linux-kernel - Re: [PATCH v5 16/18] timer: Implement the hierarchical pull model

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Message-ID: <20230321154040.GC2272870@hirez.programming.kicks-ass.net>
Date:   Tue, 21 Mar 2023 16:40:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.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>
Subject: Re: [PATCH v5 16/18] timer: Implement the hierarchical pull model

On Wed, Mar 01, 2023 at 03:17:42PM +0100, Anna-Maria Behnsen wrote:
> +static bool tmigr_requires_handle_remote_up(struct tmigr_group *group,
> +					 struct tmigr_group *child,
> +					 void *ptr)
> +{
> +	struct tmigr_remote_data *data = ptr;
> +	u32 childmask;
> +
> +	childmask = data->childmask;
> +
> +	/*
> +	 * Handle the group only if child 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;
> +
> +	/*
> +	 * Racy lockless check for next_expiry
> +	 */
> +	if (data->now >= group->next_expiry) {

I'm not far enough along to tell; but on 32bit this can/will suffer from
split loads and basically turn into a random number generator. I'm
presuming the 'check = 1' thing here covers that case?

> +		data->check = 1;
> +		return true;
> +	}
> +
> +	/* Update of childmask for next level */
> +	data->childmask = group->childmask;
> +	return false;
> +}