[PATCH v3 08/11] io_uring: wake up optimisations

Pavel Begunkov
2023-01-09T14:51:36Z

Flush completions is done either from the submit syscall or by the
task_work, both are in the context of the submitter task, and when it
goes for a single threaded rings like implied by ->task_complete, there
won't be any waiters on ->cq_wait but the master task. That means that
there can be no tasks sleeping on cq_wait while we run
__io_submit_flush_completions() and so waking up can be skipped.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
io_uring/io_uring.c | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index a6ed022c1356..0dabd0f3271f 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -621,6 +621,25 @@ static inline void __io_cq_unlock_post(struct io_ring_ctx *ctx)
io_cqring_wake(ctx);
}

+static inline void __io_cq_unlock_post_flush(struct io_ring_ctx *ctx)
+ __releases(ctx->completion_lock)
+{
+ io_commit_cqring(ctx);
+ __io_cq_unlock(ctx);
+ io_commit_cqring_flush(ctx);
+
+ /*
+ * As ->task_complete implies that the ring is single tasked, cq_wait
+ * may only be waited on by the current in io_cqring_wait(), but since
+ * it will re-check the wakeup conditions once we return we can safely
+ * skip waking it up.
+ */
+ if (!(ctx->flags & IORING_SETUP_DEFER_TASKRUN)) {
+ smp_mb();
+ __io_cqring_wake(ctx);
+ }
+}
+
void io_cq_unlock_post(struct io_ring_ctx *ctx)
__releases(ctx->completion_lock)
{
@@ -1480,7 +1499,7 @@ static void __io_submit_flush_completions(struct io_ring_ctx *ctx)
}
}
}
- __io_cq_unlock_post(ctx);
+ __io_cq_unlock_post_flush(ctx);

if (!wq_list_empty(&ctx->submit_state.compl_reqs)) {
io_free_batch_list(ctx, state->compl_reqs.first);
--
2.38.1