io_uring: optimise kiocb layout

We want ->comp_list in the second cacheline, which is hotter comparing
to the 3rd. Swap the field with ->link, which is not as hot and
controlled by flags and so not accessed unless there is a link.

By the way add a couple of comments for io_kiocb fields.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9d9dde31f8f62279a5f48c575bbc27b8290edc0c.1633373302.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 5b3b4f8..ab07a08 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -863,19 +863,22 @@ struct io_kiocb {
        struct task_struct              *task;
        u64                             user_data;
 
-       struct io_kiocb                 *link;
        struct percpu_ref               *fixed_rsrc_refs;
 
-       /* used with ctx->iopoll_list with reads/writes */
+       /* used by request caches, completion batching and iopoll */
        struct io_wq_work_node          comp_list;
+       struct io_kiocb                 *link;
        struct io_task_work             io_task_work;
        /* for polled requests, i.e. IORING_OP_POLL_ADD and async armed poll */
        struct hlist_node               hash_node;
+       /* internal polling, see IORING_FEAT_FAST_POLL */
        struct async_poll               *apoll;
        /* store used ubuf, so we can prevent reloading */
        struct io_mapped_ubuf           *imu;
        struct io_wq_work               work;
+       /* custom credentials, valid IFF REQ_F_CREDS is set */
        const struct cred               *creds;
+       /* stores selected buf, valid IFF REQ_F_BUFFER_SELECTED is set */
        struct io_buffer                *kbuf;
 };