Re: support suspend/resume SQPOLL on inflight io-uring instance

[Jens Axboe <axboe@kernel.dk>]

On 6/4/20 9:38 PM, 陈云星 wrote:
> io-uring instance can be set flag : IO_URING_SETUP_SQPOLL to enable
> kernel thread polling SQ. but some application have random io
> behavior, for example:
>      a). a burst read/write for a while,
>      b). then many scatter small io operations.
> 
> during a) period, use sqpoll will be beneficial to performance.
> but during b) period, use sqpoll will waste cpu time for empty busy poll.
> 
> If we can have a method to tell kernel start/stop poll sq when we
> need, it is good for both of a) and b):
>      in a) period, io_uring_enter tell kernel. start to poll
>      after a) , io_uring_enter tell kernel stop polling and.

We already have the start side of it, that's io_uring_enter() which should
be done when IORING_SQ_NEED_WAKEUP is set. I do like your suggestion of
making it go to sleep on demand instead of always waiting for the grace
period.

We recently added IORING_CQ_EVENTFD_DISABLED, and we can use the same
area for this as well. Eg add IORING_CQ_SQPOLL_SLEEP or something like
that, and have the sq thread look for it. If set, it'll stop busy looping
and go to sleep.

-- 
Jens Axboe