Re: [PATCH v2 3/5] rust: percpu: add a rust per-CPU variable test

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

On Wed, Jul 16, 2025 at 07:21:32PM +0200, Benno Lossin wrote:
> 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 agree with you, and I like what you're proposing, but I think design
work can be done at "PATCH" stage, right? And sometimes, it's also OK to
do some design work even at some version like "v12" ;-)

Also I want to see more forward-progress actions about the design work
improvement. For example, we can examine every case that makes
unsafe_get_per_cpu!() unsafe, and see if we can improve that by typing
or something else. We always can "do better", but the important part is
how to get there ;-)

> >> >> >> 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?
> 

In VMBus driver, it's actually isolate, i.e. each CPU only access it's
own work_struct, so synchronization between CPUs is not needed.

Regards,
Boqun

> >> > 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