Re: [PATCH 3/5] io_uring: implement our own schedule timeout handling

[David Wei <dw@davidwei.uk>]

On 2024-08-20 14:39, Jens Axboe wrote:
> On 8/20/24 3:37 PM, David Wei wrote:
>> On 2024-08-20 14:34, Jens Axboe wrote:
>>> On 8/20/24 2:08 PM, David Wei wrote:
>>>> On 2024-08-19 16:28, Jens Axboe wrote:
>>>>> In preparation for having two distinct timeouts and avoid waking the
>>>>> task if we don't need to.
>>>>>
>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>> ---
>>>>>  io_uring/io_uring.c | 41 ++++++++++++++++++++++++++++++++++++-----
>>>>>  io_uring/io_uring.h |  2 ++
>>>>>  2 files changed, 38 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>>>>> index 9e2b8d4c05db..ddfbe04c61ed 100644
>>>>> --- a/io_uring/io_uring.c
>>>>> +++ b/io_uring/io_uring.c
>>>>> @@ -2322,7 +2322,7 @@ static int io_wake_function(struct wait_queue_entry *curr, unsigned int mode,
>>>>>  	 * Cannot safely flush overflowed CQEs from here, ensure we wake up
>>>>>  	 * the task, and the next invocation will do it.
>>>>>  	 */
>>>>> -	if (io_should_wake(iowq) || io_has_work(iowq->ctx))
>>>>> +	if (io_should_wake(iowq) || io_has_work(iowq->ctx) || iowq->hit_timeout)
>>>>
>>>> iowq->hit_timeout may be modified in a timer softirq context, while this
>>>> wait_queue_func_t (AIUI) may get called from any context e.g.
>>>> net_rx_softirq for sockets. Does this need a READ_ONLY()?
>>>
>>> Yes probably not a bad idea to make it READ_ONCE().
>>>
>>>>>  		return autoremove_wake_function(curr, mode, wake_flags, key);
>>>>>  	return -1;
>>>>>  }
>>>>> @@ -2350,6 +2350,38 @@ static bool current_pending_io(void)
>>>>>  	return percpu_counter_read_positive(&tctx->inflight);
>>>>>  }
>>>>>  
>>>>> +static enum hrtimer_restart io_cqring_timer_wakeup(struct hrtimer *timer)
>>>>> +{
>>>>> +	struct io_wait_queue *iowq = container_of(timer, struct io_wait_queue, t);
>>>>> +	struct io_ring_ctx *ctx = iowq->ctx;
>>>>> +
>>>>> +	WRITE_ONCE(iowq->hit_timeout, 1);
>>>>> +	if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
>>>>> +		wake_up_process(ctx->submitter_task);
>>>>> +	else
>>>>> +		io_cqring_wake(ctx);
>>>>
>>>> This is a bit different to schedule_hrtimeout_range_clock(). Why is
>>>> io_cqring_wake() needed here for non-DEFER_TASKRUN?
>>>
>>> That's how the wakeups work - for defer taskrun, the task isn't on a
>>> waitqueue at all. Hence we need to wake the task itself. For any other
>>> setup, they will be on the waitqueue, and we just call io_cqring_wake()
>>> to wake up anyone waiting on the waitqueue. That will iterate the wake
>>> queue and call handlers for each item. Having a separate handler for
>>> that will allow to NOT wake up the task if we don't need to.
>>> taskrun, the waker
>>
>> To rephase the question, why is the original code calling
>> schedule_hrtimeout_range_clock() not needing to differentiate behaviour
>> between defer taskrun and not?
> 
> Because that part is the same, the task schedules out and goes to sleep.
> That has always been the same regardless of how the ring is setup. Only
> difference is that DEFER_TASKRUN doesn't add itself to ctx->wait, and
> hence cannot be woken by a wake_up(ctx->wait). We have to wake the task
> manually.
> 

io_cqring_timer_wakeup() is the timer expired callback which calls
wake_up_process() or io_cqring_wake() depending on DEFER_TASKRUN.

The original code calling schedule_hrtimeout_range_clock() uses
hrtimer_sleeper instead, which has a default timer expired callback set
to hrtimer_wakeup().

hrtimer_wakeup() only calls wake_up_process().

My question is: why this asymmetry? Why does the new custom callback
require io_cqring_wake()?