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

[Jens Axboe <axboe@kernel.dk>]

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something that is outdated.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.

-- 
Jens Axboe