Re: [PATCH 1/2] vhost: Reduce vhost_work_flush() wakeup latency

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 14, 2013 at 09:02:32AM +0200, Bart Van Assche wrote:
> If the TIF_NEED_RESCHED task flag is set, wake up any vhost_work_flush()
> waiters before rescheduling instead of after rescheduling.
> 
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Asias He <asias@redhat.com>

Why exactly? It's not like flush needs to be extra fast ...

> ---
>  drivers/vhost/vhost.c |   42 +++++++++++++++++++-----------------------
>  1 file changed, 19 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index e58cf00..e7ffc10 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -201,47 +201,43 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  static int vhost_worker(void *data)
>  {
>  	struct vhost_dev *dev = data;
> -	struct vhost_work *work = NULL;
> -	unsigned uninitialized_var(seq);
> +	struct vhost_work *work;
> +	unsigned seq;
>  	mm_segment_t oldfs = get_fs();
>  
>  	set_fs(USER_DS);
>  	use_mm(dev->mm);
>  
> -	for (;;) {
> +	spin_lock_irq(&dev->work_lock);
> +	while (!kthread_should_stop()) {
>  		/* mb paired w/ kthread_stop */
>  		set_current_state(TASK_INTERRUPTIBLE);
> -
> -		spin_lock_irq(&dev->work_lock);
> -		if (work) {
> -			work->done_seq = seq;
> -			if (work->flushing)
> -				wake_up_all(&work->done);
> -		}
> -
> -		if (kthread_should_stop()) {
> -			spin_unlock_irq(&dev->work_lock);
> -			__set_current_state(TASK_RUNNING);
> -			break;
> -		}
>  		if (!list_empty(&dev->work_list)) {
>  			work = list_first_entry(&dev->work_list,
>  						struct vhost_work, node);
>  			list_del_init(&work->node);
>  			seq = work->queue_seq;
> -		} else
> -			work = NULL;
> -		spin_unlock_irq(&dev->work_lock);
> +			spin_unlock_irq(&dev->work_lock);
>  
> -		if (work) {
>  			__set_current_state(TASK_RUNNING);
>  			work->fn(work);
> -			if (need_resched())
> -				schedule();
> -		} else
> +
> +			spin_lock_irq(&dev->work_lock);
> +			work->done_seq = seq;
> +			if (work->flushing)
> +				wake_up_all(&work->done);
> +		}
> +		if (list_empty(&dev->work_list) || need_resched()) {
> +			spin_unlock_irq(&dev->work_lock);
> +
>  			schedule();
>  
> +			spin_lock_irq(&dev->work_lock);
> +		}
>  	}
> +	spin_unlock_irq(&dev->work_lock);
> +
> +	__set_current_state(TASK_RUNNING);
>  	unuse_mm(dev->mm);
>  	set_fs(oldfs);
>  	return 0;
> -- 
> 1.7.10.4