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

[Benno Lossin]

On 10.10.24 11:06, Andreas Hindborg wrote:
> Andreas Hindborg <a.hindborg@xxxxxxxxxx> writes:
> 
>> Andreas Hindborg <a.hindborg@xxxxxxxxxx> writes:
>>
>>> "Gary Guo" <gary@xxxxxxxxxxx> writes:
>>>
>>>> On Sat, 5 Oct 2024 13:59:44 +0200
>>>> Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>>>>
>>>>> On Sat, Oct 5, 2024 at 11:49 AM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote:
>>>>>>
>>>>>> Hi Greg,
>>>>>>
>>>>>> "Greg KH" <gregkh@xxxxxxxxxxxxxxxxxxx> 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.

Was the benchmark run together with the LTO helper patches?

---
Cheers,
Benno