Should io_sq_thread belongs to specific cpu, not io_uring instance

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

hi，

Currently we can create multiple io_uring instances which all have SQPOLL
enabled and make them run in the same cpu core by setting sq_thread_cpu
argument, but I think this behaviour maybe not efficient. Say we create two
io_uring instances, which both have sq_thread_cpu set to 1 and sq_thread_idle
set to 1000 milliseconds, there maybe such scene below:
   For example, in 0-1s time interval, io_uring instance0 has neither sqes
nor cqes, so it just busy waits for new sqes in 0-1s time interval, but
io_uring instance1 have work to do, submitting sqes or polling issued requests,
then io_uring instance0 will impact io_uring instance1. Of cource io_uring
instance1 may impact iouring instance0 as well, which is not efficient. I think
the complete disorder of multiple io_uring instances running in same cpu core is
not good.

How about we create one io_sq_thread for user specified cpu for multiple io_uring
instances which try to share this cpu core, that means this io_sq_thread does not
belong to specific io_uring instance, it belongs to specific cpu and will
handle requests from mulpile io_uring instance, see simple running flow:
   1, for cpu 1, now there are no io_uring instances bind to it, so do not create io_sq_thread
   2, io_uring instance1 is created and bind to cpu 1, then create cpu1's io_sq_thread
   3, io_sq_thread will handle io_uring instance1's requests
   4, io_uring instance2 is created and bind to cpu 1, since there are already an
      io_sq_thread for cpu 1, will not create an io_sq_thread for cpu1.
   5. now io_sq_thread in cpu1 will handle both io_uring instances' requests.

What do you think about it? Thanks.

Regards,
Xiaoguang Wang