Re: [RFC PATCH for-next] io_uring: support multiple rings to share same poll thread by specifying same cpu

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi,

> On 9/10/20 1:03 AM, Xiaoguang Wang wrote:
>> We have already supported multiple rings to share one same poll thread
>> by passing IORING_SETUP_ATTACH_WQ, but it's not that convenient to use.
>> IORING_SETUP_ATTACH_WQ needs users to ensure that a parent ring instance
>> has already existed, that means it will require app to regulate the
>> creation oder between uring instances.
>>
>> Currently we can make this a bit simpler, for those rings which will
>> have SQPOLL enabled and are willing to be bound to one same cpu, add a
>> capability that these rings can share one poll thread by specifying
>> a new IORING_SETUP_SQPOLL_PERCPU flag, then we have 3 cases
>>    1, IORING_SETUP_ATTACH_WQ: if user specifies this flag, we'll always
>> try to attach this ring to an existing ring's corresponding poll thread,
>> no matter whether IORING_SETUP_SQ_AFF or IORING_SETUP_SQPOLL_PERCPU is
>> set.
>>    2, IORING_SETUP_SQ_AFF and IORING_SETUP_SQPOLL_PERCPU are both enabled,
>> for this case, we'll create a single poll thread to be shared by these
>> rings, and this poll thread is bound to a fixed cpu.
>>    3, for any other cases, we'll just create one new poll thread for the
>> corresponding ring.
>>
>> And for case 2, don't need to regulate creation oder of multiple uring
>> instances, we use a mutex to synchronize creation, for example, say five
>> rings which all have IORING_SETUP_SQ_AFF & IORING_SETUP_SQPOLL_PERCPU
>> enabled, and are willing to be bound same cpu, one ring that gets the
>> mutex lock will create one poll thread, the other four rings will just
>> attach themselves the previous created poll thread.
>>
>> To implement above function, add one global hlist_head hash table, only
>> sqd that is created for IORING_SETUP_SQ_AFF & IORING_SETUP_SQPOLL_PERCPU
>> will be added to this global list, and its search key are current->files
>> and cpu number.
> 
> Can you resend this against the current tree? Looks like it's against
> something that is outdated. That'll make it easier to test and review.
Sorry, I forgot to update my local tree to the current tree.
It's a little late today, I'll send a V2 against the current tree tomorrow, thanks.

Regards,
Xiaoguang Wang

>