io_uring: optimise head checks in io_get_sqring()

A user may ask to submit more than there is in the ring, and then
io_uring will submit as much as it can. However, in the last iteration
it will allocate an io_kiocb and immediately free it. It could do
better and adjust @to_submit to what is in the ring.

And since the ring's head is already checked here, there is no need to
do it in the loop, spamming with smp_load_acquire()'s barriers

Signed-off-by: Pavel Begunkov <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 497ed61..3398f40 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4522,7 +4522,6 @@ static void io_commit_sqring(struct io_ring_ctx *ctx)
 static bool io_get_sqring(struct io_ring_ctx *ctx, struct io_kiocb *req,
                          const struct io_uring_sqe **sqe_ptr)
 {
-       struct io_rings *rings = ctx->rings;
        u32 *sq_array = ctx->sq_array;
        unsigned head;
 
@@ -4534,12 +4533,7 @@ static bool io_get_sqring(struct io_ring_ctx *ctx, struct io_kiocb *req,
         * 2) allows the kernel side to track the head on its own, even
         *    though the application is the one updating it.
         */
-       head = ctx->cached_sq_head;
-       /* make sure SQ entry isn't read before tail */
-       if (unlikely(head == smp_load_acquire(&rings->sq.tail)))
-               return false;
-
-       head = READ_ONCE(sq_array[head & ctx->sq_mask]);
+       head = READ_ONCE(sq_array[ctx->cached_sq_head & ctx->sq_mask]);
        if (likely(head < ctx->sq_entries)) {
                /*
                 * All io need record the previous position, if LINK vs DARIN,
@@ -4557,7 +4551,7 @@ static bool io_get_sqring(struct io_ring_ctx *ctx, struct io_kiocb *req,
        /* drop invalid entries */
        ctx->cached_sq_head++;
        ctx->cached_sq_dropped++;
-       WRITE_ONCE(rings->sq_dropped, ctx->cached_sq_dropped);
+       WRITE_ONCE(ctx->rings->sq_dropped, ctx->cached_sq_dropped);
        return false;
 }
 
@@ -4577,7 +4571,8 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
                        return -EBUSY;
        }
 
-       nr = min(nr, ctx->sq_entries);
+       /* make sure SQ entry isn't read before tail */
+       nr = min3(nr, ctx->sq_entries, io_sqring_entries(ctx));
 
        if (!percpu_ref_tryget_many(&ctx->refs, nr))
                return -EAGAIN;