linux-kernel - Re: [PATCH v1] rust: time: Avoid 64-bit integer division

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Message-ID: <aBNrJgLFpswcgOEK@Mac.home>
Date: Thu, 1 May 2025 05:37:58 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	a.hindborg@...sung.com, frederic@...nel.org, lyude@...hat.com,
	tglx@...utronix.de, anna-maria@...utronix.de, jstultz@...gle.com,
	sboyd@...nel.org, ojeda@...nel.org, alex.gaynor@...il.com,
	gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	aliceryhl@...gle.com, tmgross@...ch.edu, chrisi.schrefl@...il.com,
	arnd@...db.de, linux@...linux.org.uk
Subject: Re: [PATCH v1] rust: time: Avoid 64-bit integer division

On Thu, May 01, 2025 at 05:26:54AM -0700, Boqun Feng wrote:
> On Thu, May 01, 2025 at 10:58:18AM +0900, FUJITA Tomonori wrote:
> > Avoid 64-bit integer division that 32-bit architectures don't
> > implement generally. This uses ktime_to_ms() and ktime_to_us()
> > instead.
> > 
> > The timer abstraction needs i64 / u32 division so C's div_s64() can be
> > used but ktime_to_ms() and ktime_to_us() provide a simpler solution
> > for this timer abstraction problem. On some architectures, there is
> > room to optimize the implementation of them, but such optimization can
> > be done if and when it becomes necessary.
> > 
> 
> Nacked-by: Boqun Feng <boqun.feng@...il.com>
> 
> As I said a few times, we should rely on compiler's optimization when
> available, i.e. it's a problem that ARM compiler doesn't have this
> optimization, don't punish other architecture of no reason.
> 
> Regards,
> Boqun
> 
> > One downside of calling the C's functions is that the as_micros/millis
> > methods can no longer be const fn. We stick with the simpler approach
> > unless there's a compelling need for a const fn.
> > 

Btw, I think you're missing the Suggested-by tag from Arnd, of course if
Arnd is not against it.

Regards,
Boqun

> > Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
> > ---
> >  rust/helpers/helpers.c |  1 +
> >  rust/helpers/time.c    | 13 +++++++++++++
> >  rust/kernel/time.rs    | 10 ++++++----
> >  3 files changed, 20 insertions(+), 4 deletions(-)
> >  create mode 100644 rust/helpers/time.c
> > 
> > diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> > index 1e7c84df7252..2ac088de050f 100644
> > --- a/rust/helpers/helpers.c
> > +++ b/rust/helpers/helpers.c
> > @@ -34,6 +34,7 @@
> >  #include "spinlock.c"
> >  #include "sync.c"
> >  #include "task.c"
> > +#include "time.c"
> >  #include "uaccess.c"
> >  #include "vmalloc.c"
> >  #include "wait.c"
> > diff --git a/rust/helpers/time.c b/rust/helpers/time.c
> > new file mode 100644
> > index 000000000000..0a5d1773a07c
> > --- /dev/null
> > +++ b/rust/helpers/time.c
> > @@ -0,0 +1,13 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +#include <linux/ktime.h>
> > +
> > +s64 rust_helper_ktime_to_us(const ktime_t kt)
> > +{
> > +	return ktime_divns(kt, NSEC_PER_USEC);
> > +}
> > +
> > +s64 rust_helper_ktime_to_ms(const ktime_t kt)
> > +{
> > +	return ktime_divns(kt, NSEC_PER_MSEC);
> > +}
> > diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> > index a8089a98da9e..e3008f6324ea 100644
> > --- a/rust/kernel/time.rs
> > +++ b/rust/kernel/time.rs
> > @@ -228,13 +228,15 @@ pub const fn as_nanos(self) -> i64 {
> >      /// Return the smallest number of microseconds greater than or equal
> >      /// to the value in the [`Delta`].
> >      #[inline]
> > -    pub const fn as_micros_ceil(self) -> i64 {
> > -        self.as_nanos().saturating_add(NSEC_PER_USEC - 1) / NSEC_PER_USEC
> > +    pub fn as_micros_ceil(self) -> i64 {
> > +        // SAFETY: It is always safe to call `ktime_to_us()` with any value.
> > +        unsafe { bindings::ktime_to_us(self.as_nanos().saturating_add(NSEC_PER_USEC - 1)) }
> >      }
> >  
> >      /// Return the number of milliseconds in the [`Delta`].
> >      #[inline]
> > -    pub const fn as_millis(self) -> i64 {
> > -        self.as_nanos() / NSEC_PER_MSEC
> > +    pub fn as_millis(self) -> i64 {
> > +        // SAFETY: It is always safe to call `ktime_to_ms()` with any value.
> > +        unsafe { bindings::ktime_to_ms(self.nanos) }
> >      }
> >  }
> > 
> > base-commit: 679185904972421c570a1c337a8266835045012d
> > -- 
> > 2.43.0
> > 
> > 
>