Re: [PATCH] io_uring: drain next sqe instead of shadowing

[Pavel Begunkov <asml.silence@gmail.com>]

On 11/21/2019 12:26 PM, Jackie Liu wrote:
> 2019年11月21日 16:54，Pavel Begunkov <asml.silence@gmail.com> 写道：
>>
>> If there is a DRAIN in the middle of a link, it uses shadow req. Defer
>> the next request/link instead. This:
>>
>> Pros:
>> 1. removes semi-duplicated code
>> 2. doesn't allocate memory for shadows
>> 3. works better if only the head marked for drain
> 
> I thought about this before, just only drain the head, but if the latter IO depends
> on the link-list, then latter IO will run in front of the link-list. If we think it
> is acceptable, then I think it is ok for me.

If I got your point right, latter requests won't run ahead of the
link-list. There shouldn't be change of behaviour.

The purpose of shadow requests is to mark some request right ahead of
the link for draining. This patch uses not a specially added shadow
request, but the following regular one. And, as drained IO shouldn't be
issued until every request behind completed, this should give the same
effect.

Am I missed something?

Just to notice, @drain_next is in @ctx, so it's preserved across
multiple io_enter_uring().
> --
> BR, Jackie Liu
> 

-- 
Pavel Begunkov