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

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Date:   Fri, 17 Jul 2020 14:55:56 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.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

On Tue, Jul 14, 2020 at 11:13:26AM +0200, Thomas Gleixner wrote:
> 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.

Me too...

> 
>         /*
>          * 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?

That's relieving, I'm updating the patch.

Thanks!