Re: [PATCH for-5.13] io_uring: maintain drain requests' logic

[Hao Xu <haoxu@linux.alibaba.com>]

在 2021/4/1 上午6:06, Pavel Begunkov 写道:
> 
> 
> On 31/03/2021 10:01, Hao Xu wrote:
>> Now that we have multishot poll requests, one sqe can emit multiple
>> cqes. given below example:
>>      sqe0(multishot poll)-->sqe1-->sqe2(drain req)
>> sqe2 is designed to issue after sqe0 and sqe1 completed, but since sqe0
>> is a multishot poll request, sqe2 may be issued after sqe0's event
>> triggered twice before sqe1 completed. This isn't what users leverage
>> drain requests for.
>> Here a simple solution is to ignore all multishot poll cqes, which means
>> drain requests  won't wait those request to be done.
>>
>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>> ---
>>   fs/io_uring.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 513096759445..cd6d44cf5940 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -455,6 +455,7 @@ struct io_ring_ctx {
>>   	struct callback_head		*exit_task_work;
>>   
>>   	struct wait_queue_head		hash_wait;
>> +	unsigned                        multishot_cqes;
>>   
>>   	/* Keep this last, we don't need it for the fast path */
>>   	struct work_struct		exit_work;
>> @@ -1181,8 +1182,8 @@ static bool req_need_defer(struct io_kiocb *req, u32 seq)
>>   	if (unlikely(req->flags & REQ_F_IO_DRAIN)) {
>>   		struct io_ring_ctx *ctx = req->ctx;
>>   
>> -		return seq != ctx->cached_cq_tail
>> -				+ READ_ONCE(ctx->cached_cq_overflow);
>> +		return seq + ctx->multishot_cqes != ctx->cached_cq_tail
>> +			+ READ_ONCE(ctx->cached_cq_overflow);
>>   	}
>>   
>>   	return false;
>> @@ -4897,6 +4898,7 @@ static bool io_poll_complete(struct io_kiocb *req, __poll_t mask, int error)
>>   {
>>   	struct io_ring_ctx *ctx = req->ctx;
>>   	unsigned flags = IORING_CQE_F_MORE;
>> +	bool multishot_poll = !(req->poll.events & EPOLLONESHOT);
>>   
>>   	if (!error && req->poll.canceled) {
>>   		error = -ECANCELED;
>> @@ -4911,6 +4913,9 @@ static bool io_poll_complete(struct io_kiocb *req, __poll_t mask, int error)
>>   		req->poll.done = true;
>>   		flags = 0;
>>   	}
>> +	if (multishot_poll)
>> +		ctx->multishot_cqes++;
>> +
> 
> We need to make sure we do that only for a non-final complete, i.e.
> not killing request, otherwise it'll double account the last one.
Hi Pavel, I saw a killing request like iopoll_remove or async_cancel 
call io_cqring_fill_event() to create an ECANCELED cqe for the original 
poll request. So there could be cases like(even for single poll request):
   (1). add poll --> cancel poll, an ECANCELED cqe.
                                                   1sqe:1cqe   all good
   (2). add poll --> trigger event(queued to task_work) --> cancel poll, 
            an ECANCELED cqe --> task_work runs, another ECANCELED cqe.
                                                   1sqe:2cqes
I suggest we shall only emit one ECANCELED cqe.
Currently I only account cqe through io_poll_complete(), so ECANCELED 
cqe from io_poll_remove or async_cancel etc are not counted in.
> E.g. is failed __io_cqring_fill_event() in io_poll_complete() fine?
> Other places?
a failed __io_cqring_fill_event() doesn't produce a cqe but increment 
ctx->cached_cq_overflow, as long as a cqe is produced or 
cached_cq_overflow is +=1, it is ok.
> 
> Btw, we can use some tests :)
I'll do more tests.
> 
> 
>>   	io_commit_cqring(ctx);
>>   	return !(flags & IORING_CQE_F_MORE);
>>   }
>>
>