Currently we drop completion events, if the CQ ring is full. That's fine
for requests with bounded completion times, but it may make it harder to
use io_uring with networked IO where request completion times are
generally unbounded. Or with POLL, for example, which is also unbounded.
This patch adds IORING_SETUP_CQ_NODROP, which changes the behavior a bit
for CQ ring overflows. First of all, it doesn't overflow the ring, it
simply stores a backlog of completions that we weren't able to put into
the CQ ring. To prevent the backlog from growing indefinitely, if the
backlog is non-empty, we apply back pressure on IO submissions. Any
attempt to submit new IO with a non-empty backlog will get an -EBUSY
return from the kernel. This is a signal to the application that it has
backlogged CQ events, and that it must reap those before being allowed
to submit more IO.
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
fs/io_uring.c | 103 ++++++++++++++++++++++++++++------
include/uapi/linux/io_uring.h | 1 +
2 files changed, 87 insertions(+), 17 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index f69d9794ce17..ff0f79a57f7b 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -207,6 +207,7 @@ struct io_ring_ctx {
struct list_head defer_list;
struct list_head timeout_list;
+ struct list_head cq_overflow_list;
wait_queue_head_t inflight_wait;
} ____cacheline_aligned_in_smp;
@@ -414,6 +415,7 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
ctx->flags = p->flags;
init_waitqueue_head(&ctx->cq_wait);
+ INIT_LIST_HEAD(&ctx->cq_overflow_list);
init_completion(&ctx->ctx_done);
init_completion(&ctx->sqo_thread_started);
mutex_init(&ctx->uring_lock);
@@ -588,6 +590,72 @@ static struct io_uring_cqe *io_get_cqring(struct io_ring_ctx *ctx)
return &rings->cqes[tail & ctx->cq_mask];
}
+static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
+{
+ if (waitqueue_active(&ctx->wait))
+ wake_up(&ctx->wait);
+ if (waitqueue_active(&ctx->sqo_wait))
+ wake_up(&ctx->sqo_wait);
+ if (ctx->cq_ev_fd)
+ eventfd_signal(ctx->cq_ev_fd, 1);
+}
+
+static void io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
+{
+ struct io_rings *rings = ctx->rings;
+ struct io_uring_cqe *cqe;
+ struct io_kiocb *req;
+ unsigned long flags;
+ LIST_HEAD(list);
+
+ if (list_empty_careful(&ctx->cq_overflow_list))
+ return;
+ if (ctx->cached_cq_tail - READ_ONCE(rings->cq.head) ==
+ rings->cq_ring_entries)
+ return;
+
+ spin_lock_irqsave(&ctx->completion_lock, flags);
+
+ while (!list_empty(&ctx->cq_overflow_list)) {
+ cqe = io_get_cqring(ctx);
+ if (!cqe && !force)
+ break;
+
+ req = list_first_entry(&ctx->cq_overflow_list, struct io_kiocb,
+ list);
+ list_move(&req->list, &list);
+ if (cqe) {
+ WRITE_ONCE(cqe->user_data, req->user_data);
+ WRITE_ONCE(cqe->res, req->result);
+ WRITE_ONCE(cqe->flags, 0);
+ }
+ }
+
+ io_commit_cqring(ctx);
+ spin_unlock_irqrestore(&ctx->completion_lock, flags);
+ io_cqring_ev_posted(ctx);
+
+ while (!list_empty(&list)) {
+ req = list_first_entry(&list, struct io_kiocb, list);
+ list_del(&req->list);
+ io_put_req(req, NULL);
+ }
+}
+
+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 {
+ refcount_inc(&req->refs);
+ req->result = res;
+ list_add_tail(&req->list, &ctx->cq_overflow_list);
+ }
+}
+
static void io_cqring_fill_event(struct io_kiocb *req, long res)
{
struct io_ring_ctx *ctx = req->ctx;
@@ -601,26 +669,15 @@ static void io_cqring_fill_event(struct io_kiocb *req, long res)
* the ring.
*/
cqe = io_get_cqring(ctx);
- if (cqe) {
+ if (likely(cqe)) {
WRITE_ONCE(cqe->user_data, req->user_data);
WRITE_ONCE(cqe->res, res);
WRITE_ONCE(cqe->flags, 0);
} else {
- WRITE_ONCE(ctx->rings->cq_overflow,
- atomic_inc_return(&ctx->cached_cq_overflow));
+ io_cqring_overflow(ctx, req, res);
}
}
-static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
-{
- if (waitqueue_active(&ctx->wait))
- wake_up(&ctx->wait);
- if (waitqueue_active(&ctx->sqo_wait))
- wake_up(&ctx->sqo_wait);
- if (ctx->cq_ev_fd)
- eventfd_signal(ctx->cq_ev_fd, 1);
-}
-
static void io_cqring_add_event(struct io_kiocb *req, long res)
{
struct io_ring_ctx *ctx = req->ctx;
@@ -877,6 +934,9 @@ static unsigned io_cqring_events(struct io_ring_ctx *ctx)
{
struct io_rings *rings = ctx->rings;
+ if (ctx->flags & IORING_SETUP_CQ_NODROP)
+ io_cqring_overflow_flush(ctx, false);
+
/* See comment at the top of this file */
smp_rmb();
return READ_ONCE(rings->cq.tail) - READ_ONCE(rings->cq.head);
@@ -2876,6 +2936,10 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
int i, submitted = 0;
bool mm_fault = false;
+ if ((ctx->flags & IORING_SETUP_CQ_NODROP) &&
+ !list_empty(&ctx->cq_overflow_list))
+ return -EBUSY;
+
if (nr > IO_PLUG_THRESHOLD) {
io_submit_state_start(&state, ctx, nr);
statep = &state;
@@ -2967,6 +3031,7 @@ static int io_sq_thread(void *data)
timeout = inflight = 0;
while (!kthread_should_park()) {
unsigned int to_submit;
+ int ret;
if (inflight) {
unsigned nr_events = 0;
@@ -3051,8 +3116,9 @@ static int io_sq_thread(void *data)
}
to_submit = min(to_submit, ctx->sq_entries);
- inflight += io_submit_sqes(ctx, to_submit, NULL, -1, &cur_mm,
- true);
+ ret = io_submit_sqes(ctx, to_submit, NULL, -1, &cur_mm, true);
+ if (ret > 0)
+ inflight += ret;
}
set_fs(old_fs);
@@ -4116,8 +4182,10 @@ static int io_uring_flush(struct file *file, void *data)
struct io_ring_ctx *ctx = file->private_data;
io_uring_cancel_files(ctx, data);
- if (fatal_signal_pending(current) || (current->flags & PF_EXITING))
+ if (fatal_signal_pending(current) || (current->flags & PF_EXITING)) {
+ io_cqring_overflow_flush(ctx, true);
io_wq_cancel_all(ctx->io_wq);
+ }
return 0;
}
@@ -4418,7 +4486,8 @@ static long io_uring_setup(u32 entries, struct io_uring_params __user *params)
}
if (p.flags & ~(IORING_SETUP_IOPOLL | IORING_SETUP_SQPOLL |
- IORING_SETUP_SQ_AFF | IORING_SETUP_CQSIZE))
+ IORING_SETUP_SQ_AFF | IORING_SETUP_CQSIZE |
+ IORING_SETUP_CQ_NODROP))
return -EINVAL;
ret = io_uring_create(entries, &p);
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index f1a118b01d18..3d8517eb376e 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -56,6 +56,7 @@ struct io_uring_sqe {
#define IORING_SETUP_SQPOLL (1U << 1) /* SQ poll thread */
#define IORING_SETUP_SQ_AFF (1U << 2) /* sq_thread_cpu is valid */
#define IORING_SETUP_CQSIZE (1U << 3) /* app defines CQ size */
+#define IORING_SETUP_CQ_NODROP (1U << 4) /* no CQ drops */
#define IORING_OP_NOP 0
#define IORING_OP_READV 1
--
2.24.0
