data;
+ struct tmigr_cpu *tmc;
+
+ tmc = per_cpu_ptr(&tmigr_cpu, cpu);
+
+ raw_spin_lock_irq(&tmc->lock);
+
+ /*
+ * If the remote CPU is offline then the timers have been migrated to
+ * another CPU.
+ *
+ * If tmigr_cpu::remote is set, at the moment another CPU already
+ * expires the timers of the remote CPU.
+ *
+ * If tmigr_event::ignore is set, then the CPU returns from idle and
+ * takes care of its timers.
+ *
+ * If the next event expires in the future, then the event has been
+ * updated and there are no timers to expire right now. The CPU which
+ * updated the event takes care when hierarchy is completely
+ * idle. Otherwise the migrator does it as the event is enqueued.
+ */
+ if (!tmc->online || tmc->remote || tmc->cpuevt.ignore ||
+ now < tmc->cpuevt.nextevt.expires) {
+ raw_spin_unlock_irq(&tmc->lock);
+ return;
+ }
+
+ tmc->remote = true;
+ WRITE_ONCE(tmc->wakeup, KTIME_MAX);
+
+ /* Drop the lock to allow the remote CPU to exit idle */
+ raw_spin_unlock_irq(&tmc->lock);
+
+ if (cpu != smp_processor_id())
+ timer_expire_remote(cpu);
+
+ /*
+ * Lock ordering needs to be preserved - timer_base locks before tmigr
+ * related locks (see section "Locking rules" in the documentation at
+ * the top). During fetching the next timer interrupt, also tmc->lock
+ * needs to be held. Otherwise there is a possible race window against
+ * the CPU itself when it comes out of idle, updates the first timer in
+ * the hierarchy and goes back to idle.
+ *
+ * timer base locks are dropped as fast as possible: After checking
+ * whether the remote CPU went offline in the meantime and after
+ * fetching the next remote timer interrupt. Dropping the locks as fast
+ * as possible keeps the locking region small and prevents holding
+ * several (unnecessary) locks during walking the hierarchy for updating
+ * the timerqueue and group events.
+ */
+ local_irq_disable();
+ timer_lock_remote_bases(cpu);
+ raw_spin_lock(&tmc->lock);
+
+ /*
+ * When the CPU went offline in the meantime, no hierarchy walk has to
+ * be done for updating the queued events, because the walk was
+ * already done during marking the CPU offline in the hierarchy.
+ *
+ * When the CPU is no longer idle, the CPU takes care of the timers and
+ * also of the timers in the hierarchy.
+ *
+ * (See also section "Required event and timerqueue update after a
+ * remote expiry" in the documentation at the top)
+ */
+ if (!tmc->online || !tmc->idle) {
+ timer_unlock_remote_bases(cpu);
+ goto unlock;
+ }
+
+ /* next event of CPU */
+ fetch_next_timer_interrupt_remote(jif, now, &tevt, cpu);
+ timer_unlock_remote_bases(cpu);
+
+ data.nextexp = tevt.global;
+ data.firstexp = KTIME_MAX;
+ data.evt = &tmc->cpuevt;
+ data.remote = true;
+
+ /*
+ * The update is done even when there is no 'new' global timer pending
+ * on the remote CPU (see section "Required event and timerqueue update
+ * after a remote expiry" in the documentation at the top)
+ */
+ walk_groups(&tmigr_new_timer_up, &data, tmc);
+
+unlock:
+ tmc->remote = false;
+ raw_spin_unlock_irq(&tmc->lock);
+}
+
+static bool tmigr_handle_remote_up(struct tmigr_group *group,
+ struct tmigr_group *child,
+ void *ptr)
+{
+ struct tmigr_remote_data *data = ptr;
+ struct tmigr_event *evt;
+ unsigned long jif;
+ u8 childmask;
+ u64 now;
+
+ 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_irq(&group->lock);
+
+ evt = tmigr_next_expired_groupevt(group, now);
+
+ if (evt) {
+ unsigned int remote_cpu = evt->cpu;
+
+ raw_spin_unlock_irq(&group->lock);
+
+ tmigr_handle_remote_cpu(remote_cpu, now, jif);
+
+ /* check if there is another event, that needs to be handled */
+ goto again;
+ }
+
+ /*
+ * Update of childmask for the next level and keep track of the expiry
+ * of the first event that needs to be handled (group->next_expiry was
+ * updated by tmigr_next_expired_groupevt(), next was set by
+ * tmigr_handle_remote_cpu()).
+ */
+ data->childmask = group->childmask;
+ data->firstexp = group->next_expiry;
+
+ raw_spin_unlock_irq(&group->lock);
+
+ return false;
+}
+
+/**
+ * tmigr_handle_remote() - Handle global timers of 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;
+
+ if (tmigr_is_not_available(tmc))
+ return;
+
+ data.childmask = tmc->childmask;
+ data.firstexp = KTIME_MAX;
+
+ /*
+ * NOTE: This is a doubled check because the migrator test will be done
+ * in tmigr_handle_remote_up() anyway. Keep this check to speed up the
+ * return when nothing has to be done.
+ */
+ if (!tmigr_check_migrator(tmc->tmgroup, tmc->childmask))
+ return;
+
+ data.now = get_jiffies_update(&data.basej);
+
+ /*
+ * Update @tmc->wakeup only at the end and do not reset @tmc->wakeup to
+ * KTIME_MAX. Even if tmc->lock is not held during the whole remote
+ * handling, tmc->wakeup is fine to be stale as it is called in
+ * interrupt context and tick_nohz_next_event() is executed in interrupt
+ * exit path only after processing the last pending interrupt.
+ */
+
+ __walk_groups(&tmigr_handle_remote_up, &data, tmc);
+
+ raw_spin_lock_irq(&tmc->lock);
+ WRITE_ONCE(tmc->wakeup, data.firstexp);
+ raw_spin_unlock_irq(&tmc->lock);
+}
+
+static bool tmigr_requires_handle_remote_up(struct tmigr_group *group,
+ struct tmigr_group *child,
+ void *ptr)
+{
+ struct tmigr_remote_data *data = ptr;
+ u8 childmask;
+
+ childmask = data->childmask;
+
+ /*
+ * Handle the group only if the 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;
+
+ /*
+ * When there is a parent group and the CPU which triggered the
+ * hierarchy walk is not active, proceed the walk to reach the top level
+ * group before reading the next_expiry value.
+ */
+ if (group->parent && !data->tmc_active)
+ goto out;
+
+ /*
+ * The lock is required on 32bit architectures to read the variable
+ * consistently with a concurrent writer. On 64bit the lock is not
+ * required because the read operation is not split and so it is always
+ * consistent.
+ */
+ if (IS_ENABLED(CONFIG_64BIT)) {
+ data->firstexp = READ_ONCE(group->next_expiry);
+ if (data->now >= data->firstexp) {
+ data->check = true;
+ return true;
+ }
+ } else {
+ raw_spin_lock(&group->lock);
+ data->firstexp = group->next_expiry;
+ if (data->now >= group->next_expiry) {
+ data->check = true;
+ raw_spin_unlock(&group->lock);
+ return true;
+ }
+ raw_spin_unlock(&group->lock);
+ }
+
+out:
+ /* Update of childmask for the next level */
+ data->childmask = group->childmask;
+ return false;
+}
+
+/**
+ * tmigr_requires_handle_remote() - Check the need of remote timer handling
+ *
+ * Must be called with interrupts disabled.
+ */
+bool tmigr_requires_handle_remote(void)
+{
+ struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+ struct tmigr_remote_data data;
+ unsigned long jif;
+ bool ret = false;
+
+ if (tmigr_is_not_available(tmc))
+ return ret;
+
+ data.now = get_jiffies_update(&jif);
+ data.childmask = tmc->childmask;
+ data.firstexp = KTIME_MAX;
+ data.tmc_active = !tmc->idle;
+ data.check = false;
+
+ /*
+ * If the CPU is active, walk the hierarchy to check whether a remote
+ * expiry is required.
+ *
+ * Check is done lockless as interrupts are disabled and @tmc->idle is
+ * set only by the local CPU.
+ */
+ if (!tmc->idle) {
+ __walk_groups(&tmigr_requires_handle_remote_up, &data, tmc);
+
+ return data.check;
+ }
+
+ /*
+ * When the CPU is idle, compare @tmc->wakeup with @data.now. The lock
+ * is required on 32bit architectures to read the variable consistently
+ * with a concurrent writer. On 64bit the lock is not required because
+ * the read operation is not split and so it is always consistent.
+ */
+ if (IS_ENABLED(CONFIG_64BIT)) {
+ if (data.now >= READ_ONCE(tmc->wakeup))
+ return true;
+ } else {
+ raw_spin_lock(&tmc->lock);
+ if (data.now >= tmc->wakeup)
+ ret = true;
+ raw_spin_unlock(&tmc->lock);
+ }
+
+ return ret;
+}
+
+/**
+ * tmigr_cpu_new_timer() - enqueue next global timer into hierarchy (idle tmc)
+ * @nextexp: Next expiry of global timer (or KTIME_MAX if not)
+ *
+ * The CPU is already deactivated in the timer migration
+ * hierarchy. tick_nohz_get_sleep_length() calls tick_nohz_next_event()
+ * and thereby the timer idle path is executed once more. @tmc->wakeup
+ * holds the first timer, when the timer migration hierarchy is
+ * completely idle.
+ *
+ * Returns the first timer that needs to be handled by this CPU or KTIME_MAX if
+ * nothing needs to be done.
+ */
+u64 tmigr_cpu_new_timer(u64 nextexp)
+{
+ struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+ u64 ret;
+
+ if (tmigr_is_not_available(tmc))
+ return nextexp;
+
+ raw_spin_lock(&tmc->lock);
+
+ ret = READ_ONCE(tmc->wakeup);
+ if (nextexp != KTIME_MAX) {
+ if (nextexp != tmc->cpuevt.nextevt.expires ||
+ tmc->cpuevt.ignore) {
+ ret = tmigr_new_timer(tmc, nextexp);
+ }
+ }
+ /*
+ * Make sure the reevaluation of timers in idle path will not miss an
+ * event.
+ */
+ WRITE_ONCE(tmc->wakeup, ret);
+
+ raw_spin_unlock(&tmc->lock);
+ return ret;
+}
+
+static bool tmigr_inactive_up(struct tmigr_group *group,
+ struct tmigr_group *child,
+ void *ptr)
+{
+ union tmigr_state curstate, newstate, childstate;
+ struct tmigr_walk *data = ptr;
+ bool walk_done;
+ u8 childmask;
+
+ childmask = data->childmask;
+ childstate.state = 0;
+
+ /*
+ * The memory barrier is paired with the cmpxchg() in tmigr_active_up()
+ * to make sure the updates of child and group states are ordered. The
+ * ordering is mandatory, as the group state change depends on the child
+ * state.
+ */
+ curstate.state = atomic_read_acquire(&group->migr_state);
+
+ for (;;) {
+ if (child)
+ childstate.state = atomic_read(&child->migr_state);
+
+ newstate = curstate;
+ walk_done = true;
+
+ /* Reset active bit when the child is no longer active */
+ if (!childstate.active)
+ newstate.active &= ~childmask;
+
+ if (newstate.migrator == childmask) {
+ /*
+ * Find a new migrator for the group, because the child
+ * group is idle!
+ */
+ if (!childstate.active) {
+ unsigned long new_migr_bit, active = newstate.active;
+
+ new_migr_bit = find_first_bit(&active, BIT_CNT);
+
+ if (new_migr_bit != BIT_CNT) {
+ newstate.migrator = BIT(new_migr_bit);
+ } else {
+ newstate.migrator = TMIGR_NONE;
+
+ /* Changes need to be propagated */
+ walk_done = false;
+ }
+ }
+ }
+
+ newstate.seq++;
+
+ WARN_ON_ONCE((newstate.migrator != TMIGR_NONE) && !(newstate.active));
+
+ if (atomic_try_cmpxchg(&group->migr_state, &curstate.state,
+ newstate.state))
+ break;
+
+ /*
+ * The memory barrier is paired with the cmpxchg() in
+ * tmigr_active_up() to make sure the updates of child and group
+ * states are ordered. It is required only when the above
+ * try_cmpxchg() fails.
+ */
+ smp_mb__after_atomic();
+ }
+
+ data->remote = false;
+
+ /* Event Handling */
+ tmigr_update_events(group, child, data);
+
+ if (group->parent && (walk_done == false))
+ data->childmask = group->childmask;
+
+ /*
+ * data->firstexp was set by tmigr_update_events() and contains the
+ * expiry of the first global event which needs to be handled. It
+ * differs from KTIME_MAX if:
+ * - group is the top level group and
+ * - group is idle (which means CPU was the last active CPU in the
+ * hierarchy) and
+ * - there is a pending event in the hierarchy
+ */
+ WARN_ON_ONCE(data->firstexp != KTIME_MAX && group->parent);
+
+ return walk_done;
+}
+
+static u64 __tmigr_cpu_deactivate(struct tmigr_cpu *tmc, u64 nextexp)
+{
+ struct tmigr_walk data = { .nextexp = nextexp,
+ .firstexp = KTIME_MAX,
+ .evt = &tmc->cpuevt,
+ .childmask = tmc->childmask };
+
+ /*
+ * If nextexp is KTIME_MAX, the CPU event will be ignored because the
+ * local timer expires before the global timer, no global timer is set
+ * or CPU goes offline.
+ */
+ if (nextexp != KTIME_MAX)
+ tmc->cpuevt.ignore = false;
+
+ walk_groups(&tmigr_inactive_up, &data, tmc);
+ return data.firstexp;
+}
+
+/**
+ * tmigr_cpu_deactivate() - Put current CPU into inactive state
+ * @nextexp: The next global timer expiry of the current CPU
+ *
+ * Must be called with interrupts disabled.
+ *
+ * Return: the next event expiry of the current CPU or the next event expiry
+ * from the hierarchy if this CPU is the top level migrator or the hierarchy is
+ * completely idle.
+ */
+u64 tmigr_cpu_deactivate(u64 nextexp)
+{
+ struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+ u64 ret;
+
+ if (tmigr_is_not_available(tmc))
+ return nextexp;
+
+ raw_spin_lock(&tmc->lock);
+
+ ret = __tmigr_cpu_deactivate(tmc, nextexp);
+
+ tmc->idle = true;
+
+ /*
+ * Make sure the reevaluation of timers in idle path will not miss an
+ * event.
+ */
+ WRITE_ONCE(tmc->wakeup, ret);
+
+ raw_spin_unlock(&tmc->lock);
+ return ret;
+}
+
+/**
+ * tmigr_quick_check() - Quick forecast of next tmigr event when CPU wants to
+ * go idle
+ * @nextevt: The next global timer expiry of the current CPU
+ *
+ * Return:
+ * * KTIME_MAX - when it is probable that nothing has to be done (not
+ * the only one in the level 0 group; and if it is the
+ * only one in level 0 group, but there are more than a
+ * single group active on the way to top level)
+ * * nextevt - when CPU is offline and has to handle timer on his own
+ * or when on the way to top in every group only a single
+ * child is active and but @nextevt is before next_expiry
+ * of top level group
+ * * next_expiry (top) - value of top level group, when on the way to top in
+ * every group only a single child is active and @nextevt
+ * is after this value active child.
+ */
+u64 tmigr_quick_check(u64 nextevt)
+{
+ struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+ struct tmigr_group *group = tmc->tmgroup;
+
+ if (tmigr_is_not_available(tmc))
+ return nextevt;
+
+ if (WARN_ON_ONCE(tmc->idle))
+ return nextevt;
+
+ if (!tmigr_check_migrator_and_lonely(tmc->tmgroup, tmc->childmask))
+ return KTIME_MAX;
+
+ do {
+ if (!tmigr_check_lonely(group)) {
+ return KTIME_MAX;
+ } else if (!group->parent) {
+ u64 first_global = READ_ONCE(group->next_expiry);
+
+ return min_t(u64, nextevt, first_global);
+ }
+ group = group->parent;
+ } while (group);
+
+ return KTIME_MAX;
+}
+
+static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,
+ int node)
+{
+ 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_children = 0;
+
+ s.migrator = TMIGR_NONE;
+ s.active = 0;
+ s.seq = 0;
+ atomic_set(&group->migr_state, s.state);
+
+ timerqueue_init_head(&group->events);
+ timerqueue_init(&group->groupevt.nextevt);
+ group->groupevt.nextevt.expires = KTIME_MAX;
+ WRITE_ONCE(group->next_expiry, KTIME_MAX);
+ group->groupevt.ignore = true;
+}
+
+static struct tmigr_group *tmigr_get_group(unsigned int cpu, int node,
+ unsigned int lvl)
+{
+ struct tmigr_group *tmp, *group = NULL;
+
+ lockdep_assert_held(&tmigr_mutex);
+
+ /* Try to attach to an existing 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_children >= TMIGR_CHILDREN_PER_GROUP)
+ continue;
+
+ /*
+ * TODO: A possible further improvement: Make sure that all CPU
+ * siblings end up in the same group of the lowest level of the
+ * hierarchy. Rely on the topology sibling mask would be a
+ * reasonable solution.
+ */
+
+ group = tmp;
+ break;
+ }
+
+ if (group)
+ return group;
+
+ /* Allocate and set up a new group */
+ group = kzalloc_node(sizeof(*group), GFP_KERNEL, node);
+ if (!group)
+ return ERR_PTR(-ENOMEM);
+
+ tmigr_init_group(group, lvl, node);
+
+ /* 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;
+
+ raw_spin_lock_irq(&child->lock);
+ raw_spin_lock_nested(&parent->lock, SINGLE_DEPTH_NESTING);
+
+ child->parent = parent;
+ child->childmask = BIT(parent->num_children++);
+
+ raw_spin_unlock(&parent->lock);
+ raw_spin_unlock_irq(&child->lock);
+
+ /*
+ * To prevent inconsistent states, active children need to be active in
+ * the new parent as well. Inactive children are already marked inactive
+ * in the parent group:
+ *
+ * * When new groups were created by tmigr_setup_groups() starting from
+ * the lowest level (and not higher then one level below the current
+ * top level), then they are not active. They will be set active when
+ * the new online CPU comes active.
+ *
+ * * But if a new group above the current top level is required, it is
+ * mandatory to propagate the active state of the already existing
+ * child to the new parent. So tmigr_connect_child_parent() is
+ * executed with the formerly top level group (child) and the newly
+ * created group (parent).
+ */
+ childstate.state = atomic_read(&child->migr_state);
+ if (childstate.migrator != TMIGR_NONE) {
+ struct tmigr_walk data;
+
+ data.childmask = child->childmask;
+
+ /*
+ * There is only one new level per time. When connecting the
+ * child and the parent and set the child active when the parent
+ * is inactive, the parent needs to be the uppermost
+ * 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;
+
+ stack = kcalloc(tmigr_hierarchy_levels, sizeof(*stack), GFP_KERNEL);
+ if (!stack)
+ return -ENOMEM;
+
+ do {
+ group = tmigr_get_group(cpu, node, i);
+ if (IS_ERR(group)) {
+ err = PTR_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;
+ }
+
+ WARN_ON_ONCE(i != group->level);
+
+ /*
+ * Update tmc -> group / child -> group connection
+ */
+ if (i == 0) {
+ struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+
+ raw_spin_lock_irq(&group->lock);
+
+ tmc->tmgroup = group;
+ tmc->childmask = BIT(group->num_children++);
+
+ raw_spin_unlock_irq(&group->lock);
+
+ /* There are no children that need to be connected */
+ continue;
+ } else {
+ child = stack[i - 1];
+ tmigr_connect_child_parent(child, group);
+ }
+
+ /* check if uppermost level was newly created */
+ if (top != i)
+ continue;
+
+ WARN_ON_ONCE(top == 0);
+
+ lvllist = &tmigr_level_list[top];
+ if (group->num_children == 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)
+{
+ 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);
+ 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;
+ tmc->cpuevt.ignore = true;
+ tmc->cpuevt.cpu = cpu;
+
+ tmc->remote = false;
+ WRITE_ONCE(tmc->wakeup, KTIME_MAX);
+ }
+ raw_spin_lock_irq(&tmc->lock);
+ tmc->idle = timer_base_is_idle();
+ if (!tmc->idle)
+ __tmigr_cpu_activate(tmc);
+ tmc->online = true;
+ raw_spin_unlock_irq(&tmc->lock);
+ return 0;
+}
+
+/*
+ * tmigr_trigger_active() - trigger a CPU to become active again
+ *
+ * This function is executed on a CPU which is part of cpu_online_mask, when the
+ * last active CPU in the hierarchy is offlining. With this, it is ensured that
+ * the other CPU is active and takes over the migrator duty.
+ */
+static long tmigr_trigger_active(void *unused)
+{
+ struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+
+ WARN_ON_ONCE(!tmc->online || tmc->idle);
+
+ return 0;
+}
+
+static int tmigr_cpu_offline(unsigned int cpu)
+{
+ struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
+ int migrator;
+ u64 firstexp;
+
+ raw_spin_lock_irq(&tmc->lock);
+ tmc->online = false;
+ WRITE_ONCE(tmc->wakeup, KTIME_MAX);
+
+ /*
+ * CPU has to handle the local events on his own, when on the way to
+ * offline; Therefore nextevt value is set to KTIME_MAX
+ */
+ firstexp = __tmigr_cpu_deactivate(tmc, KTIME_MAX);
+ raw_spin_unlock_irq(&tmc->lock);
+
+ if (firstexp != KTIME_MAX) {
+ migrator = cpumask_any_but(cpu_online_mask, cpu);
+ work_on_cpu(migrator, tmigr_trigger_active, NULL);
+ }
+
+ return 0;
+}
+
+static int __init tmigr_init(void)
+{
+ unsigned int cpulvl, nodelvl, cpus_per_node, i;
+ unsigned int nnodes = num_possible_nodes();
+ unsigned int ncpus = num_possible_cpus();
+ int ret = -ENOMEM;
+
+ BUILD_BUG_ON_NOT_POWER_OF_2(TMIGR_CHILDREN_PER_GROUP);
+
+ /* Nothing to do if running on UP */
+ if (ncpus == 1)
+ return 0;
+
+ /*
+ * 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 across
+ * nodes. We cannot rely on cpumask_of_node() because it only works for
+ * online CPUs.
+ */
+ 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_CHILDREN_PER_GROUP));
+
+ /* Calculate the extra levels to connect all nodes */
+ nodelvl = DIV_ROUND_UP(order_base_2(nnodes),
+ ilog2(TMIGR_CHILDREN_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 across NUMA nodes. Store
+ * this information for the setup code to decide in which level node
+ * matching is no longer required.
+ */
+ tmigr_crossnode_level = cpulvl;
+
+ tmigr_level_list = kcalloc(tmigr_hierarchy_levels, sizeof(struct list_head), 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 children per group;"
+ " %d crossnode level\n",
+ tmigr_hierarchy_levels, TMIGR_CHILDREN_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;
+
+ return 0;
+
+err:
+ pr_err("Timer migration setup failed\n");
+ return ret;
+}
+late_initcall(tmigr_init);
new file mode 100644
@@ -0,0 +1,140 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _KERNEL_TIME_MIGRATION_H
+#define _KERNEL_TIME_MIGRATION_H
+
+/* Per group capacity. Must be a power of 2! */
+#define TMIGR_CHILDREN_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;
+ bool ignore;
+};
+
+/**
+ * struct tmigr_group - timer migration hierarchy group
+ * @lock: Lock protecting the event information and group hierarchy
+ * information during setup
+ * @parent: Pointer to the parent group
+ * @groupevt: Next event of the group which is only used when the
+ * group is !active. The group event is then queued into
+ * the parent timer queue.
+ * Ignore bit of @groupevt is set when the group is active.
+ * @next_expiry: Base monotonic expiry time of the next event of the
+ * group; It is used for the racy lockless check whether a
+ * remote expiry is required; it is always reliable
+ * @events: Timer queue for child events queued in the group
+ * @migr_state: State of the group (see union tmigr_state)
+ * @level: Hierarchy level of the group; Required during setup
+ * @numa_node: Required for setup only to make sure CPU and low level
+ * group information is NUMA local. It is set to NUMA node
+ * as long as the group level is per NUMA node (level <
+ * tmigr_crossnode_level); otherwise it is set to
+ * NUMA_NO_NODE
+ * @num_children: Counter of group children to make sure the group is only
+ * filled with TMIGR_CHILDREN_PER_GROUP; Required for setup
+ * only
+ * @childmask: childmask of the group in the parent group; is set
+ * during setup and will never change; can be read
+ * lockless
+ * @list: List head that is added to the per level
+ * tmigr_level_list; is required during setup when a
+ * new group needs to be connected to the existing
+ * hierarchy groups
+ */
+struct tmigr_group {
+ raw_spinlock_t lock;
+ struct tmigr_group *parent;
+ struct tmigr_event groupevt;
+ u64 next_expiry;
+ struct timerqueue_head events;
+ atomic_t migr_state;
+ unsigned int level;
+ int numa_node;
+ unsigned int num_children;
+ u8 childmask;
+ struct list_head list;
+};
+
+/**
+ * struct tmigr_cpu - timer migration per CPU group
+ * @lock: Lock protecting the tmigr_cpu group information
+ * @online: Indicates whether the CPU is online; In deactivate path
+ * it is required to know whether the migrator in the top
+ * level group is to be set offline, while a timer is
+ * pending. Then another online CPU needs to be notified to
+ * take over the migrator role. Furthermore the information
+ * is required in CPU hotplug path as the CPU is able to go
+ * idle before the timer migration hierarchy hotplug AP is
+ * reached. During this phase, the CPU has to handle the
+ * global timers on its own and must not act as a migrator.
+ * @idle: Indicates whether the CPU is idle in the timer migration
+ * hierarchy
+ * @remote: Is set when timers of the CPU are expired remotely
+ * @tmgroup: Pointer to the parent group
+ * @childmask: childmask of tmigr_cpu in the 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 in the idle path and is only
+ * used in idle path.
+ * @cpuevt: CPU event which could be enqueued into the parent group
+ */
+struct tmigr_cpu {
+ raw_spinlock_t lock;
+ bool online;
+ bool idle;
+ bool remote;
+ struct tmigr_group *tmgroup;
+ u8 childmask;
+ u64 wakeup;
+ struct tmigr_event cpuevt;
+};
+
+/**
+ * union tmigr_state - state of tmigr_group
+ * @state: Combined version of the state - only used for atomic
+ * read/cmpxchg function
+ * @struct: Split version of the state - only use the struct members to
+ * update information to stay independent of endianness
+ */
+union tmigr_state {
+ u32 state;
+ /**
+ * struct - split state of tmigr_group
+ * @active: Contains each childmask bit of the active children
+ * @migrator: Contains childmask of the child which is migrator
+ * @seq: Sequence counter needs to be increased when an update
+ * to the tmigr_state is done. It prevents a race when
+ * updates in the child groups are propagated in changed
+ * order. Detailed information about the scenario is
+ * given in the documentation at the begin of
+ * timer_migration.c.
+ */
+ struct {
+ u8 active;
+ u8 migrator;
+ u16 seq;
+ } __packed;
+};
+
+#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
+extern void tmigr_handle_remote(void);
+extern bool tmigr_requires_handle_remote(void);
+extern void tmigr_cpu_activate(void);
+extern u64 tmigr_cpu_deactivate(u64 nextevt);
+extern u64 tmigr_cpu_new_timer(u64 nextevt);
+extern u64 tmigr_quick_check(u64 nextevt);
+#else
+static inline void tmigr_handle_remote(void) { }
+static inline bool tmigr_requires_handle_remote(void) { return false; }
+static inline void tmigr_cpu_activate(void) { }
+#endif
+
+#endif