linux-kernel - Re: [PATCH v2] rust: time: add Ktime

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Date: Thu, 11 Apr 2024 08:56:58 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, John Stultz <jstultz@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stephen Boyd <sboyd@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rust: time: add Ktime

On Fri, Mar 22, 2024 at 08:59:38AM +0000, Alice Ryhl wrote:
> Introduce a wrapper around `ktime_t` with a few different useful
> methods.
> 
> Rust Binder will use these bindings to compute how many milliseconds a
> transaction has been active for when dumping the current state of the
> Binder driver. This replicates the logic in C Binder [1].
> 
> For a usage example in Rust Binder, see [2].
> 
> The `ktime_get` method cannot be safely called in NMI context. This
> requirement is not checked by these abstractions, but it is intended
> that klint [3] or a similar tool will be used to check it in the future.
> 
> Link: https://lore.kernel.org/lkml/5ac8c0d09392290be789423f0dd78a520b830fab.1682333709.git.zhangchuang3@xiaomi.com/ [1]
> Link: https://r.android.com/3004103 [2]
> Link: https://rust-for-linux.com/klint [3]
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
> Changes in v2:
> - Mention that ktime_get cannot be safely called in NMI context.
> - Link to v1: https://lore.kernel.org/r/20240320-rust-ktime_ms_delta-v1-1-ccb8672a0941@google.com
> ---
>  rust/kernel/time.rs | 60 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 60 insertions(+)
> 
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> index 25a896eed468..6811d5cadbd4 100644
> --- a/rust/kernel/time.rs
> +++ b/rust/kernel/time.rs
> @@ -5,6 +5,9 @@
>  //! This module contains the kernel APIs related to time and timers that
>  //! have been ported or wrapped for usage by Rust code in the kernel.
>  
> +/// The number of nanoseconds per millisecond.
> +pub const NSEC_PER_MSEC: i64 = bindings::NSEC_PER_MSEC as i64;
> +
>  /// The time unit of Linux kernel. One jiffy equals (1/HZ) second.
>  pub type Jiffies = core::ffi::c_ulong;
>  
> @@ -18,3 +21,60 @@ pub fn msecs_to_jiffies(msecs: Msecs) -> Jiffies {
>      // matter what the argument is.
>      unsafe { bindings::__msecs_to_jiffies(msecs) }
>  }
> +
> +/// A Rust wrapper around a `ktime_t`.
> +#[repr(transparent)]
> +#[derive(Copy, Clone)]
> +pub struct Ktime {
> +    inner: bindings::ktime_t,
> +}
> +
> +impl Ktime {
> +    /// Create a `Ktime` from a raw `ktime_t`.
> +    #[inline]
> +    pub fn from_raw(inner: bindings::ktime_t) -> Self {
> +        Self { inner }
> +    }
> +
> +    /// Get the current time using `CLOCK_MONOTONIC`.
> +    #[inline]
> +    pub fn ktime_get() -> Self {
> +        // SAFETY: It is always safe to call `ktime_get` outside of NMI context.
> +        Self::from_raw(unsafe { bindings::ktime_get() })
> +    }
> +
> +    /// Divide the number of nanoseconds by a compile-time constant.
> +    #[inline]
> +    fn divns_constant<const DIV: i64>(self) -> i64 {
> +        self.to_ns() / DIV
> +    }
> +
> +    /// Returns the number of nanoseconds.
> +    #[inline]
> +    pub fn to_ns(self) -> i64 {
> +        self.inner
> +    }
> +
> +    /// Returns the number of milliseconds.
> +    #[inline]
> +    pub fn to_ms(self) -> i64 {
> +        self.divns_constant::<NSEC_PER_MSEC>()
> +    }
> +}
> +
> +/// Returns the number of milliseconds between two ktimes.
> +#[inline]
> +pub fn ktime_ms_delta(later: Ktime, earlier: Ktime) -> i64 {
> +    (later - earlier).to_ms()
> +}
> +
> +impl core::ops::Sub for Ktime {
> +    type Output = Ktime;
> +
> +    #[inline]
> +    fn sub(self, other: Ktime) -> Ktime {
> +        Self {
> +            inner: self.inner - other.inner,

Nit: although we use "Release mode" to compile Rust code in kernel, so
i64 substraction behaves as 2's complement wrap, I think it's better we
use wrapping_sub here:
		
        self.inner.wrapping_sub(other.inner)

however it's not a correctness issue for now, so with or without it,

Reviewed-by: Boqun Feng <boqun.feng@...il.com>

Regards,
Boqun

> +        }
> +    }
> +}
> 
> ---
> base-commit: 768409cff6cc89fe1194da880537a09857b6e4db
> change-id: 20240320-rust-ktime_ms_delta-74b00c9ab872
> 
> Best regards,
> -- 
> Alice Ryhl <aliceryhl@...gle.com>
>