io_uring: batch task_work

Batching task work up is an important performance optimisation, as
task_work_add is expensive.

In order to keep the semantics replace the task_list with a fake node
while processing the old list, and then do a cmpxchg at the end to see if
there is more work.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220622134028.2013417-6-dylany@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 356000255211fbe2e8417bf8d1ea5f129ca56bf8..9d523fafacb72dd312c30b1aa262602005f137f9 100644 (file)
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -986,11 +986,11 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx, bool *locked)
        percpu_ref_put(&ctx->refs);
 }
 
-
 static void handle_tw_list(struct llist_node *node,
-                          struct io_ring_ctx **ctx, bool *locked)
+                          struct io_ring_ctx **ctx, bool *locked,
+                          struct llist_node *last)
 {
-       do {
+       while (node != last) {
                struct llist_node *next = node->next;
                struct io_kiocb *req = container_of(node, struct io_kiocb,
                                                    io_task_work.node);
@@ -1006,7 +1006,7 @@ static void handle_tw_list(struct llist_node *node,
                }
                req->io_task_work.func(req, locked);
                node = next;
-       } while (node);
+       }
 }
 
 /**
@@ -1045,11 +1045,15 @@ void tctx_task_work(struct callback_head *cb)
        struct io_ring_ctx *ctx = NULL;
        struct io_uring_task *tctx = container_of(cb, struct io_uring_task,
                                                  task_work);
-       struct llist_node *node = llist_del_all(&tctx->task_list);
-
-       if (node) {
-               handle_tw_list(node, &ctx, &uring_locked);
-               cond_resched();
+       struct llist_node fake = {};
+       struct llist_node *node = io_llist_xchg(&tctx->task_list, &fake);
+
+       handle_tw_list(node, &ctx, &uring_locked, NULL);
+       node = io_llist_cmpxchg(&tctx->task_list, &fake, NULL);
+       while (node != &fake) {
+               node = io_llist_xchg(&tctx->task_list, &fake);
+               handle_tw_list(node, &ctx, &uring_locked, &fake);
+               node = io_llist_cmpxchg(&tctx->task_list, &fake, NULL);
        }
 
        ctx_flush_and_put(ctx, &uring_locked);