Re: [PATCH net-next RFC 3/4] vhost: interrupt coalescing support

On 2017年03月07日 01:31, Willem de Bruijn wrote:

On Mon, Mar 6, 2017 at 4:28 AM, Jason Wang [off-list ref] wrote:

quoted

On 2017年03月03日 22:39, Willem de Bruijn wrote:

quoted

+void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq);
+static enum hrtimer_restart vhost_coalesce_timer(struct hrtimer *timer)
+{
+       struct vhost_virtqueue *vq =
+               container_of(timer, struct vhost_virtqueue, ctimer);
+
+       if (mutex_trylock(&vq->mutex)) {
+               vq->coalesce_frames = vq->max_coalesce_frames;
+               vhost_signal(vq->dev, vq);
+               mutex_unlock(&vq->mutex);
+       }
+
+       /* TODO: restart if lock failed and not held by handle_tx */
+       return HRTIMER_NORESTART;
+}
+

Then we may lose an interrupt forever if no new tx request? I believe we
need e.g vhost_poll_queue() here.

Absolutely, I need to fix this. The common case for failing to grab
the lock is competition with handle_tx. With careful coding we can
probably avoid scheduling another run with vhost_poll_queue in
the common case.

Yes, probably add some checking after releasing the mutex_lock in 
handle_tx().

Thans

Your patch v7 cancels the pending hrtimer at the start of handle_tx.
I need to reintroduce that, and also only schedule a timer at the end
of handle_tx, not immediately when vq->coalesce_frames becomes
non-zero.