Re: [PATCH 2/2] rust: sync: add `CondVar::wait_timeout`

[Alice Ryhl]

On Wed, Dec 6, 2023 at 5:53 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Dec 06, 2023 at 05:42:29PM +0100, Alice Ryhl wrote:
> > On Wed, Dec 6, 2023 at 5:39 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > On Wed, Dec 06, 2023 at 08:30:06AM -0800, Boqun Feng wrote:
> > > > On Wed, Dec 06, 2023 at 10:09:24AM +0000, Alice Ryhl wrote:
> > > > [...]
> > > > > +
> > > > > +/// The return type of `wait_timeout`.
> > > > > +pub enum CondVarTimeoutResult {
> > > > > +    /// The timeout was reached.
> > > > > +    Timeout,
> > > > > +    /// Somebody woke us up.
> > > > > +    Woken {
> > > > > +        /// Remaining sleep duration.
> > > > > +        jiffies: u64,
> > > >
> > > > I have a Jiffies definition in the my upcoming timer patchset:
> > > >
> > > > /// The time unit of Linux kernel. One jiffy equals (1/HZ) second.
> > > > pub type Jiffies = core::ffi::c_ulong;
> > > >
> > > > Maybe you can add that (in a separate patch) in kernel::time?
> > >
> > > Urgh, why are we using jiffies in 2023?
> >
> > I assumed that the correct thing here would be to accept the same unit
> > as what schedule_timeout takes. Should I be doing something else?
>
> Bah, so we have schedule_hrtimeout() that takes ktime/u64 nsec. But the
> 'problem' is that hrtimers are written with the expectation to fire,
> while the old timers are written with the expectation to not fire.
>
> Timeouts are typically best done with the latter, so in that regard
> using schedule_timeout() is right. But it is sad to inflict the
> brain-damage that is jiffies onto new code.
>
> Perhaps add schedule_timeout_*msec() wrappers around schedule_timeout*()
> and use a consistent sane time unit?
>
> Thomas?

Hmm, looking over my usage in Rust Binder again ... the unit I need
*is* msec, but when we are woken up, we sometimes just go to sleep
again, which means that we need to be able to pass the remaining
duration back to `wait_timeout` to continue sleeping. I'm guessing
that I would lose precision if I converted back/forth to msecs here?

Alice