linux-kernel - Re: Is adding requeue_delayed

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Date:	Mon, 24 Aug 2009 14:11:00 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Is adding requeue_delayed_work() a good idea


 > > In my particular case it doesn't really matter.  In the queued case it
 > > could leave it to run whenever it gets to the head of the workqueue.  In
 > > the already running case then I think the timer should be reset.  The
 > > main point is that if I do requeue_delayed_work() I want to make sure
 > > the work runs all the way through from the beginning at some point in
 > > the future.  The pattern I have in mind is something like:
 > >
 > > 	spin_lock_irqsave(&mydata_lock);
 > > 	new_timeout = add_item_to_timeout_list();
 > > 	requeue_delayed_work(wq, &process_timeout_list_work, new_timeout);
 > > 	spin_unlock_irqsave(&mydata_lock);
 > >
 > > so if the process_timeout_list_work runs early or twice it doesn't
 > > matter; I just want to make sure that the work runs from the beginning
 > > and sees the new item I added to the list at some point after the
 > > requeue.
 > 
 > Hmm. But, asuming that process_timeout_list_work->func() takes mydata_lock
 > too, you can just use queue_delayed_work() ?
 > 
 > process_timeout_list_work can't miss new items, queue_delayed_work()
 > can only fail if dwork is pending and its ->func has not started yet.

Maybe I misunderstand the code or misunderstand you, but looking at it:

	int queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
				struct delayed_work *dwork, unsigned long delay)
	{
		int ret = 0;
		struct timer_list *timer = &dwork->timer;
		struct work_struct *work = &dwork->work;
	
		if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work))) {
			BUG_ON(timer_pending(timer));
			BUG_ON(!list_empty(&work->entry));
			//...
		}
		return ret;
	}

if I have dwork already queued and try to requeue it for earlier it
seems that this will just silently return with the later expiration
still in effect.

Maybe I wasn't totally clear on what I want to happen; an example would
be if, say, I have delayed work queued with timer set to expire in 100
seconds and then a new piece of work is added that needs to expire in 1
second.  So I want to requeue delayed work to run in 1 second, not wait
the 100 seconds for the original timeout work.

 > I think we need requeue_xxx helper(s), but the exact semantics is not
 > clear to me.

Not clear to me either unfortunately.

 - R.
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