Subject: Re: [PATCH 1/3] vhost: replace vhost_workqueue with per-vhost kthread

From: Oleg Nesterov <[email-masked]>
Date: Mon, 31 May 2010 16:39:45 +0200

On 05/30, Tejun Heo wrote:
>
> This conversion is to make each vhost use a dedicated kthread so that
> resource control via cgroup can be applied.

Personally, I agree. I think This is better than play with workqueue thread.

A couple of simple questions after the quick glance at the unapplied patch...

>  void vhost_poll_flush(struct vhost_poll *poll)
>  {
> -	flush_work(&poll->work);
> +	int seq = poll->queue_seq;
> +
> +	if (seq - poll->done_seq > 0)
> +		wait_event(poll->done, seq - poll->done_seq <= 0);

The check before wait_event() is not needed, please note that wait_event()
checks the condition before __wait_event().

What I can't understand is why we do have ->queue_seq and ->done_seq.

Isn't the single "bool poll->active" enough? vhost_poll_queue() sets
->active == T, vhost_poller() clears it before wake_up_all(poll->done).

> +static int vhost_poller(void *data)
> +{
> +	struct vhost_dev *dev = data;
> +	struct vhost_poll *poll;
> +
> +repeat:
> +	set_current_state(TASK_INTERRUPTIBLE);	/* mb paired w/ kthread_stop */

I don't understand the comment... why do we need this barrier?

> +	if (kthread_should_stop()) {
> +		__set_current_state(TASK_RUNNING);
> +		return 0;
> +	}
> +
> +	poll = NULL;
> +	spin_lock(&dev->poller_lock);
> +	if (!list_empty(&dev->poll_list)) {
> +		poll = list_first_entry(&dev->poll_list,
> +					struct vhost_poll, node);
> +		list_del_init(&poll->node);
> +	}
> +	spin_unlock(&dev->poller_lock);
> +
> +	if (poll) {
> +		__set_current_state(TASK_RUNNING);
> +		poll->fn(poll);
> +		smp_wmb();	/* paired with rmb in vhost_poll_flush() */
> +		poll->done_seq = poll->queue_seq;
> +		wake_up_all(&poll->done);
> +	} else
> +		schedule();
> +
> +	goto repeat;
> +}

Given that vhost_poll_queue() does list_add() and wake_up_process() under
->poller_lock, I don't think we need any barriers to change ->state.

IOW, can't vhost_poller() simply do

	while(!kthread_should_stop()) {

		poll = NULL;
		spin_lock(&dev->poller_lock);
		if (!list_empty(&dev->poll_list)) {
			...
		} else
			 __set_current_state(TASK_INTERRUPTIBLE);
		spin_unlock(&dev->poller_lock);

		if (poll) {
			...
		} else
			schedule();
	}

?

Oleg.