"Benno Lossin" <benno.lossin@xxxxxxxxx> writes: > 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? No, that the effect of applying only this patch set alone. I did apply the helper LTO patches downstream a few times, but I don't carry them in my default tree. But I guess I can start doing that now. Best regards, Andreas