drm/modes: Fix drm_mode_vrefres() docs

[drm/drm-misc.git] / kernel / time / timer_migration.c

// 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"

#define CREATE_TRACE_POINTS
#include <trace/events/timer_migration.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;
}

/**
 * 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
 *                      hierarchy is completely idle.  When CPU itself was the
 *                      last going idle, information makes sure, that CPU will
 *                      be back in time. When using this value in the remote
 *                      expiry case, firstexp is stored in the per CPU tmigr_cpu
 *                      struct of CPU which expires remote timers. It is updated
 *                      in top level group only. Be aware, there could occur a
 *                      new top level of the hierarchy between the 'top level
 *                      call' in tmigr_update_events() and the check for the
 *                      parent group in walk_groups(). Then @firstexp might
 *                      contain a value != KTIME_MAX even if it was not the
 *                      final top level. This is not a problem, as the worst
 *                      outcome is a CPU which might wake up a little early.
 * @evt:                Pointer to tmigr_event which needs to be queued (of idle
 *                      child group)
 * @childmask:          groupmask of child group
 * @remote:             Is set, when the new timer path is executed in
 *                      tmigr_handle_remote_cpu()
 * @basej:              timer base in jiffies
 * @now:                timer base monotonic
 * @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_walk {
        u64                     nextexp;
        u64                     firstexp;
        struct tmigr_event      *evt;
        u8                      childmask;
        bool                    remote;
        unsigned long           basej;
        u64                     now;
        bool                    check;
        bool                    tmc_active;
};

typedef bool (*up_f)(struct tmigr_group *, struct tmigr_group *, struct tmigr_walk *);

static void __walk_groups(up_f up, struct tmigr_walk *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;
                data->childmask = child->groupmask;
        } while (group);
}

static void walk_groups(up_f up, struct tmigr_walk *data, struct tmigr_cpu *tmc)
{
        lockdep_assert_held(&tmc->lock);

        __walk_groups(up, data, tmc);
}

/*
 * 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,
                            struct tmigr_walk *data)
{
        union tmigr_state curstate, newstate;
        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));

        trace_tmigr_group_set_cpu_active(group, newstate, 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->groupmask;

        trace_tmigr_cpu_active(tmc);

        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 possible if there is a parent, sparing group
                 *    locking at this level, because the upper walking call to
                 *    the parent will take care about removing this event from
                 *    within the group and update next_expiry accordingly.
                 *
                 * However if there is no parent, ie: the hierarchy has only a
                 * single level so @group is the top level group, make sure the
                 * first event information of the group is updated properly and
                 * also handled properly, so skip this fast return path.
                 */
                if (evt->ignore && !remote && group->parent)
                        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) {
                        /* Make sure not to miss a new CPU event with the same expiry */
                        evt->cpu = first_childevt->cpu;
                        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);
        }

        trace_tmigr_update_events(child, group, childstate, groupstate,
                                  nextexp);

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,
                               struct tmigr_walk *data)
{
        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;

        trace_tmigr_cpu_new_timer(tmc);

        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;
        }

        trace_tmigr_handle_remote_cpu(tmc);

        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,
                                   struct tmigr_walk *data)
{
        struct tmigr_event *evt;
        unsigned long jif;
        u8 childmask;
        u64 now;

        jif = data->basej;
        now = data->now;

        childmask = data->childmask;

        trace_tmigr_handle_remote(group);
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;
        }

        /*
         * 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->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_walk data;

        if (tmigr_is_not_available(tmc))
                return;

        data.childmask = tmc->groupmask;
        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->groupmask)) {
                /*
                 * If this CPU was an idle migrator, make sure to clear its wakeup
                 * value so it won't chase timers that have already expired elsewhere.
                 * This avoids endless requeue from tmigr_new_timer().
                 */
                if (READ_ONCE(tmc->wakeup) == KTIME_MAX)
                        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,
                                            struct tmigr_walk *data)
{
        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)
                return false;

        /*
         * 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);
        }

        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_walk data;
        unsigned long jif;
        bool ret = false;

        if (tmigr_is_not_available(tmc))
                return ret;

        data.now = get_jiffies_update(&jif);
        data.childmask = tmc->groupmask;
        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);
                }
        }
        trace_tmigr_cpu_new_timer_idle(tmc, nextexp);
        raw_spin_unlock(&tmc->lock);
        return ret;
}

static bool tmigr_inactive_up(struct tmigr_group *group,
                              struct tmigr_group *child,
                              struct tmigr_walk *data)
{
        union tmigr_state curstate, newstate, childstate;
        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)) {
                        trace_tmigr_group_set_cpu_inactive(group, newstate, childmask);
                        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);

        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->groupmask };

        /*
         * 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);

        trace_tmigr_cpu_idle(tmc, nextexp);
        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 its own
 *                        or when on the way to top in every group only a single
 *                        child is active but @nextevt is before the lowest
 *                        next_expiry encountered while walking up to top level.
 * * next_expiry        - value of lowest expiry encountered while walking groups
 *                        if only a single child is active on each and @nextevt
 *                        is after this lowest expiry.
 */
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->groupmask))
                return KTIME_MAX;

        do {
                if (!tmigr_check_lonely(group)) {
                        return KTIME_MAX;
                } else {
                        /*
                         * Since current CPU is active, events may not be sorted
                         * from bottom to the top because the CPU's event is ignored
                         * up to the top and its sibling's events not propagated upwards.
                         * Thus keep track of the lowest observed expiry.
                         */
                        nextevt = min_t(u64, nextevt, READ_ONCE(group->next_expiry));
                        if (!group->parent)
                                return nextevt;
                }
                group = group->parent;
        } while (group);

        return KTIME_MAX;
}

/*
 * 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);
        trace_tmigr_cpu_offline(tmc);
        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 tmigr_cpu_online(unsigned int cpu)
{
        struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);

        /* Check whether CPU data was successfully initialized */
        if (WARN_ON_ONCE(!tmc->tmgroup))
                return -EINVAL;

        raw_spin_lock_irq(&tmc->lock);
        trace_tmigr_cpu_online(tmc);
        tmc->idle = timer_base_is_idle();
        if (!tmc->idle)
                __tmigr_cpu_activate(tmc);
        tmc->online = true;
        raw_spin_unlock_irq(&tmc->lock);
        return 0;
}

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]);
        trace_tmigr_group_set(group);
        return group;
}

static void tmigr_connect_child_parent(struct tmigr_group *child,
                                       struct tmigr_group *parent,
                                       bool activate)
{
        struct tmigr_walk data;

        raw_spin_lock_irq(&child->lock);
        raw_spin_lock_nested(&parent->lock, SINGLE_DEPTH_NESTING);

        child->parent = parent;
        child->groupmask = BIT(parent->num_children++);

        raw_spin_unlock(&parent->lock);
        raw_spin_unlock_irq(&child->lock);

        trace_tmigr_connect_child_parent(child);

        if (!activate)
                return;

        /*
         * 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).
         *
         * * It is ensured that the child is active, as this setup path is
         *   executed in hotplug prepare callback. This is exectued by an
         *   already connected and !idle CPU. Even if all other CPUs go idle,
         *   the CPU executing the setup will be responsible up to current top
         *   level group. And the next time it goes inactive, it will release
         *   the new childmask and parent to subsequent walkers through this
         *   @child. Therefore propagate active state unconditionally.
         */
        data.childmask = child->groupmask;

        /*
         * There is only one new level per time (which is protected by
         * tmigr_mutex). 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);

        while (i > 0) {
                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 = per_cpu_ptr(&tmigr_cpu, cpu);

                        raw_spin_lock_irq(&group->lock);

                        tmc->tmgroup = group;
                        tmc->groupmask = BIT(group->num_children++);

                        raw_spin_unlock_irq(&group->lock);

                        trace_tmigr_connect_cpu_parent(tmc);

                        /* There are no children that need to be connected */
                        continue;
                } else {
                        child = stack[i - 1];
                        /* Will be activated at online time */
                        tmigr_connect_child_parent(child, group, false);
                }

                /* 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)) {
                        /*
                         * The target CPU must never do the prepare work, except
                         * on early boot when the boot CPU is the target. Otherwise
                         * it may spuriously activate the old top level group inside
                         * the new one (nevertheless whether old top level group is
                         * active or not) and/or release an uninitialized childmask.
                         */
                        WARN_ON_ONCE(cpu == raw_smp_processor_id());

                        lvllist = &tmigr_level_list[top - 1];
                        list_for_each_entry(child, lvllist, list) {
                                if (child->parent)
                                        continue;

                                tmigr_connect_child_parent(child, group, true);
                        }
                }
        }

        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_prepare(unsigned int cpu)
{
        struct tmigr_cpu *tmc = per_cpu_ptr(&tmigr_cpu, cpu);
        int ret = 0;

        /* Not first online attempt? */
        if (tmc->tmgroup)
                return ret;

        raw_spin_lock_init(&tmc->lock);
        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);

        ret = tmigr_add_cpu(cpu);
        if (ret < 0)
                return ret;

        if (tmc->groupmask == 0)
                return -EINVAL;

        return ret;
}

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_TMIGR_PREPARE, "tmigr:prepare",
                                tmigr_cpu_prepare, NULL);
        if (ret)
                goto err;

        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;
}
early_initcall(tmigr_init);