Re: [ISSUE] The time cost of IOSQE_IO_LINK

[Jens Axboe <axboe@kernel.dk>]

On 2/14/20 8:32 AM, Peter Zijlstra wrote:
> On Thu, Feb 13, 2020 at 10:03:54PM -0700, Jens Axboe wrote:
> 
>> CC'ing peterz for some cluebat knowledge. Peter, is there a nice way to
>> currently do something like this? Only thing I'm currently aware of is
>> the preempt in/out notifiers, but they don't quite provide what I need,
>> since I need to pass some data (a request) as well.
> 
> Whee, nothing quite like this around I think.

Probably not ;-)

>> The full detail on what I'm trying here is:
>>
>> io_uring can have linked requests. One obvious use case for that is to
>> queue a POLLIN on a socket, and then link a read/recv to that. When the
>> poll completes, we want to run the read/recv. io_uring hooks into the
>> waitqueue wakeup handler to finish the poll request, and since we're
>> deep in waitqueue wakeup code, it queues the linked read/recv for
>> execution via an async thread. This is not optimal, obviously, as it
>> relies on a switch to a new thread to perform this read. This hack
>> queues a backlog to the task itself, and runs it when it's scheduled in.
>> Probably want to do the same for sched out as well, currently I just
>> hack that in the io_uring wait part...
> 
> I'll definitely need to think more about this, but a few comments on the
> below.
> 
>> +static void __io_uring_task_handler(struct list_head *list)
>> +{
>> +	struct io_kiocb *req;
>> +
>> +	while (!list_empty(list)) {
>> +		req = list_first_entry(list, struct io_kiocb, list);
>> +		list_del(&req->list);
>> +
>> +		__io_queue_sqe(req, NULL);
>> +	}
>> +}
>> +
>> +void io_uring_task_handler(struct task_struct *tsk)
>> +{
>> +	LIST_HEAD(list);
>> +
>> +	raw_spin_lock_irq(&tsk->uring_lock);
>> +	if (!list_empty(&tsk->uring_work))
>> +		list_splice_init(&tsk->uring_work, &list);
>> +	raw_spin_unlock_irq(&tsk->uring_lock);
>> +
>> +	__io_uring_task_handler(&list);
>> +}
> 
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index fc1dfc007604..b60f081cac17 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2717,6 +2717,11 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
>>  	INIT_HLIST_HEAD(&p->preempt_notifiers);
>>  #endif
>>  
>> +#ifdef CONFIG_IO_URING
>> +	INIT_LIST_HEAD(&p->uring_work);
>> +	raw_spin_lock_init(&p->uring_lock);
>> +#endif
>> +
>>  #ifdef CONFIG_COMPACTION
>>  	p->capture_control = NULL;
>>  #endif
>> @@ -3069,6 +3074,20 @@ fire_sched_out_preempt_notifiers(struct task_struct *curr,
>>  
>>  #endif /* CONFIG_PREEMPT_NOTIFIERS */
>>  
>> +#ifdef CONFIG_IO_URING
>> +extern void io_uring_task_handler(struct task_struct *tsk);
>> +
>> +static inline void io_uring_handler(struct task_struct *tsk)
>> +{
>> +	if (!list_empty(&tsk->uring_work))
>> +		io_uring_task_handler(tsk);
>> +}
>> +#else /* !CONFIG_IO_URING */
>> +static inline void io_uring_handler(struct task_struct *tsk)
>> +{
>> +}
>> +#endif
>> +
>>  static inline void prepare_task(struct task_struct *next)
>>  {
>>  #ifdef CONFIG_SMP
>> @@ -3322,6 +3341,8 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
>>  	balance_callback(rq);
>>  	preempt_enable();
>>  
>> +	io_uring_handler(current);
>> +
>>  	if (current->set_child_tid)
>>  		put_user(task_pid_vnr(current), current->set_child_tid);
>>  
> 
> I suspect you meant to put that in finish_task_switch() which is the
> tail end of every schedule(), schedule_tail() is the tail end of
> clone().
> 
> Or maybe you meant to put it in (and rename) sched_update_worker() which
> is after every schedule() but in a preemptible context -- much saner
> since you don't want to go add an unbounded amount of work in a
> non-preemptible context.
> 
> At which point you already have your callback: io_wq_worker_running(),
> or is this for any random task?

Let me try and clarify - this isn't for the worker tasks, this is for
any task that is using io_uring. In fact, it's particularly not for the
worker threads, just the task itself.

I basically want the handler to be called when:

1) The task is scheduled in. The poll will complete and stuff some items
   on that task list, and I want to task to process them as it wakes up.

2) The task is going to sleep, don't want to leave entries around while
   the task is sleeping.

3) I need it to be called from "normal" context, with ints enabled,
   preempt enabled, etc.

sched_update_worker() (with a rename) looks ideal for #1, and the
context is sane for me. Just need a good spot to put the hook call for
schedule out. I think this:

	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
		preempt_disable();
		if (tsk->flags & PF_WQ_WORKER)
			wq_worker_sleeping(tsk);
		else
			io_wq_worker_sleeping(tsk);
		preempt_enable_no_resched();
	}

just needs to go into another helper, and then I can call it there
outside of the preempt.

I'm sure there are daemons lurking here, but I'll test and see how it
goes...

-- 
Jens Axboe