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

[Usama Arif <usama.arif@bytedance.com>]

On 03/02/2022 15:48, Pavel Begunkov wrote:
> 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.
> 

I guess this should be ok now from v3?

Thanks,
Usama
> if (ctx->ev_fd) {
>      rcu_read_lock();
>          ev_fd = rcu_deref(...);
>          ...
>          rcu_read_unlock();
> }
>