Re: [RFC] io_uring: avoid ring quiesce while registering/unregistering eventfd

[Jens Axboe <axboe@kernel.dk>]

On 2/2/22 8:59 AM, Usama Arif wrote:
> Acquire completion_lock at the start of __io_uring_register before
> registering/unregistering eventfd and release it at the end. Hence
> all calls to io_cqring_ev_posted which adds to the eventfd counter
> will finish before acquiring the spin_lock in io_uring_register, and
> all new calls will wait till the eventfd is registered. This avoids
> ring quiesce which is much more expensive than acquiring the spin_lock.
> 
> On the system tested with this patch, io_uring_reigster with
> IORING_REGISTER_EVENTFD takes less than 1ms, compared to 15ms before.

This seems like optimizing for the wrong thing, so I've got a few
questions. Are you doing a lot of eventfd registrations (and unregister)
in your workload? Or is it just the initial pain of registering one? In
talking to Pavel, he suggested that RCU might be a good use case here,
and I think so too. That would still remove the need to quiesce, and the
posted side just needs a fairly cheap rcu read lock/unlock around it.

-- 
Jens Axboe