Re: [RFC] io_uring: wake up optimisations

[Jens Axboe <axboe@kernel.dk>]

On 12/20/22 11:06 AM, Pavel Begunkov wrote:
> On 12/20/22 17:58, Pavel Begunkov wrote:
>> NOT FOR INCLUSION, needs some ring poll workarounds
>>
>> Flush completions is done either from the submit syscall or by the
>> task_work, both are in the context of the submitter task, and when it
>> goes for a single threaded rings like implied by ->task_complete, there
>> won't be any waiters on ->cq_wait but the master task. That means that
>> there can be no tasks sleeping on cq_wait while we run
>> __io_submit_flush_completions() and so waking up can be skipped.
> 
> Not trivial to benchmark as we need something to emulate a task_work
> coming in the middle of waiting. I used the diff below to complete nops
> in tw and removed preliminary tw runs for the "in the middle of waiting"
> part. IORING_SETUP_SKIP_CQWAKE controls whether we use optimisation or
> not.
> 
> It gets around 15% more IOPS (6769526 -> 7803304), which correlates
> to 10% of wakeup cost in profiles. Another interesting part is that
> waitqueues are excessive for our purposes and we can replace cq_wait
> with something less heavier, e.g. atomic bit set

I was thinking something like that the other day, for most purposes
the wait infra is too heavy handed for our case. If we exclude poll
for a second, everything else is internal and eg doesn't need IRQ
safe locking at all. That's just one part of it. But I didn't have
a good idea for the poll() side of things, which would be required
to make some progress there.

-- 
Jens Axboe