linux-kernel - Re: [PATCH v9 04/13] rust: hrtimer: allow timer restart from timer handler

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Message-ID: <6c2a299ec594fbaae294037e36c78cb37aaa7f33.camel@redhat.com>
Date: Tue, 25 Feb 2025 16:46:56 -0500
From: Lyude Paul <lyude@...hat.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Anna-Maria Behnsen	
 <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, 
 Thomas Gleixner <tglx@...utronix.de>, Danilo Krummrich <dakr@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,  Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>, Björn Roy Baron	
 <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, Alice
 Ryhl	 <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Guangbo Cui
	 <2407018371@...com>, Dirk Behme <dirk.behme@...il.com>, Daniel Almeida	
 <daniel.almeida@...labora.com>, Tamir Duberstein <tamird@...il.com>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 04/13] rust: hrtimer: allow timer restart from timer
 handler

On Tue, 2025-02-25 at 09:58 +0100, Andreas Hindborg wrote:
> "Lyude Paul" <lyude@...hat.com> writes:
> 
> > On Mon, 2025-02-24 at 13:03 +0100, Andreas Hindborg wrote:
> > > This patch allows timer handlers to report that they want a timer to be
> > > restarted after the timer handler has finished executing.
> > > 
> > > Also update the `hrtimer` documentation to showcase the new feature.
> > > 
> > > Acked-by: Frederic Weisbecker <frederic@...nel.org>
> > > Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> > > ---
> > >  rust/kernel/time/hrtimer.rs     | 19 ++++++++++++++++++-
> > >  rust/kernel/time/hrtimer/arc.rs |  4 +---
> > >  2 files changed, 19 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
> > > index d08fd7de158d..a431c8b728ae 100644
> > > --- a/rust/kernel/time/hrtimer.rs
> > > +++ b/rust/kernel/time/hrtimer.rs
> > > @@ -207,7 +207,7 @@ pub trait HrTimerCallback {
> > >      type Pointer<'a>: RawHrTimerCallback;
> > > 
> > >      /// Called by the timer logic when the timer fires.
> > > -    fn run(this: <Self::Pointer<'_> as RawHrTimerCallback>::CallbackTarget<'_>)
> > > +    fn run(this: <Self::Pointer<'_> as RawHrTimerCallback>::CallbackTarget<'_>) -> HrTimerRestart
> > >      where
> > >          Self: Sized;
> > >  }
> > > @@ -313,6 +313,23 @@ unsafe fn start(self_ptr: *const Self, expires: Ktime) {
> > >      }
> > >  }
> > > 
> > > +/// Restart policy for timers.
> > > +pub enum HrTimerRestart {
> > > +    /// Timer should not be restarted.
> > > +    NoRestart,
> > > +    /// Timer should be restarted.
> > > +    Restart,
> > > +}
> > 
> > Should we have #[derive(Copy, Clone, PartialEq, Eq)] here?
> 
> Yea, lets do that. `Debug` as well?

sgtm

> 
> > 
> > Also, I feel like I might have asked this a few versions ago so hopefully i'm
> > not asking again: but what's the reason for us not just using the
> > discriminants of `HrTimerRestart` here:
> > 
> > /// Restart policy for timers.
> > #[repr(u32)]
> > pub enum HrTimerRestart {
> >     /// Timer should not be restarted.
> >     NoRestart = bindings::hrtimer_restart_HRTIMER_NORESTART,
> >     /// Timer should be restarted.
> >     Restart = bindings::hrtimer_restart_HRTIMER_RESTART,
> > }
> 
> I forget if we discussed this, but it does not make much of a
> difference, does it?
> 
> With a Rust enum, we get a smaller storage type maybe with better
> support for niche optimizations? And then pay a bit more for conversion.
> All in all, I don't think it makes much difference.

No idea about performance wise, but I -think- it would actually cut down on
the code that you need - particularly for the larger enums here. Mainly
because you only would need to manually specify each variant for converting
from bindings::hrtimer_restart to HrTimerRestart, but not the other way
around:

   /// Restart policy for timers.
   #[repr(u32)]
   pub enum HrTimerRestart {
       /// Timer should not be restarted.
       NoRestart = bindings::hrtimer_restart_HRTIMER_NORESTART,
       /// Timer should be restarted.
       Restart = bindings::hrtimer_restart_HRTIMER_RESTART,
   }
   
   impl From<bindings::hrtimer_restart> for HrTimerRestart {
       fn from(value: u32) -> Self {
           match value {
               bindings::hrtimer_restart_HRTIMER_NORESTART => Self::NoRestart,
               _ => Self::Restart,
           }
       }
   }
   
   impl From<HrTimerRestart> for bindings::hrtimer_restart {
       fn from(value: HrTimerRestart) -> Self {
           value as Self
       }
   }

> 
> 
> Best regards,
> Andreas Hindborg
> 
> 

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

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