Re: [PATCH v5 16/18] timer: Implement the hierarchical pull model

Peter Zijlstra
2023-03-21T15:36:44Z

On Wed, Mar 01, 2023 at 03:17:42PM +0100, Anna-Maria Behnsen wrote:
> +static bool tmigr_inactive_up(struct tmigr_group *group,
> + struct tmigr_group *child,
> + void *ptr)
> +{
> + union tmigr_state curstate, newstate;
> + struct tmigr_walk *data = ptr;
> + bool walk_done;
> + u32 childmask;
> +
> + childmask = data->childmask;
> + newstate = curstate = data->groupstate;
> +
> +retry:
> + walk_done = true;
> +
> + /* Reset active bit when child is no longer active */
> + if (!data->childstate.active)
> + newstate.active &= ~(u8)childmask;
> +
> + if (newstate.migrator == (u8)childmask) {
> + /*
> + * Find a new migrator for the group, because child group
> + * is idle!
> + */
> + if (!data->childstate.active) {
> + unsigned long new_migr_bit, active = newstate.active;
> +
> + new_migr_bit = find_first_bit(&active, BIT_CNT);
> +
> + /* Changes need to be propagated */
> + walk_done = false;
> +
> + if (new_migr_bit != BIT_CNT)
> + newstate.migrator = BIT(new_migr_bit);
> + else
> + newstate.migrator = TMIGR_NONE;
> + }
> + }
> +
> + newstate.seq++;
> +
> + DBG_BUG_ON((newstate.migrator != TMIGR_NONE) && !(newstate.active));
> +
> + if (atomic_cmpxchg(group->migr_state, curstate.state, newstate.state) != curstate.state) {
> + /*
> + * Something changed in child/parent group in the meantime,
> + * reread the state of child and parent; Update of
> + * data->childstate is required for event handling;
> + */
> + if (child)
> + data->childstate.state = atomic_read(child->migr_state);
> + newstate.state = curstate.state = atomic_read(group->migr_state);
> +
> + goto retry;
> + }

Idem:

if (!atomic_try_cmpxchg(group->migr_state, &curstate.state, newstate.state)) {
newstate.state = curstate.state;
if (child)
data->childstate.state = atomic_read(child->migr_state);
goto retry;
}

> +
> + data->groupstate = newstate;
> + data->remote = false;
> +
> + /* Event Handling */
> + tmigr_update_events(group, child, data);
> +
> + if (group->parent && (walk_done == false)) {
> + data->childmask = group->childmask;
> + data->childstate = newstate;
> + data->groupstate.state = atomic_read(group->parent->migr_state);
> + }
> +
> + /*
> + * data->nextexp was set by tmigr_update_events() and contains the
> + * expiry of first global event which needs to be handled
> + */
> + if (data->nextexp != KTIME_MAX) {
> + DBG_BUG_ON(group->parent);
> + /*
> + * Toplevel path - If this cpu is about going offline wake
> + * up some random other cpu so it will take over the
> + * migrator duty and program its timer properly. Ideally
> + * wake the cpu with the closest expiry time, but that's
> + * overkill to figure out.
> + */
> + if (!(this_cpu_ptr(&tmigr_cpu)->online)) {
> + unsigned int cpu = smp_processor_id();
> +
> + cpu = cpumask_any_but(cpu_online_mask, cpu);
> + smp_send_reschedule(cpu);
> + }
> + }
> +
> + return walk_done;
> +}