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

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Message-ID: <ZICMBw-Fo-nzqGXb@2a01cb0980007dab8d6b2773c13fd2df.ipv6.abo.wanadoo.fr>
Date:   Wed, 7 Jun 2023 15:54:15 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
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>,
        Frederic Weisbecker <fweisbec@...il.com>,
        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>
Subject: Re: [PATCH v7 19/21] timer: Implement the hierarchical pull model

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?

Thanks.

> +			return true;