Merge tag 'io_uring-futex-2023-10-30' of git://git.kernel.dk/linux

Pull io_uring futex support from Jens Axboe: "This adds support for using futexes through io_uring - first futex wake and wait, and then the vectored variant of waiting, futex waitv. For both wait/wake/waitv, we support the bitset variant, as the 'normal' variants can be easily implemented on top of that. PI and requeue are not supported through io_uring, just the above mentioned parts. This may change in the future, but in the spirit of keeping this small (and based on what people have been asking for), this is what we currently have. Wake support is pretty straight forward, most of the thought has gone into the wait side to avoid needing to offload wait operations to a blocking context. Instead, we rely on the usual callbacks to retry and post a completion event, when appropriate. As far as I can recall, the first request for futex support with io_uring came from Andres Freund, working on postgres. His aio rework of postgres was one of the early adopters of io_uring, and futex support was a natural extension for that. This is relevant from both a usability point of view, as well as for effiency and performance. In Andres's words, for the former: Futex wait support in io_uring makes it a lot easier to avoid deadlocks in concurrent programs that have their own buffer pool: Obviously pages in the application buffer pool have to be locked during IO. If the initiator of IO A needs to wait for a held lock B, the holder of lock B might wait for the IO A to complete. The ability to wait for a lock and IO completions at the same time provides an efficient way to avoid such deadlocks and in terms of effiency, even without unlocking the full potential yet, Andres says: Futex wake support in io_uring is useful because it allows for more efficient directed wakeups. For some "locks" postgres has queues implemented in userspace, with wakeup logic that cannot easily be implemented with FUTEX_WAKE_BITSET on a single "futex word" (imagine waiting for journal flushes to have completed up to a certain point). Thus a "lock release" sometimes need to wake up many processes in a row. A quick-and-dirty conversion to doing these wakeups via io_uring lead to a 3% throughput increase, with 12% fewer context switches, albeit in a fairly extreme workload" * tag 'io_uring-futex-2023-10-30' of git://git.kernel.dk/linux: io_uring: add support for vectored futex waits futex: make the vectored futex operations available futex: make futex_parse_waitv() available as a helper futex: add wake_data to struct futex_q io_uring: add support for futex wake and wait futex: abstract out a __futex_wake_mark() helper futex: factor out the futex wake handling futex: move FUTEX2_VALID_MASK to futex.h

LTLinus Torvalds committed 3 years ago