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

Frederic Weisbecker
2024-02-06T11:11:42Z

Le Tue, Feb 06, 2024 at 12:03:32PM +0100, Anna-Maria Behnsen a écrit :
> Frederic Weisbecker <frederic@kernel.org> writes:
>
> > Le Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen a écrit :
> >> +/*
> >> + * 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.
> >> + *
> >> + * 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, union tmigr_state childstate,
> >> + union tmigr_state groupstate)
> >> +{
> >> + struct tmigr_event *evt, *first_childevt;
> >> + bool walk_done, remote = data->remote;
> >> + bool leftmost_change = false;
> >> + u64 nextexp;
> >> +
> >> + if (child) {
> >> + raw_spin_lock(&child->lock);
> >> + raw_spin_lock_nested(&group->lock, SINGLE_DEPTH_NESTING);
> >> +
> >> + if (childstate.active) {
> >> + walk_done = true;
> >> + goto unlock;
> >> + }
> >> +
> >> + first_childevt = tmigr_next_groupevt(child);
> >> + nextexp = child->next_expiry;
> >> + evt = &child->groupevt;
> >> + } 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);
> >> + }
> >> +
> >> + if (nextexp == KTIME_MAX) {
> >> + evt->ignore = true;
> >> +
> >> + /*
> >> + * 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;
> >> + goto unlock;
> >> + }
> >> + } else {
> >> + /*
> >> + * An update of @evt->cpu and @evt->ignore flag is required only
> >> + * when @child is set (the child is equal or higher than lvl0),
> >> + * but it doesn't matter if it is written once more to the per
> >> + * CPU event; make the update unconditional.
> >> + */
> >> + evt->cpu = first_childevt->cpu;
> >> + evt->ignore = false;
> >> + }
> >> +
> >> + walk_done = !group->parent;
> >> +
> >> + /*
> >> + * If the child event is already queued in the group, remove it from the
> >> + * queue when the expiry time changed only.
> >> + */
> >> + if (timerqueue_node_queued(&evt->nextevt)) {
> >> + if (evt->nextevt.expires == nextexp)
> >> + goto check_toplvl;
> >> +
> >> + leftmost_change = timerqueue_getnext(&group->events) == &evt->nextevt;
> >> + if (!timerqueue_del(&group->events, &evt->nextevt))
> >> + WRITE_ONCE(group->next_expiry, KTIME_MAX);
> >> + }
> >> +
> >> + evt->nextevt.expires = nextexp;
> >> +
> >> + if (timerqueue_add(&group->events, &evt->nextevt)) {
> >> + leftmost_change = true;
> >> + WRITE_ONCE(group->next_expiry, nextexp);
> >> + }
> >> +
> >> +check_toplvl:
> >> + if (walk_done && (groupstate.migrator == TMIGR_NONE)) {
> >> + /*
> >> + * Nothing to do when first event didn't changed and update was
> >> + * done during remote timer handling.
> >> + */
> >> + if (remote && !leftmost_change)
> >
> > So if the first timer in the list hasn't changed, and that first timer belongs
> > to another CPU (and another group) than the tmc for which we are remotely
> > handling timers and re-propagating timers up, then data->firstexp will be
> > after the leftmost timer expiration (data->firstexp could even be KTIME_MAX
> > in the worst case), and so will be tmc->wakeup for the caller of
> > tmigr_handle_remote()?
> >
>
> This is related to the discussion regarding tmigr_handle_remote_up(). So
> this should be also covered by the change I proposed there.
>
> And then we definitely do not need the update of data->firstevt here, as
> we are still on the way to top to make sure all events are handled. And
> the first event which needs to be handled by the migrator CPU is set by
> the call to tmigr_next_expired_groupevt().

Sounds good!

Thanks.

>
> >
> >> + 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;
> >> +}