Messages in this thread | | | Date | Thu, 3 Feb 2022 15:48:35 +0000 | Subject | Re: [PATCH 1/2] io_uring: avoid ring quiesce while registering/unregistering eventfd | From | Pavel Begunkov <> |
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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@bytedance.com> > --- > 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
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