Re: [PATCH v2 2/8] rust: hrtimer: Add HrTimer::raw_forward() and forward()

[Lyude Paul <lyude@redhat.com>]

On Wed, 2025-04-23 at 14:57 +0200, Andreas Hindborg wrote:
> > +
> > +    /// Forward the timer expiry so it expires at `duration` after `now`.
> > +    ///
> > +    /// This is mainly useful for timer types that can start off providing a mutable reference (e.g.
> > +    /// `Pin<Box<…>>`) before the timer is started.
> > +    ///
> > +    /// Note that this does not requeue the timer, it simply updates its expiry value. It returns
> > +    /// the number of overruns that have occurred as a result of the expiry change.
> 
> Looking at C `hrtimer_forward`, I don't think the description is
> correct:
> 
>     u64 hrtimer_forward(struct hrtimer *timer, ktime_t now, ktime_t interval)
>     {
>       u64 orun = 1;
>       ktime_t delta;
> 
>       delta = ktime_sub(now, hrtimer_get_expires(timer));
> 
>       if (delta < 0)
>         return 0;
> 
>       if (WARN_ON(timer->state & HRTIMER_STATE_ENQUEUED))
>         return 0;
> 
>       if (interval < hrtimer_resolution)
>         interval = hrtimer_resolution;
> 
>       if (unlikely(delta >= interval)) {
>         s64 incr = ktime_to_ns(interval);
> 
>         orun = ktime_divns(delta, incr);
>         hrtimer_add_expires_ns(timer, incr * orun);
>         if (hrtimer_get_expires_tv64(timer) > now)
>           return orun;
>         /*
>         * This (and the ktime_add() below) is the
>         * correction for exact:
>         */
>         orun++;
>       }
>       hrtimer_add_expires(timer, interval);
> 
>       return orun;
>     }
> 
> As I read the code:
> 
>   If the timer expires 2s after `now` and `interval` is 6s, then the new expiry
>   time is moved 6s forward. Not to 6s after `now`. Return value will be 0.
> 
>   If the timer expires 3s after `now` and `interval` is 2s, then the
>   expiry time is moved 2s forward and the return value is 1.
> 
>   If the timer expires 4s after `now` and `interval` is 2s, then the
>   expiry time is moved 4s forward and the return value is 2.
> 
>   If the timer expires 5s after `now` and `interval` is 2s, then the
>   expiry time is moved 4s forward and the return value is 2.
> 
> Can you capture this behavior in the docs?

Perhaps I will understand this at some point after sending this email, but as
I'm writing this I have to admit I'm very confused. This is the first time
I've actually looked directly at the hrtimer_forward() source and I have to
say this is 100% not what I expected the term "overrun" to mean. Honestly,
enough so I'm kind of wondering if overrun is even the right word for the C
documentation to be using here.

To make sure I'm understanding this right, an overrun is not "how many times
we would have executed the timer between now and the new execution time" (e.g.
"how many times did our new expiration value overrun the previous expiry
interval"). Instead it's actually "if the timer's next execution time is
greater than the previous expiry time then the timer will be forwarded by
`interval`, but if the timer's execution time is shorter than the previous
expiry time then the new execution time will be determined by figuring out if
the timer were to execute at `interval` what the closest expiry time at that
interval to the previous expiry time would be". Which, I'm afraid to admit
doesn't actually make any sense to me and makes me feel like "overrun" is
entirely the wrong word to be used here.

I'm having a little trouble understanding how I'd really describe this in the
documentation because I'm also having a lot of trouble understanding why this
behavior is the way it is and why someone would want it to work like this.
Should this be something like "Forward the timer to the closest expiry time to
the current expiry time that can be reached if the timer were to execute at
the given interval"?. Or should I maybe just copy the C documentation as close
as possible and just leave this strange behavior as an exercise for the
reader?

> 
> 
> Best regards,
> Andreas Hindborg
> 
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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