Commit 1cd15904 authored Apr 12, 2022 by Pavel Begunkov Committed by Jens Axboe Apr 24, 2022

io_uring: optimise submission left counting



Considering all inlining io_submit_sqe() is huge and usually ends up
calling some other functions.

We decrement @left in io_submit_sqes() just before calling
io_submit_sqe() and use it later after the call. Considering how huge
io_submit_sqe() is, there is not much hope @left will be treated
gracefully by compilers.

Decrement it after the call, not only it's easier on register spilling
and probably saves stack write/read, but also at least for x64 uses
CPU flags set by the dec instead of doing (read/write and tests).

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/807f9a276b54ee8ff4e42e2b78721484f1c71743.1649771823.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent 8e6971a8

fs/io_uring.c

+10 −10

Original line number	Diff line number	Diff line
		@@ -7877,17 +7877,17 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
		io_req_add_to_cache(req, ctx);
		break;
		}
		/* will complete beyond this point, count as submitted */
		left--;
		if (io_submit_sqe(ctx, req, sqe)) {

		/*
		* Continue submitting even for sqe failure if the
		* ring was setup with IORING_SETUP_SUBMIT_ALL
		*/
		if (!(ctx->flags & IORING_SETUP_SUBMIT_ALL))
		if (unlikely(io_submit_sqe(ctx, req, sqe)) &&
		!(ctx->flags & IORING_SETUP_SUBMIT_ALL)) {
		left--;
		break;
		}
		} while (left);
		} while (--left);

		if (unlikely(left)) {
		ret -= left;