linux-kernel - Re: [PATCH 1/2] io_uring: avoid ring quiesce while registering/unregistering eventfd

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Date:   Thu, 3 Feb 2022 15:48:35 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Usama Arif <usama.arif@...edance.com>, io-uring@...r.kernel.org,
        axboe@...nel.dk, linux-kernel@...r.kernel.org
Cc:     fam.zheng@...edance.com
Subject: Re: [PATCH 1/2] io_uring: avoid ring quiesce while
 registering/unregistering eventfd

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@...edance.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