linux-kernel - Re: [PATCH 2/9] timer: Add comments about calc

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Message-ID: <87tuyav4qx.fsf@nanos.tec.linutronix.de>
Date:   Tue, 14 Jul 2020 11:13:26 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH 2/9] timer: Add comments about calc_index() ceiling work

Frederic Weisbecker <frederic@...nel.org> writes:
>  static inline unsigned calc_index(unsigned expires, unsigned lvl)
>  {
> +	/*
> +	 * Time may have past since the clock last reached an index of
> +	 * this @lvl. And that time, below LVL_GRAN(@lvl), is going to
> +	 * be substracted from the delta until we reach @expires. To
> +	 * fix that we must add one level granularity unit to make sure
> +	 * we rather expire late than early. Prefer ceil over floor.

This comment confuses the hell out of me.

        /*
         * The timer wheel has to guarantee that a timer does not fire
         * early. Early expiry can happen due to:
         * - Timer is armed at the edge of a tick
         * - Truncation of the expiry time in the outer wheel levels
         *
         * Round up with level granularity to prevent this.
         */

Hmm?

Thanks,

        tglx