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

[Jens Axboe <axboe@kernel.dk>]

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

-- 
Jens Axboe