I botched subject tags, should be [LSF/MM/BPF TOPIC]. On 6/5/21 10:08 AM, Pavel Begunkov wrote: > One of the core ideas behind io_uring is passing requests via memory > shared b/w the userspace and the kernel, a.k.a. queues or rings. That > serves a purpose of reducing number of context switches or bypassing > them, but the userspace is responsible for controlling the flow, > reaping and processing completions (a.k.a. Completion Queue Entry, CQE), > and submitting new requests, adding extra context switches even if there > is not much work to do. A simple illustration is read(open()), where > io_uring is unable to propagate the returned fd to the read, with more > cases piling up. > > The big picture idea stays the same since last year, to give out some > of this control to BPF, allow it to check results of completed requests, > manipulate memory if needed and submit new requests. Apart from being > just a glue between two requests, it might even offer more flexibility > like keeping a QD, doing reduce/broadcast and so on. > > The prototype [1,2] is in a good shape but some work need to be done. > However, the main concern is getting an understanding what features and > functionality have to be added to be flexible enough. Various toy > examples can be found at [3] ([1] includes an overview of cases). > > Discussion points: > - Use cases, feature requests, benchmarking > - Userspace programming model, code reuse (e.g. liburing) > - BPF-BPF and userspace-BPF synchronisation. There is > CQE based notification approach and plans (see design > notes), however need to discuss what else might be > needed. > - Do we need more contexts passed apart from user_data? > e.g. specifying a BPF map/array/etc fd io_uring requests? > - Userspace atomics and efficiency of userspace reads/writes. If > proved to be not performant enough there are potential ways to take > on it, e.g. inlining, having it in BPF ISA, and pre-verifying > userspace pointers. > > [1] https://lore.kernel.org/io-uring/a83f147b-ea9d-e693-a2e9-c6ce16659749@xxxxxxxxx/T/#m31d0a2ac6e2213f912a200f5e8d88bd74f81406b > [2] https://github.com/isilence/linux/tree/ebpf_v2 > [3] https://github.com/isilence/liburing/tree/ebpf_v2/examples/bpf > > > ----------------------------------------------------------------------- > Design notes: > > Instead of basing it on hooks it adds support of a new type of io_uring > requests as it gives a better control and let's to reuse internal > infrastructure. These requests run a new type of io_uring BPF programs > wired with a bunch of new helpers for submitting requests and dealing > with CQEs, are allowed to read/write userspace memory in virtue of a > recently added sleepable BPF feature. and also provided with a token > (generic io_uring token, aka user_data, specified at submission and > returned in an CQE), which may be used to pass a userspace pointer used > as a context. > > Besides running BPF programs, they are able to request waiting. > Currently it supports CQ waiting for a number of completions, but others > might be added and/or needed, e.g. futex and/or requeueing the current > BPF request onto an io_uring request/link being submitted. That hides > the overhead of creating BPF requests by keeping them alive and > invoking multiple times. > > Another big chunk solved is figuring out a good way of feeding CQEs > (potentially many) to a BPF program. The current approach > is to enable multiple completion queues (CQ), and specify for each > request to which one steer its CQE, so all the synchronisation > is in control of the userspace. For instance, there may be a separate > CQ per each in-flight BPF request, and they can work with their own > queues and send an CQE to the main CQ so notifying the userspace. > It also opens up a notification-like sync through CQE posting to > neighbours' CQs. > > -- Pavel Begunkov
