linux-kernel - Re: [RFC PATCH 10/10] timer: Lower base clock forwarding threshold

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Message-ID: <20200703001219.GD27417@lenoir>
Date:   Fri, 3 Jul 2020 02:12:20 +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: [RFC PATCH 10/10] timer: Lower base clock forwarding threshold

On Thu, Jul 02, 2020 at 05:14:23PM +0200, Thomas Gleixner wrote:
> Frederic Weisbecker <frederic@...nel.org> writes:
> > On Thu, Jul 02, 2020 at 03:21:35PM +0200, Thomas Gleixner wrote:
> > The following part:
> >
> >> >      * Also while executing timers, base->clk is 1 offset ahead
> >> >      * of jiffies to avoid endless requeuing to current jffies.
> >> >      */
> >
> > relates to situation when (long)(jnow - base->clk) < 0
> 
> This still is inconsistent with your changelog:

Right.

> 
> > There is no apparent reason for not forwarding base->clk when it's 2
> > jiffies late
> 
> Let's do the math:
> 
>  jiffies = 4
>  base->clk = 2
> 
>  4 - 2 = 2
> 
> which means it is forwarded when it's 2 jiffies late with the original
> code, because 2 < 2.
> 
> The reason for this < 2 is historical and goes back to the oddities of
> the original timer wheel before the big rewrite.

Ok. And is it still needed today or can we now forward even with a 1 delta?