Re: [PATCH 3/3] rust: block: convert `block::mq` to use `Refcount`

[Andreas Hindborg <a.hindborg@kernel.org>]

Andreas Hindborg <a.hindborg@kernel.org> writes:

> Andreas Hindborg <a.hindborg@kernel.org> writes:
>
>> "Gary Guo" <gary@garyguo.net> writes:
>>
>>> On Sat, 5 Oct 2024 13:59:44 +0200
>>> Alice Ryhl <aliceryhl@google.com> wrote:
>>>
>>>> On Sat, Oct 5, 2024 at 11:49 AM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>>> >
>>>> > Hi Greg,
>>>> >
>>>> > "Greg KH" <gregkh@linuxfoundation.org> writes:
>>>> >
>>>> > > On Fri, Oct 04, 2024 at 04:52:24PM +0100, Gary Guo wrote:
>>>> > >> There is an operation needed by `block::mq`, atomically decreasing
>>>> > >> refcount from 2 to 0, which is not available through refcount.h, so
>>>> > >> I exposed `Refcount::as_atomic` which allows accessing the refcount
>>>> > >> directly.
>>>> > >
>>>> > > That's scary, and of course feels wrong on many levels, but:
>>>> > >
>>>> > >
>>>> > >> @@ -91,13 +95,17 @@ pub(crate) unsafe fn start_unchecked(this: &ARef<Self>) {
>>>> > >>      /// C `struct request`. If the operation fails, `this` is returned in the
>>>> > >>      /// `Err` variant.
>>>> > >>      fn try_set_end(this: ARef<Self>) -> Result<*mut bindings::request, ARef<Self>> {
>>>> > >> -        // We can race with `TagSet::tag_to_rq`
>>>> > >> -        if let Err(_old) = this.wrapper_ref().refcount().compare_exchange(
>>>> > >> -            2,
>>>> > >> -            0,
>>>> > >> -            Ordering::Relaxed,
>>>> > >> -            Ordering::Relaxed,
>>>> > >> -        ) {
>>>> > >> +        // To hand back the ownership, we need the current refcount to be 2.
>>>> > >> +        // Since we can race with `TagSet::tag_to_rq`, this needs to atomically reduce
>>>> > >> +        // refcount to 0. `Refcount` does not provide a way to do this, so use the underlying
>>>> > >> +        // atomics directly.
>>>> > >> +        if this
>>>> > >> +            .wrapper_ref()
>>>> > >> +            .refcount()
>>>> > >> +            .as_atomic()
>>>> > >> +            .compare_exchange(2, 0, Ordering::Relaxed, Ordering::Relaxed)
>>>> > >> +            .is_err()
>>>> > >
>>>> > > Why not just call rust_helper_refcount_set()?  Or is the issue that you
>>>> > > think you might not be 2 here?  And if you HAVE to be 2, why that magic
>>>> > > value (i.e. why not just always be 1 and rely on normal
>>>> > > increment/decrement?)
>>>> > >
>>>> > > I know some refcounts are odd in the kernel, but I don't see where the
>>>> > > block layer is caring about 2 as a refcount anywhere, what am I missing?
>>>> >
>>>> > It is in the documentation, rendered version available here [1]. Let me
>>>> > know if it is still unclear, then I guess we need to update the docs.
>>>> >
>>>> > Also, my session from Recipes has a little bit of discussion regarding
>>>> > this refcount and it's use [2].
>>>> >
>>>> > Best regards,
>>>> > Andreas
>>>> >
>>>> >
>>>> > [1] https://rust.docs.kernel.org/kernel/block/mq/struct.Request.html#implementation-details
>>>> > [2] https://youtu.be/1LEvgkhU-t4?si=B1XnJhzCCNnUtRsI&t=1685
>>>>
>>>> So it sounds like there is one refcount from the C side, and some
>>>> number of references from the Rust side. The function checks whether
>>>> there's only one Rust reference left, and if so, takes ownership of
>>>> the value, correct?
>>>>
>>>> In that case, the CAS should have an acquire ordering to synchronize
>>>> with dropping the refcount 3->2 on another thread. Otherwise, you
>>>> might have a data race with the operations that happened just before
>>>> the 3->2 refcount drop.
>>>>
>>>> Alice
>>>
>>> The code as is is fine since there's no data protected in
>>> `RequestDataWrapper` yet (in fact it's not even generic yet). I know
>>> Andreas does want to introduce driver-specific data into that, so in
>>> the long term the acquire would be necessary.
>>>
>>> Andreas, please let me know if you want me to make the change now, or
>>> you'd rather change the ordering when you introduce data to
>>> `RequestDataWrapper`.
>>
>> I guess we will have said data dependencies when we are going to run
>> drop for fields in the private data area. Thanks for pointing that out.
>> I will update the ordering when I submit that patch.
>>
>> As I mentioned before, I would rather we do not apply this patch before
>> we get a way to inline helpers.
>
> As discussed offline, the code that suffers the performance regression
> is downstream, and since this change seems to be important, I can apply
> the helper LTO patch downstream as well.
>
> Since the plan for the downstream code _is_ to move upstream, I really
> hope to see the helper LTO patch upstream, so we don't get a performance
> regression because of these refcounts.
>
> If we cannot figure out a way to get the LTO patches (or an alternative
> solution) upstream, we can always revert back to a more performant
> solution in block.

I forgot to report the result of the benchmarks. Over the usual
benchmark workload that I run for `rnull` I see an average 0.8 percent
performance penalty with this patch. For some configurations
I see 95% CI N=40 [-18%;-5%]. So it is not insignificant.


BR Andreas