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

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Message-ID: <681124ff.050a0220.13a0e7.2c80@mx.google.com>
Date: Tue, 29 Apr 2025 12:14:04 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Arnd Bergmann <arnd@...db.de>
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 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