On 2/3/22 15:11, Usama Arif wrote:
This is done by creating a new RCU data structure (io_ev_fd) as part of
io_ring_ctx that holds the eventfd_ctx.
The function io_eventfd_signal is executed under rcu_read_lock with a
single rcu_dereference to io_ev_fd so that if another thread unregisters
the eventfd while io_eventfd_signal is still being executed, the
eventfd_signal for which io_eventfd_signal was called completes
successfully.
The process of registering/unregistering eventfd is done under a lock
so multiple threads don't enter a race condition while
registering/unregistering eventfd.
With the above approach ring quiesce can be avoided which is much more
expensive then using RCU lock. On the system tested, io_uring_reigster with
IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared to 15ms
before with ring quiesce.
Signed-off-by: Usama Arif <usama.arif@xxxxxxxxxxxxx>
---
fs/io_uring.c | 103 +++++++++++++++++++++++++++++++++++++++-----------
1 file changed, 80 insertions(+), 23 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 2e04f718319d..f07cfbb387a6 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -326,6 +326,12 @@ struct io_submit_state {
struct blk_plug plug;
};
-static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
+static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx, struct io_ev_fd *ev_fd)
{
- if (likely(!ctx->cq_ev_fd))
+ if (likely(!ev_fd))
return false;
if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
return false;
return !ctx->eventfd_async || io_wq_current_is_worker();
}
+static void io_eventfd_signal(struct io_ring_ctx *ctx)
+{
+ struct io_ev_fd *ev_fd;
+
+ rcu_read_lock();
Please always think about the fast path, which is not set eventfd.
We don't want extra overhead here.
if (ctx->ev_fd) {
rcu_read_lock();
ev_fd = rcu_deref(...);
...
rcu_read_unlock();
}
--
Pavel Begunkov
