[PATCH] io_uring: account overflows even with IORING_SETUP_CQ_NODROP

[Jens Axboe <axboe@kernel.dk>]

It's useful for the application to know if the kernel had to dip into
using the backlog to prevent overflows. Let's keep on accounting any
overflow in cq_ring->overflow, even if we handled it correctly. As it's
impossible to get dropped events with IORING_SETUP_CQ_NODROP, overflow
with CQ_NODROP enabled simply provides a hint to the application that it
may reconsider using a bigger ring.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

Since this hasn't been released yet, we can tweak the behavior a bit. I
think it makes sense to still account the overflows, even if we handled
it correctly. If the application doesn't care, it simply doesn't need to
look at cq_ring->overflow if it is using CQ_NODROP. But it may care, as
it is less efficient than a suitably sized ring.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 94ec44caac00..aa3b6149dfe9 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -666,10 +666,10 @@ static void io_cqring_overflow(struct io_ring_ctx *ctx, struct io_kiocb *req,
 			       long res)
 	__must_hold(&ctx->completion_lock)
 {
-	if (!(ctx->flags & IORING_SETUP_CQ_NODROP)) {
-		WRITE_ONCE(ctx->rings->cq_overflow,
-				atomic_inc_return(&ctx->cached_cq_overflow));
-	} else {
+	WRITE_ONCE(ctx->rings->cq_overflow,
+			atomic_inc_return(&ctx->cached_cq_overflow));
+
+	if (ctx->flags & IORING_SETUP_CQ_NODROP) {
 		refcount_inc(&req->refs);
 		req->result = res;
 		list_add_tail(&req->list, &ctx->cq_overflow_list);

-- 
Jens Axboe