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

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Message-Id: <2d89292f-b02c-47b1-9299-92c5f4ba4c9d@app.fastmail.com>
Date: Tue, 29 Apr 2025 20:33:47 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Boqun Feng" <boqun.feng@...il.com>
Cc: "FUJITA Tomonori" <fujita.tomonori@...il.com>,
 "Andreas Hindborg" <a.hindborg@...nel.org>,
 rust-for-linux@...r.kernel.org, "Gary Guo" <gary@...yguo.net>,
 "Alice Ryhl" <aliceryhl@...gle.com>, me@...enk.dev,
 daniel.almeida@...labora.com, linux-kernel@...r.kernel.org,
 Netdev <netdev@...r.kernel.org>, "Andrew Lunn" <andrew@...n.ch>,
 "Heiner Kallweit" <hkallweit1@...il.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Miguel Ojeda" <ojeda@...nel.org>,
 "Alex Gaynor" <alex.gaynor@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 "Benno Lossin" <benno.lossin@...ton.me>,
 "Andreas Hindborg" <a.hindborg@...sung.com>,
 "Anna-Maria Gleixner" <anna-maria@...utronix.de>,
 "Frederic Weisbecker" <frederic@...nel.org>,
 "Thomas Gleixner" <tglx@...utronix.de>,
 "John Stultz" <jstultz@...gle.com>, "Stephen Boyd" <sboyd@...nel.org>,
 "Ingo Molnar" <mingo@...hat.com>,
 "Peter Zijlstra" <peterz@...radead.org>,
 "Juri Lelli" <juri.lelli@...hat.com>,
 "Vincent Guittot" <vincent.guittot@...aro.org>,
 "Dietmar Eggemann" <dietmar.eggemann@....com>,
 "Steven Rostedt" <rostedt@...dmis.org>,
 "Benjamin Segall" <bsegall@...gle.com>, "Mel Gorman" <mgorman@...e.de>,
 "Valentin Schneider" <vschneid@...hat.com>, tgunders@...hat.com,
 david.laight.linux@...il.com, "Paolo Bonzini" <pbonzini@...hat.com>,
 "Jocelyn Falempe" <jfalempe@...hat.com>,
 "Russell King" <linux@...linux.org.uk>,
 "Christian Schrefl" <chrisi.schrefl@...il.com>,
 "Linus Walleij" <linus.walleij@...aro.org>
Subject: Re: [PATCH v15 5/6] rust: time: Add wrapper for fsleep() function

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.

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?

     Arnd