Re: [PATCH v7 19/21] timer: Implement the hierarchical pull model

Anna-Maria Behnsen
2023-06-12T12:30:08Z

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On Wed, 7 Jun 2023, Frederic Weisbecker wrote:

> Le Wed, May 24, 2023 at 09:06:27AM +0200, Anna-Maria Behnsen a écrit :
> > +/*
> > + * Returns true, if there is nothing to be propagated to the next level
> > + *
> > + * @data->nextexp is reset to KTIME_MAX; it is reused for first global
> > + * event which needs to be handled by migrator (in toplevel group)
> > + *
> > + * This is the only place where 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;
> > + bool walk_done, remote = data->remote;
> > + u64 nextexp;
> > +
> > + if (child) {
> > + if (data->childstate.active)
> > + return true;
> > +
> > + raw_spin_lock(&child->lock);
> > + raw_spin_lock_nested(&group->lock, SINGLE_DEPTH_NESTING);
> > +
> > + first_childevt = tmigr_next_groupevt(child);
> > + nextexp = child->next_expiry;
> > + evt = &child->groupevt;
> > + } else {
> > + nextexp = data->nextexp;
> > +
> > + /*
> > + * Set @data->nextexp to KTIME_MAX; it is reused for first
> > + * global event which needs to be handled by migrator (in
> > + * toplevel group)
> > + */
> > + data->nextexp = KTIME_MAX;
> > +
> > + first_childevt = evt = data->evt;
> > +
> > + /*
> > + * Walking the hierarchy is required in any case, when a
> > + * remote expiry was done before.
>
> You can probably remove that comma because it feels like breaking the condition link.
>
> > + * This ensures to not lost
>
> lose
>
> > + * already queued events in non active groups (see section
> > + * "Required event and timerqueue update after remote
> > + * expiry" in documentation at the top).
> > + */
> > + if (evt->ignore && !remote)
>
> It looks like in the case of !remote, this branch will never end up
> stopping the propagation up because either:
>
> * We come here from tmigr_inactive_up() which takes care of the propagation.
>
> or
>
> * We come here from tmigr_new_timer() and ->ignore can't be set.
>
> If I'm right, can we add a comment about that so that the poor reviewer
> doesn't stutter on that for too long?
>

Right. It will never stop the propagation - but the condition could be
fulfilled when call site is tmigr_inactive_up(). My proposal for expanding
the comment is the following:

/*
* 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 remote
* expiry" in 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 propagation by itself
* and ignores return value. But an immediate return is
* required because nothing has to be done in this level as
* event could be ignored.
*/

Thanks,

Anna-Maria