io_uring: account overflows even with IORING_SETUP_CQ_NODROP

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 <[email protected]>

---

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.

fs/io_uring.c +0 -0

index 94ec44caac00..aa3b6149dfe9 100644
@@ -666,10 +666,10 @@ static void io_cqring_overflow(struct io_ring_ctx *ctx, struct io_kiocb *req,
 			       long res)

fs/io_uring.c +0 -0

 		refcount_inc(&req->refs);
 		req->result = res;
 		list_add_tail(&req->list, &ctx->cq_overflow_list);

Quoted 23 lines · Jens Axboe

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 <[email protected]>

---

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.

fs/io_uring.c +0 -0

index 94ec44caac00..aa3b6149dfe9 100644
@@ -666,10 +666,10 @@ static void io_cqring_overflow(struct io_ring_ctx *ctx, struct io_kiocb *req,
  			       long res)

fs/io_uring.c static void io_cqring_overflow(struct io_ring_ctx *ctx, struct io_kiocb *req, · line 672

Quoted 10 lines

fs/io_uring.c +4 -4

  	__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) {

We used cq_overflow to fix __io_sequence_defer().
This breaks the assumption:
cached_cq_tail + cached_cq_overflow ==
total number of handled completions

First, we account overflow, and then add it to cq_ring (i.e. cached_cq_tail++) in io_cqring_overflow_flush()