io_uring: optimise deferred tw execution

We needed fake nodes in __io_run_local_work() and to avoid unecessary wake
ups while the task already running task_works, but we don't need them
anymore since wake ups are protected by cq_waiting, which is always
cleared by the time we're executing deferred task_work items.

Note that because of loose sync around cq_waiting clearing
io_req_local_work_add() may wake the task more than once, but that's
fine and should be rare to not hurt perf.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8839534891f0a2f1076e78554a31ea7e099f7de5.1673274244.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 3f92028a7ee86765ad30a2a7f23532a10c879dcc..6533617462037feaa685550a4dbbf0f84c332807 100644 (file)
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1322,17 +1322,16 @@ static void __cold io_move_task_work_from_local(struct io_ring_ctx *ctx)
 static int __io_run_local_work(struct io_ring_ctx *ctx, bool *locked)
 {
        struct llist_node *node;
-       struct llist_node fake;
-       struct llist_node *current_final = NULL;
+       unsigned int loops = 0;
        int ret = 0;
-       unsigned int loops = 1;
 
        if (WARN_ON_ONCE(ctx->submitter_task != current))
                return -EEXIST;
-
-       node = io_llist_xchg(&ctx->work_llist, &fake);
+       if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
+               atomic_andnot(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
 again:
-       while (node != current_final) {
+       node = io_llist_xchg(&ctx->work_llist, NULL);
+       while (node) {
                struct llist_node *next = node->next;
                struct io_kiocb *req = container_of(node, struct io_kiocb,
                                                    io_task_work.node);
@@ -1341,23 +1340,14 @@ again:
                ret++;
                node = next;
        }
+       loops++;
 
-       if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
-               atomic_andnot(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
-
-       node = io_llist_cmpxchg(&ctx->work_llist, &fake, NULL);
-       if (node != &fake) {
-               loops++;
-               current_final = &fake;
-               node = io_llist_xchg(&ctx->work_llist, &fake);
+       if (!llist_empty(&ctx->work_llist))
                goto again;
-       }
-
        if (*locked)
                io_submit_flush_completions(ctx);
        trace_io_uring_local_work_run(ctx, ret, loops);
        return ret;
-
 }
 
 static inline int io_run_local_work_locked(struct io_ring_ctx *ctx)