linux-kernel - Re: Question on expiring HRtimer in-kernel

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Date:   Thu, 3 Mar 2022 14:43:22 +0200
From:   Matti Vaittinen <mazziesaccount@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>,
        linux-power <linux-power@...rohmeurope.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>
Cc:     Sebastian Reichel <sre@...nel.org>,
        Code Kipper <codekipper@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Lee Jones <lee.jones@...aro.org>
Subject: Re: Question on expiring HRtimer in-kernel

Hi Linus,

On 3/1/22 14:19, Linus Walleij wrote:
> I have a problem with a premature expiring HRtimer.
> 
> The HRtimer hrtimer_set_expires_range() is used in two places in
> the upstream kernel:
> kernel/futex/core.c
> drivers/power/supply/ab8500_chargalg.c
> 
> Now I am testing the code in the latter, and it has seen some
> bitrot since merged in 2012. Maybe it was correct at one point.
> The timer is started like this:
> 
>      hrtimer_init(&di->safety_timer, CLOCK_REALTIME, HRTIMER_MODE_ABS);
> (...)
>      hrtimer_set_expires_range(&di->safety_timer,
>          ktime_set(timer_expiration * ONE_HOUR_IN_SECONDS, 0),
>          ktime_set(FIVE_MINUTES_IN_SECONDS, 0));
>      hrtimer_start_expires(&di->safety_timer, HRTIMER_MODE_REL);
> 
> What the author wanted to achieve is a very definitive callback in one
> hour relative to now +/- 5 min, and that is one hour later in the
> physical world,
> as this deals with battery charging.
> 
> However sometimes this fires almost immediately rather than in an hour.
> 
> My first thought is to pass HRTIMER_MODE_REL also to init as
> hrtimer_set_expires_range() could make things happen immediately
> if we have ABS set, but this is all just intuitive.
> 
> Any hints? Better ways to create a definitive event in one hour?
> 

_a lot_ of water has been flowing in the Oulu river since I last touched 
on any code like this. Unfortunately I can't go back to my old code as 
it was left in proprietary world. So no promises I am at all helpful here ;)

In any case, I remember few cases where I hit nasty issues because I 
used CLOCK_REALTIME - which (AFAIR) is subject to the time adjustments. 
NTP, GPS-time and so on can make the time tick in a strange way :) I 
guess you would have noticed if time was set when timer did expire.

Anyways, I guess the battery charging should rather be tied CLOCK_MONOTONIC.

No guarantees though - please ignore me if I make no sense. Oh, 
actually, please don't - please correct me instead :)


-- Matti