Re: [PATCH 0/3] tw mutex & IRQ rw completion batching

Pavel Begunkov
2021-08-19T16:36:07Z

On 8/19/21 4:53 PM, Jens Axboe wrote:
> On 8/18/21 5:42 AM, Pavel Begunkov wrote:
>> In essence, it's about two features. The first one is implemented by
>> 1-2 and saves ->uring_lock lock/unlock in a single call of
>> tctx_task_work(). Should be useful for links, apolls and BPF requests
>> at some moment.
>>
>> The second feature (3/3) is batching freeing and completing of
>> IRQ-based read/write requests.
>>
>> Haven't got numbers yet, but just throwing it for public discussion.
>
> I ran some numbers and it looks good to me, it's a nice boost for the
> IRQ completions. It's funny how the initial move to task_work for IRQ
> completions took a small hit, but there's so many optimizations that it
> unlocks that it's already better than before.
>
> I'd like to apply 1/3 for now, but it depends on both master and
> for-5.15/io_uring. Hence I think it'd be better to defer that one until
> after the initial batch has gone in.
>
> For the batched locking, the principle is sound and measures out to be a
> nice win. But I have a hard time getting over the passed lock state, I
> do wonder if there's a cleaner way to accomplish this...

The initial idea was to have a request flag telling whether a task_work
function may need a lock, but setting/clearing it would be more subtle.
Then there is io_poll_task_func -> io_req_task_submit -> lock, and
reads/writes based using trylock, because otherwise I'd rather be
afraid of it hurting latency.

This version looks good enough, apart from conditional locking always
being a pain. We can hide bool into a struct, and with a bunch of
helpers leave no visibility into it. Though, I don't think it helps
anything.

--
Pavel Begunkov