Re: [PATCH for-next 5/7] io_uring: post msg_ring CQE in task context

[Jens Axboe <axboe@kernel.dk>]

On 12/5/22 8:12?AM, Dylan Yudaken wrote:
> On Mon, 2022-12-05 at 04:53 -0700, Jens Axboe wrote:
>> On 12/4/22 7:44?PM, Pavel Begunkov wrote:
>>> We want to limit post_aux_cqe() to the task context when -
>>>> task_complete
>>> is set, and so we can't just deliver a IORING_OP_MSG_RING CQE to
>>> another
>>> thread. Instead of trying to invent a new delayed CQE posting
>>> mechanism
>>> push them into the overflow list.
>>
>> This is really the only one out of the series that I'm not a big fan
>> of.
>> If we always rely on overflow for msg_ring, then that basically
>> removes
>> it from being usable in a higher performance setting.
>>
>> The natural way to do this would be to post the cqe via task_work for
>> the target, ring, but we also don't any storage available for that.
>> Might still be better to alloc something ala
>>
>> struct tw_cqe_post {
>> ????????struct task_work work;
>> ????????s32 res;
>> ????????u32 flags;
>> ????????u64 user_data;
>> }
>>
>> and post it with that?
>>
> 
> It might work to post the whole request to the target, post the cqe,
> and then return the request back to the originating ring via tw for the
> msg_ring CQE and cleanup.

I did consider that, but then you need to ref that request as well as
bounce it twice via task_work. Probably easier to just alloc at that
point? Though if you do that, then the target cqe would post later than
the original. And potentially lose -EOVERFLOW if the target ring is
overflown...

-- 
Jens Axboe