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

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Date:   Mon, 6 Mar 2017 12:31:38 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next RFC 3/4] vhost: interrupt coalescing support

On Mon, Mar 6, 2017 at 4:28 AM, Jason Wang <jasowang@...hat.com> wrote:
>
>
> On 2017年03月03日 22:39, Willem de Bruijn wrote:
>>
>> +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.

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.