Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

Anna-Maria Behnsen
2024-02-05T15:59:48Z

Frederic Weisbecker <frederic@kernel.org> writes:

> Le Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen a écrit :
>> +static bool tmigr_handle_remote_up(struct tmigr_group *group,
>> + struct tmigr_group *child,
>> + void *ptr)
>> +{
>> + struct tmigr_remote_data *data = ptr;
>> + u64 now, next = KTIME_MAX;
>> + struct tmigr_event *evt;
>> + unsigned long jif;
>> + u8 childmask;
>> +
>> + jif = data->basej;
>> + now = data->now;
>> +
>> + childmask = data->childmask;
>> +
>> +again:
>> + /*
>> + * Handle the group only if @childmask is the migrator or if the
>> + * group has no migrator. Otherwise the group is active and is
>> + * handled by its own migrator.
>> + */
>> + if (!tmigr_check_migrator(group, childmask))
>> + return true;
>> +
>> + raw_spin_lock_irq(&group->lock);
>> +
>> + evt = tmigr_next_expired_groupevt(group, now);
>> +
>> + if (evt) {
>> + unsigned int remote_cpu = evt->cpu;
>> +
>> + raw_spin_unlock_irq(&group->lock);
>> +
>> + next = tmigr_handle_remote_cpu(remote_cpu, now, jif);
>> +
>> + /* check if there is another event, that needs to be handled */
>> + goto again;
>> + } else {
>> + raw_spin_unlock_irq(&group->lock);
>> + }
>> +
>> + /*
>> + * Update of childmask for the next level and keep track of the expiry
>> + * of the first event that needs to be handled
>> + */
>> + data->childmask = group->childmask;
>> + data->firstexp = next;
>
> So assume we have:
>
> [GRP1:0]
> migrator = [GRP0:0]
> active = [GRP0:0]
> nextevt = TIMER3
> / \
> [GRP0:0] [GRP0:1]
> migrator = CPU0 migrator = NONE
> active = CPU0 active = NONE
> nextevt = KTIME_MAX nextevt = TIMER3
> / \ / \
> 0 1 2 3
> idle idle idle idle (TIMER3)
>
> Then CPU 0 goes idle:
>
> [GRP1:0]
> migrator = NONE
> active = NONE
> nextevt = TIMER3
> / \
> [GRP0:0] [GRP0:1]
> migrator = NONE migrator = NONE
> active = NONE active = NONE
> nextevt = KTIME_MAX nextevt = TIMER3
> / \ / \
> 0 1 2 3
> idle idle idle idle (TIMER3)
>
> CPU 0 is the idle migrator and its tmc->wakeup is TIMER3.
> But CPU 0 has a local timer that expires before TIMER3.
>
> When that timer interrupt fires, it raises the softirq, which
> executes on IRQ tail. So CPU0 eventually calls tmigr_handle_remote()
> before TIMER3 has expired.
>
> This leads to tmigr_next_expired_groupevt() to return NULL and then
> data->firstexp = KTIME_MAX and then tmc->wakeup = KTIME_MAX.
>
> Later on, tmigr_new_timer() is called with a KTIME_MAX global
> event and so tmc->wakeup stays with KTIME_MAX, ignoring TIMER3.
>
> It looks like you need to handle the tmigr_next_expired_groupevt()
> case returning NULL.

Yes. You are right. group->next_expiry is updated via
tmigr_next_expired_groupevt(). So I should take this value to rely on
for the firstevt.

But with this, we do not need the return value of
tmigr_handle_remote_cpu(). When the upperst level is inactive and there
is a timer (e.g. propagated there by tmigr_handle_remote_cpu()), the
return value of tmigr_handle_remote_cpu() is the next_expiry of the top
level group. But this value is also noticed by walking the hierarchy up
to the top level with tmigr_handle_remote_up().

Thanks,

Anna-Maria