Date: Tue, 06 Feb 2024 15:13:23 +0100
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
 John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
 Eric Dumazet <edumazet@...gle.com>, "Rafael J . Wysocki"
 <rafael.j.wysocki@...el.com>, Arjan van de Ven <arjan@...radead.org>,
 "Paul E . McKenney" <paulmck@...nel.org>, Rik van Riel <riel@...riel.com>,
 Steven Rostedt <rostedt@...dmis.org>, Sebastian Siewior
 <bigeasy@...utronix.de>, Giovanni Gherdovich <ggherdovich@...e.cz>, Lukasz
 Luba <lukasz.luba@....com>, "Gautham R . Shenoy" <gautham.shenoy@....com>,
 Srinivas Pandruvada <srinivas.pandruvada@...el.com>, K Prateek Nayak
 <kprateek.nayak@....com>
Subject: Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

Frederic Weisbecker <frederic@...nel.org> writes:

> Le Tue, Feb 06, 2024 at 12:36:55PM +0100, Anna-Maria Behnsen a écrit :
>> Frederic Weisbecker <frederic@...nel.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;
>> >
>> > So if the current tmc going inactive was the migrator for the whole hierarchy
>> > and it is reaching here the top-level, this assumes that if none of this tmc's
>> > groups have a timer, then it can just return. But what if the top level has
>> > timers from other children? Who is going to handle them then?
>> >
>> > Should this be "goto check_toplvl" instead?
>> >
>> 
>> Simply replacing this goto will not work. Then we chould end up with a
>> 'data->firstexp' set even if we do not want to have it (when remote is
>> not set).
>
> Hmm, but the (groupstate.migrator == TMIGR_NONE) condition should prevent that,
> no?
>

When remote is not set, and the group is not active (but it's not top
level), walk_done will be set to true. Then goto check_toplevel. There
walk_done and the groupstate migrator check is both true. But it is not
top level... and the next goto unlock will also not happen and
data->firstexp will be set. No?

>> 
>> There is another issue in here. When the event could be ignored and it
>> is propagated because of e.g. remote timer handling, then the timerqueue
>> dance is done nevertheless. It's not a big problem (as the ignore flag
>> is set and event is removed of queue when revisting the timer queue),
>> but its obviously more work than it is required to have.
>
> Right. I guess it doesn't hurt to delete it from the timerqueue if present and
> then update group->next_expiry accordingly. But it's certainly not useful
> to requeue it :-)
>

This would be the plan to keep the delete but drop the requeue in this
case.

With those two things, the tmigr_update_events() wants to be changed a
little more.

Thanks

> Thanks.
>
>> 
>> Thanks
>> 
>> >> +		}
>> >> +	} 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)
>> >> +			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;
>> >> +}

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