On 6/7/21 7:51 PM, Victor Stewart wrote: > On Sat, Jun 5, 2021 at 5:09 AM Pavel Begunkov <asml.silence@xxxxxxxxx> 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 > > hi Pavel, > > coincidentally i'm tossing around in my mind at the moment an idea for > offloading > the PING/PONG of a QUIC server/client into the kernel via eBPF. > > problem being, being that QUIC is userspace run transport and that NAT-ed UDP > mappings can't be expected to stay open longer than 30 seconds, QUIC > applications > bare a large cost of context switching wake-up to conduct connection lifetime > maintenance... especially when managing a large number of mostly idle long lived > connections. so offloading this maintenance service into the kernel > would be a great > efficiency boon. > > the main impediment is that access to the kernel crypto libraries > isn't currently possible > from eBPF. that said, connection wide crypto offload into the NIC is a > frequently mentioned > subject in QUIC circles, so one could argue better to allocate the > time to NIC crypto offload > and then simply conduct this PING/PONG offload in plain text. > > CQEs would provide a great way for the offloaded service to be able to > wake up the > application when it's input is required. Interesting, want to try out the idea? All pointers are here and/or in the patchset's cv, but if anything is not clear, inconvenient, lacks needed functionality, etc. let me know > anyway food for thought. > > Victor > >> - 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 -- Pavel Begunkov
