[PATCH v3 11/11] io_uring: optimise deferred tw execution

Pavel Begunkov
2023-01-09T14:51:39Z

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>
---
io_uring/io_uring.c | 24 +++++++-----------------
1 file changed, 7 insertions(+), 17 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 9c95ceb1a9f2..1dd0fc0412c8 100644
--- 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 @@ static int __io_run_local_work(struct io_ring_ctx *ctx, bool *locked)
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)
--
2.38.1