Re: [PATCH] io_uring: account overflows even with IORING_SETUP_CQ_NODROP

[Jens Axboe <axboe@kernel.dk>]

On 11/9/19 2:59 AM, Pavel Begunkov wrote:
> On 08/11/2019 18:51, Jens Axboe wrote:
>> It's useful for the application to know if the kernel had to dip into
>> using the backlog to prevent overflows. Let's keep on accounting any
>> overflow in cq_ring->overflow, even if we handled it correctly. As it's
>> impossible to get dropped events with IORING_SETUP_CQ_NODROP, overflow
>> with CQ_NODROP enabled simply provides a hint to the application that it
>> may reconsider using a bigger ring.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> ---
>>
>> Since this hasn't been released yet, we can tweak the behavior a bit. I
>> think it makes sense to still account the overflows, even if we handled
>> it correctly. If the application doesn't care, it simply doesn't need to
>> look at cq_ring->overflow if it is using CQ_NODROP. But it may care, as
>> it is less efficient than a suitably sized ring.
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 94ec44caac00..aa3b6149dfe9 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -666,10 +666,10 @@ static void io_cqring_overflow(struct io_ring_ctx *ctx, struct io_kiocb *req,
>>   			       long res)
>>   	__must_hold(&ctx->completion_lock)
>>   {
>> -	if (!(ctx->flags & IORING_SETUP_CQ_NODROP)) {
>> -		WRITE_ONCE(ctx->rings->cq_overflow,
>> -				atomic_inc_return(&ctx->cached_cq_overflow));
>> -	} else {
>> +	WRITE_ONCE(ctx->rings->cq_overflow,
>> +			atomic_inc_return(&ctx->cached_cq_overflow));
>> +
>> +	if (ctx->flags & IORING_SETUP_CQ_NODROP) {
> 
> We used cq_overflow to fix __io_sequence_defer().
> This breaks the assumption:
> cached_cq_tail + cached_cq_overflow ==
> 	total number of handled completions
> 
> First, we account overflow, and then add it to cq_ring
> (i.e. cached_cq_tail++) in io_cqring_overflow_flush()

Yeah, I realized that later, and also the fact that it makes it awkward
to use in a program because of that. I dropped this patch, thanks
for taking a look at it!

-- 
Jens Axboe