Message-Id: <DBDNIAW09Z7W.EXO6C61HCNVP@kernel.org>
Date: Wed, 16 Jul 2025 19:21:32 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>
Cc: "Mitchell Levy" <levymitchell0@...il.com>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "Andrew Morton" <akpm@...ux-foundation.org>, "Dennis Zhou"
 <dennis@...nel.org>, "Tejun Heo" <tj@...nel.org>, "Christoph Lameter"
 <cl@...ux.com>, "Danilo Krummrich" <dakr@...nel.org>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <linux-mm@...ck.org>
Subject: Re: [PATCH v2 3/5] rust: percpu: add a rust per-CPU variable test

On Wed Jul 16, 2025 at 5:33 PM CEST, Boqun Feng wrote:
> On Wed, Jul 16, 2025 at 12:32:04PM +0200, Benno Lossin wrote:
>> On Tue Jul 15, 2025 at 11:34 PM CEST, Boqun Feng wrote:
>> > On Tue, Jul 15, 2025 at 07:44:01PM +0200, Benno Lossin wrote:
>> > [...]
>> >> >> >
>> >> >> > First of all, `thread_local!` has to be implemented by some sys-specific
>> >> >> > unsafe mechanism, right? For example on unix, I think it's using
>> >> >> > pthread_key_t:
>> >> >> >
>> >> >> > 	https://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_key_create.html
>> >> >> >
>> >> >> > what we are implementing (or wrapping) is the very basic unsafe
>> >> >> > mechanism for percpu here. Surely we can explore the design for a safe
>> >> >> > API, but the unsafe mechanism is probably necessary to look into at
>> >> >> > first.
>> >> >> 
>> >> >> But this is intended to be used by drivers, right? If so, then we should
>> >> >
>> >> > Not necessarily only for drivers, we can also use it for implementing
>> >> > other safe abstraction (e.g. hazard pointers, percpu counters etc)
>> >> 
>> >> That's fair, but then it should be `pub(crate)`.
>> >> 
>> >
>> > Fine by me, but please see below.
>> >
>> >> >> do our usual due diligence and work out a safe abstraction. Only fall
>> >> >> back to unsafe if it isn't possible.
>> >> >> 
>> >> >
>> >> > All I'm saying is instead of figuring out a safe abstraction at first,
>> >> > we should probably focus on identifying how to implement it and which
>> >> > part is really unsafe and the safety requirement for that.
>> >> 
>> >> Yeah. But then we should do that before merging :)
>> >> 
>> >
>> > Well, who's talknig about merging? ;-) I thought we just began reviewing
>> > here ;-)
>> 
>> I understand [PATCH] emails as "I want to merge this" and [RFC PATCH] as
>
> But it doesn't mean "merge as it is", right? I don't think either I or
> Mitchell implied that, I'm surprised that you had to mention that,

Yeah that is true, but it at least shows the intention :)

> also based on "I often mute those" below, making it "[PATCH]" seems to
> be a practical way to get more attention if one wants to get some
> reviews.

That is true, I do usually read the titles of RFC patches though and
sometimes take a look eg your atomics series.

>> "I want to talk about merging this". It might be that I haven't seen the
>> RFC patch series, because I often mute those.
>> 
>
> Well, then you cannot blame people to move from "RFC PATCH" to "PATCH"
> stage for more reviews, right? And you cannot make rules about what the
> difference between [PATCH] and [RFC PATCH] if you ignore one of them ;-)

I'm not trying to blame anyone. I saw a lot of unsafe in the example and
thought "we can do better" and since I haven't heard any sufficient
arguments showing that it's impossible to improve, we should do some
design work.

>> >> >> I'm not familiar with percpu, but from the name I assumed that it's
>> >> >> "just a variable for each cpu" so similar to `thread_local!`, but it's
>> >> >> bound to the specific cpu instead of the thread.
>> >> >> 
>> >> >> That in my mind should be rather easy to support in Rust at least with
>> >> >> the thread_local-style API. You just need to ensure that no reference
>> >> >> can escape the cpu, so we can make it `!Send` & `!Sync` + rely on klint
>> >> >
>> >> > Not really, in kernel, we have plenty of use cases that we read the
>> >> > other CPU's percpu variables. For example, each CPU keeps it's own
>> >> > counter and we sum them other in another CPU.
>> >> 
>> >> But then you need some sort of synchronization?
>> >> 
>> >
>> > Right, but the synchronization can exist either in the percpu operations
>> > themselves or outside the percpu operations. Some cases, the data types
>> > are small enough to fit in atomic data types, and operations are just
>> > load/store/cmpxchg etc, then operations on the current cpu and remote
>> > read will be naturally synchronized. Sometimes extra synchronization is
>> > needed.
>> 
>> Sure, so we probably want direct atomics support. What about "extra
>> synchronization"? Is that using locks or RCU or what else?
>> 
>
> It's up to the users obviously, It could be some sort of locking or RCU,
> it's case by case.

Makes sense, what do you need in the VMS driver?

>> > Keyword find all these cases are `per_cpu_ptr()`:
>> >
>> > 	https://elixir.bootlin.com/linux/v6.15.6/A/ident/per_cpu_ptr
>> 
>> Could you explain to me how to find them? I can either click on one of
>> the files with horrible C preprocessor macros or the auto-completion in
>> the search bar. But that one only shows 3 suggestions `_hyp_sym`,
>> `_nvhe_sym` and `_to_phys` which doesn't really mean much to me.
>> 
>
> You need to find the usage of `per_cpu_ptr()`, which is a function that
> gives you a pointer to a percpu variable on the other CPU, and then
> that's usually the case where a "remote" read of percpu variable
> happens.

Ahh gotcha, I thought you pointed me to some definitions of operations
on percpu pointers.

---
Cheers,
Benno

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