From 49f1c68e048f1706b71c8255faf8110113d1cc48 Mon Sep 17 00:00:00 2001 From: Pavel Begunkov Date: Thu, 23 Jun 2022 14:24:49 +0100 Subject: io_uring: optimise submission side poll_refs The final poll_refs put in __io_arm_poll_handler() takes quite some cycles. When we're arming from the original task context task_work won't be run, so in this case we can assume that we won't race with task_works and so not take the initial ownership ref. One caveat is that after arming a poll we may race with it, so we have to add a bunch of io_poll_get_ownership() hidden inside of io_poll_can_finish_inline() whenever we want to complete arming inline. For the same reason we can't just set REQ_F_DOUBLE_POLL in __io_queue_proc() and so need to sync with the first poll entry by taking its wq head lock. Signed-off-by: Pavel Begunkov Link: https://lore.kernel.org/r/8825315d7f5e182ac1578a031e546f79b1c97d01.1655990418.git.asml.silence@gmail.com Signed-off-by: Jens Axboe --- io_uring/poll.c | 88 +++++++++++++++++++++++++++++++++++++++++++-------------- 1 file changed, 67 insertions(+), 21 deletions(-) diff --git a/io_uring/poll.c b/io_uring/poll.c index 3f3ae3b1505f..eba767594dee 100644 --- a/io_uring/poll.c +++ b/io_uring/poll.c @@ -34,6 +34,7 @@ struct io_poll_table { struct io_kiocb *req; int nr_entries; int error; + bool owning; /* output value, set only if arm poll returns >0 */ __poll_t result_mask; }; @@ -374,6 +375,27 @@ static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync, return 1; } +static void io_poll_double_prepare(struct io_kiocb *req) +{ + struct wait_queue_head *head; + struct io_poll *poll = io_poll_get_single(req); + + /* head is RCU protected, see io_poll_remove_entries() comments */ + rcu_read_lock(); + head = smp_load_acquire(&poll->head); + if (head) { + /* + * poll arm may not hold ownership and so race with + * io_poll_wake() by modifying req->flags. There is only one + * poll entry queued, serialise with it by taking its head lock. + */ + spin_lock_irq(&head->lock); + req->flags |= REQ_F_DOUBLE_POLL; + spin_unlock_irq(&head->lock); + } + rcu_read_unlock(); +} + static void __io_queue_proc(struct io_poll *poll, struct io_poll_table *pt, struct wait_queue_head *head, struct io_poll **poll_ptr) @@ -405,16 +427,19 @@ static void __io_queue_proc(struct io_poll *poll, struct io_poll_table *pt, pt->error = -ENOMEM; return; } + + io_poll_double_prepare(req); /* mark as double wq entry */ wqe_private |= IO_WQE_F_DOUBLE; - req->flags |= REQ_F_DOUBLE_POLL; io_init_poll_iocb(poll, first->events, first->wait.func); *poll_ptr = poll; if (req->opcode == IORING_OP_POLL_ADD) req->flags |= REQ_F_ASYNC_DATA; + } else { + /* fine to modify, there is no poll queued to race with us */ + req->flags |= REQ_F_SINGLE_POLL; } - req->flags |= REQ_F_SINGLE_POLL; pt->nr_entries++; poll->head = head; poll->wait.private = (void *) wqe_private; @@ -435,6 +460,12 @@ static void io_poll_queue_proc(struct file *file, struct wait_queue_head *head, (struct io_poll **) &pt->req->async_data); } +static bool io_poll_can_finish_inline(struct io_kiocb *req, + struct io_poll_table *pt) +{ + return pt->owning || io_poll_get_ownership(req); +} + /* * Returns 0 when it's handed over for polling. The caller owns the requests if * it returns non-zero, but otherwise should not touch it. Negative values @@ -443,7 +474,8 @@ static void io_poll_queue_proc(struct file *file, struct wait_queue_head *head, */ static int __io_arm_poll_handler(struct io_kiocb *req, struct io_poll *poll, - struct io_poll_table *ipt, __poll_t mask) + struct io_poll_table *ipt, __poll_t mask, + unsigned issue_flags) { struct io_ring_ctx *ctx = req->ctx; int v; @@ -452,34 +484,45 @@ static int __io_arm_poll_handler(struct io_kiocb *req, req->work.cancel_seq = atomic_read(&ctx->cancel_seq); io_init_poll_iocb(poll, mask, io_poll_wake); poll->file = req->file; - req->apoll_events = poll->events; ipt->pt._key = mask; ipt->req = req; ipt->error = 0; ipt->nr_entries = 0; - /* - * Take the ownership to delay any tw execution up until we're done - * with poll arming. see io_poll_get_ownership(). + * Polling is either completed here or via task_work, so if we're in the + * task context we're naturally serialised with tw by merit of running + * the same task. When it's io-wq, take the ownership to prevent tw + * from running. However, when we're in the task context, skip taking + * it as an optimisation. + * + * Note: even though the request won't be completed/freed, without + * ownership we still can race with io_poll_wake(). + * io_poll_can_finish_inline() tries to deal with that. */ - atomic_set(&req->poll_refs, 1); + ipt->owning = issue_flags & IO_URING_F_UNLOCKED; + + atomic_set(&req->poll_refs, (int)ipt->owning); mask = vfs_poll(req->file, &ipt->pt) & poll->events; if (unlikely(ipt->error || !ipt->nr_entries)) { io_poll_remove_entries(req); - if (mask && (poll->events & EPOLLET)) { + if (!io_poll_can_finish_inline(req, ipt)) { + io_poll_mark_cancelled(req); + return 0; + } else if (mask && (poll->events & EPOLLET)) { ipt->result_mask = mask; return 1; - } else { - return ipt->error ?: -EINVAL; } + return ipt->error ?: -EINVAL; } if (mask && ((poll->events & (EPOLLET|EPOLLONESHOT)) == (EPOLLET|EPOLLONESHOT))) { + if (!io_poll_can_finish_inline(req, ipt)) + return 0; io_poll_remove_entries(req); ipt->result_mask = mask; /* no one else has access to the req, forget about the ref */ @@ -491,18 +534,21 @@ static int __io_arm_poll_handler(struct io_kiocb *req, else io_poll_req_insert(req); - if (mask && (poll->events & EPOLLET)) { + if (mask && (poll->events & EPOLLET) && + io_poll_can_finish_inline(req, ipt)) { __io_poll_execute(req, mask); return 0; } - /* - * Release ownership. If someone tried to queue a tw while it was - * locked, kick it off for them. - */ - v = atomic_dec_return(&req->poll_refs); - if (unlikely(v & IO_POLL_REF_MASK)) - __io_poll_execute(req, 0); + if (ipt->owning) { + /* + * Release ownership. If someone tried to queue a tw while it was + * locked, kick it off for them. + */ + v = atomic_dec_return(&req->poll_refs); + if (unlikely(v & IO_POLL_REF_MASK)) + __io_poll_execute(req, 0); + } return 0; } @@ -585,7 +631,7 @@ int io_arm_poll_handler(struct io_kiocb *req, unsigned issue_flags) io_kbuf_recycle(req, issue_flags); - ret = __io_arm_poll_handler(req, &apoll->poll, &ipt, mask); + ret = __io_arm_poll_handler(req, &apoll->poll, &ipt, mask, issue_flags); if (ret) return ret > 0 ? IO_APOLL_READY : IO_APOLL_ABORTED; trace_io_uring_poll_arm(req, mask, apoll->poll.events); @@ -817,7 +863,7 @@ int io_poll_add(struct io_kiocb *req, unsigned int issue_flags) else req->flags &= ~REQ_F_HASH_LOCKED; - ret = __io_arm_poll_handler(req, poll, &ipt, poll->events); + ret = __io_arm_poll_handler(req, poll, &ipt, poll->events, issue_flags); if (ret > 0) { io_req_set_res(req, ipt.result_mask, 0); return IOU_OK; -- cgit v1.2.3