[PATCH v5 3/4] io_uring: avoid ring quiesce for IORING_REGISTER_EVENTFD_ASYNC

Usama Arif
2022-02-03T23:34:49Z

This is done using the RCU data structure (io_ev_fd). eventfd_async
is moved from io_ring_ctx to io_ev_fd which is RCU protected hence
avoiding ring quiesce which is much more expensive than an RCU lock.
io_should_trigger_evfd is already under rcu_read_lock so there is
no extra RCU read-side critical section needed.

Signed-off-by: Usama Arif <usama.arif@bytedance.com>
---
fs/io_uring.c | 22 ++++++++++++----------
1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 51602bddb9a8..5ae51ea12f0f 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -329,6 +329,7 @@ struct io_submit_state {
struct io_ev_fd {
struct eventfd_ctx *cq_ev_fd;
struct io_ring_ctx *ctx;
+ unsigned int eventfd_async: 1;
struct rcu_head rcu;
bool unregistering;
};
@@ -342,7 +343,6 @@ struct io_ring_ctx {
unsigned int flags;
unsigned int compat: 1;
unsigned int drain_next: 1;
- unsigned int eventfd_async: 1;
unsigned int restricted: 1;
unsigned int off_timeout_used: 1;
unsigned int drain_active: 1;
@@ -1756,7 +1756,7 @@ static void io_eventfd_signal(struct io_ring_ctx *ctx)
if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
goto out;

- if (!ctx->eventfd_async || io_wq_current_is_worker())
+ if (!ev_fd->eventfd_async || io_wq_current_is_worker())
eventfd_signal(ev_fd->cq_ev_fd, 1);

out:
@@ -9377,7 +9377,8 @@ static int __io_sqe_buffers_update(struct io_ring_ctx *ctx,
return done ? done : err;
}

-static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg)
+static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg,
+ unsigned int eventfd_async)
{
struct io_ev_fd *ev_fd;
__s32 __user *fds = arg;
@@ -9416,6 +9417,7 @@ static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg)
}
ev_fd->ctx = ctx;
ev_fd->unregistering = false;
+ ev_fd->eventfd_async = eventfd_async;

rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
ret = 0;
@@ -11029,6 +11031,7 @@ static bool io_register_op_must_quiesce(int op)
case IORING_UNREGISTER_FILES:
case IORING_REGISTER_FILES_UPDATE:
case IORING_REGISTER_EVENTFD:
+ case IORING_REGISTER_EVENTFD_ASYNC:
case IORING_UNREGISTER_EVENTFD:
case IORING_REGISTER_PROBE:
case IORING_REGISTER_PERSONALITY:
@@ -11129,17 +11132,16 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
ret = io_register_files_update(ctx, arg, nr_args);
break;
case IORING_REGISTER_EVENTFD:
- case IORING_REGISTER_EVENTFD_ASYNC:
ret = -EINVAL;
if (nr_args != 1)
break;
- ret = io_eventfd_register(ctx, arg);
- if (ret)
+ ret = io_eventfd_register(ctx, arg, 0);
+ break;
+ case IORING_REGISTER_EVENTFD_ASYNC:
+ ret = -EINVAL;
+ if (nr_args != 1)
break;
- if (opcode == IORING_REGISTER_EVENTFD_ASYNC)
- ctx->eventfd_async = 1;
- else
- ctx->eventfd_async = 0;
+ ret = io_eventfd_register(ctx, arg, 1);
break;
case IORING_UNREGISTER_EVENTFD:
ret = -EINVAL;
--
2.25.1