Re: [PATCH v15 5/6] rust: time: Add wrapper for fsleep() function

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Apr 29, 2025 at 08:33:47PM +0200, Arnd Bergmann wrote:
> On Tue, Apr 29, 2025, at 19:15, Boqun Feng wrote:
> > On Tue, Apr 29, 2025 at 06:11:02PM +0200, Arnd Bergmann wrote:
> >> On Tue, Apr 29, 2025, at 18:03, Boqun Feng wrote:
> >
> > Would it make sense if we rely on compiler optimization when it's
> > avaiable (for x86_64, arm64, riscv, etc), and only call ktime_to_ms() if
> > not? The downside of calling ktime_to_ms() are:
> >
> > * it's a call function, and cannot be inlined with LTO or INLINE_HELPER:
> >
> > 	https://lore.kernel.org/all/20250319205141.3528424-1-gary@garyguo.net/
> >
> > * it doesn't provide the overflow checking even if
> >   CONFIG_RUST_OVERFLOW_CHECKS=y
> >
> > Thoughts?
> 
> The function call overhead is tiny compared to replacing a 64-bit
> division with a constant mult/shift.
> 

Just to be clear, are you essientially saying that even in C,
ktime_to_ms() is not worth inlining? Because the call overhead is tiny
compared to the function own cost?

My impression is that on x86 at least, function call is 10+ cycles, and
multiply is 3 cycles, so I would think that ktime_to_ms() itself is at
most 10 cycles. Maybe I'm out of date of the modern micro-architecture?

> What is the possible overflow that can happen here? For a constant
> division at least there is no chance of divide-by-zero. Do you mean
> truncating to 32 bit?
> 

I was referring the last part of Miguel's email:

	https://lore.kernel.org/rust-for-linux/CANiq72mMRpY4NC4_8v_wDpq6Z3qs99Y8gXd-7XL_3Bed58gkJg@mail.gmail.com/

Regards,
Boqun

>      Arnd