Re: [PATCH next v1 2/2] io_uring: limit local tw done

[Pavel Begunkov <asml.silence@gmail.com>]

On 11/21/24 00:52, David Wei wrote:
> On 2024-11-20 15:56, Pavel Begunkov wrote:
>> On 11/20/24 22:14, David Wei wrote:
...
>> One thing that is not so nice is that now we have this handling and
>> checks in the hot path, and __io_run_local_work_loop() most likely
>> gets uninlined.
>>
>> I wonder, can we just requeue it via task_work again? We can even
>> add a variant efficiently adding a list instead of a single entry,
>> i.e. local_task_work_add(head, tail, ...);
> 
> That was an early idea, but it means re-reversing the list and then
> atomically adding each node back to work_llist concurrently with e.g.
> io_req_local_work_add().
> 
> Using a separate retry_llist means we don't need to concurrently add to
> either retry_llist or work_llist.
> 
>>
>> I'm also curious what's the use case you've got that is hitting
>> the problem?
>>
> 
> There is a Memcache-like workload that has load shedding based on the
> time spent doing work. With epoll, the work of reading sockets and
> processing a request is done by user, which can decide after some amount
> of time to drop the remaining work if it takes too long. With io_uring,
> the work of reading sockets is done eagerly inside of task work. If
> there is a burst of work, then so much time is spent in task work
> reading from sockets that, by the time control returns to user the
> timeout has already elapsed.

Interesting, it also sounds like instead of an arbitrary 20 we
might want the user to feed it to us. Might be easier to do it
with the bpf toy not to carve another argument.

-- 
Pavel Begunkov