io_uring: remove ->flush_cqes optimisation

It's not clear how widely used IOSQE_CQE_SKIP_SUCCESS is, and how often
->flush_cqes flag prevents from completion being flushed. Sometimes it's
high level of concurrency that enables it at least for one CQE, but
sometimes it doesn't save much because nobody waiting on the CQ.

Remove ->flush_cqes flag and the optimisation, it should benefit the
normal use case. Note, that there is no spurious eventfd problem with
that as checks for spuriousness were incorporated into
io_eventfd_signal().
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/692e81eeddccc096f449a7960365fa7b4a18f8e6.1655637157.git.asml.silence@gmail.com
[axboe: remove now dead state->flush_cqes variable]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring_types.h

@@ -148,7 +148,6 @@ struct io_submit_state {
 
 	bool			plug_started;
 	bool			need_plug;
-	bool			flush_cqes;
 	unsigned short		submit_nr;
 	struct blk_plug		plug;
 };

io_uring/io_uring.c

@@ -1250,7 +1250,6 @@ static void __io_submit_flush_completions(struct io_ring_ctx *ctx)
 	struct io_wq_work_node *node, *prev;
 	struct io_submit_state *state = &ctx->submit_state;
 
-	if (state->flush_cqes) {
 	spin_lock(&ctx->completion_lock);
 	wq_list_for_each(node, prev, &state->compl_reqs) {
 		struct io_kiocb *req = container_of(node, struct io_kiocb,
@@ -1263,8 +1262,6 @@ static void __io_submit_flush_completions(struct io_ring_ctx *ctx)
 	io_commit_cqring(ctx);
 	spin_unlock(&ctx->completion_lock);
 	io_cqring_ev_posted(ctx);
-		state->flush_cqes = false;
-	}
 
 	io_free_batch_list(ctx, state->compl_reqs.first);
 	INIT_WQ_LIST(&state->compl_reqs);

io_uring/io_uring.h

@@ -219,8 +219,6 @@ static inline void io_req_add_compl_list(struct io_kiocb *req)
 {
 	struct io_submit_state *state = &req->ctx->submit_state;
 
-	if (!(req->flags & REQ_F_CQE_SKIP))
-		state->flush_cqes = true;
 	wq_list_add_tail(&req->comp_list, &state->compl_reqs);
 }