linux-kernel - Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

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Message-ID: <87tt8ned3e.fsf@kernel.org>
Date: Fri, 21 Feb 2025 12:44:53 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Frederic Weisbecker" <frederic@...nel.org>
Cc: "Benno Lossin" <benno.lossin@...ton.me>,  "Miguel Ojeda"
 <ojeda@...nel.org>,  "Anna-Maria Behnsen" <anna-maria@...utronix.de>,
  "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>,  "Alice Ryhl" <aliceryhl@...gle.com>,  "Trevor
 Gross" <tmgross@...ch.edu>,  "Lyude Paul" <lyude@...hat.com>,  "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 v8 02/14] rust: hrtimer: introduce hrtimer support

"Frederic Weisbecker" <frederic@...nel.org> writes:

> Le Thu, Feb 20, 2025 at 11:46:10PM +0000, Benno Lossin a écrit :
>> On 18.02.25 14:27, Andreas Hindborg wrote:
>> > This patch adds support for intrusive use of the hrtimer system. For now,
>> > only one timer can be embedded in a Rust struct.
>> >
>> > The hrtimer Rust API is based on the intrusive style pattern introduced by
>> > the Rust workqueue API.
>> >
>> > Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
>> > ---
>> >  rust/kernel/time.rs         |   2 +
>> >  rust/kernel/time/hrtimer.rs | 312 ++++++++++++++++++++++++++++++++++++++++++++
>> >  2 files changed, 314 insertions(+)
>> >
>> > diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
>> > index 87e47f2f5618d..2cf365cfb412e 100644
>> > --- a/rust/kernel/time.rs
>> > +++ b/rust/kernel/time.rs
>> > @@ -10,6 +10,8 @@
>> >
>> >  use core::convert::Into;
>> >
>> > +pub mod hrtimer;
>> > +
>> >  /// The number of nanoseconds per millisecond.
>> >  pub const NSEC_PER_MSEC: i64 = bindings::NSEC_PER_MSEC as i64;
>> >
>> > diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
>> > new file mode 100644
>> > index 0000000000000..a6332924efabd
>> > --- /dev/null
>> > +++ b/rust/kernel/time/hrtimer.rs
>> > @@ -0,0 +1,312 @@
>> > +// SPDX-License-Identifier: GPL-2.0
>> > +
>> > +//! Intrusive high resolution timers.
>> > +//!
>> > +//! Allows running timer callbacks without doing allocations at the time of
>> > +//! starting the timer. For now, only one timer per type is allowed.
>> > +//!
>> > +//! # Vocabulary
>> > +//!
>> > +//! States:
>> > +//!
>> > +//! * Stopped
>> > +//! * Running
>> > +//!
>> > +//! Operations:
>> > +//!
>> > +//! * Start
>> > +//! * Cancel
>> > +//! * Stop
>> > +//! * Restart
>> > +//!
>> > +//! Events:
>> > +//!
>> > +//! * Expire
>> > +//!
>> > +//! ## State Diagram
>> > +//!
>> > +//! ```text
>> > +//!                  <-- Stop ----
>> > +//!                  <-- Cancel --
>> > +//!                  --- Start -->
>> > +//!        +---------+        +---------+
>> > +//!   O--->| Stopped |        | Running |---o
>> > +//!        +---------+        +---------+   |
>> > +//!                                  ^      |
>> > +//!                  <- Expire --    |      |
>> > +//!                                  o------o
>> > +//!                                   Restart
>> > +//! ```
>> > +//!
>> > +//! A timer is initialized in the **stopped** state. A stopped timer can be
>> > +//! **started** with an **expiry** time. After the timer is started, it is
>> > +//! **running**. When the timer **expires**, the timer handler is executed.
>> > +//! After the handler has executed, the timer may be **restarted** or
>> > +//! **stopped**. A running timer can be **canceled** before it's handler is
>>
>> This confuses me a bit, in the other thread you wrote that the handler
>> decides if the timer should restart or be stopped. But What happens when
>> I call `cancel` on the `HrTimerHandle` while the handler is running, but
>> finishes shortly after with a restart request? Will it be canceled?
>>
>> I also have a bit of a wording issue with "the timer is running" IIUC,
>> this means that the timer subsystem keeps track of the expiry time and
>> when the time is elapsed, it fires the code that you registered prior.
>> At first, I thought that "the timer is running" meant that the
>> registered code is running. Maybe we should have two different terms for
>> that? I personally would prefer "active timer" for "the timer subsystem
>> is currently tracking the time and when it is elapsed, it will run the
>> code" and "running timer" for "the timer's expiry time has elapsed and
>> the timer callback is currently being executed".
>
> Good point. "Running" in the hrtimer terminology is usually to
> describe a running callback and not just an elapsing (or say started) timer.
>
> I would rather have:
>
> Stopped: initialized but not started, or cancelled, or not restarted
> Started: initialized and started or restarted
> Running: executing the callback

Alright, we can do that. That makes sense 👍



Best regards,
Andreas Hindborg