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

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Message-ID: <87zh8irltc.fsf@nanos.tec.linutronix.de>
Date:   Thu, 02 Jul 2020 17:14:23 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
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

Frederic Weisbecker <frederic@...nel.org> writes:
> On Thu, Jul 02, 2020 at 03:21:35PM +0200, Thomas Gleixner wrote:
>> Frederic Weisbecker <frederic@...nel.org> writes:
>> > @@ -883,7 +883,7 @@ static inline void forward_timer_base(struct timer_base *base)
>> >  	 * Also while executing timers, base->clk is 1 offset ahead
>> >  	 * of jiffies to avoid endless requeuing to current jffies.
>> >  	 */
>> > -	if ((long)(jnow - base->clk) < 2)
>> > +	if ((long)(jnow - base->clk) < 1)
>> >  		return;
>> 
>> The apparent reason is in the comment right above the condition ...
>
> Hmm, that's a comment I added myself in the patch before.

:)

> 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:

> 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.

Thanks,

        tglx