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>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 3e84963b14ff7cdc36b153c9084ffab056d5ed6c..c3cadb36326bca43e00f1903b16ecb270dc767b8 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -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))
-                               break;
+
+               /*
+                * Continue submitting even for sqe failure if the
+                * ring was setup with 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;