Subject: Re: [RFC PATCH net-next 2/6] virtio: introduce virtio_enable_cb_avail()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 15 Oct 2014 14:43:21 +0300

On Wed, Oct 15, 2014 at 06:58:15PM +0800, Jason Wang wrote:
> On 10/15/2014 06:41 PM, Michael S. Tsirkin wrote:
> > On Wed, Oct 15, 2014 at 06:19:15PM +0800, Jason Wang wrote:
> >> On 10/15/2014 05:28 PM, Michael S. Tsirkin wrote:
> >>> On Wed, Oct 15, 2014 at 03:25:26PM +0800, Jason Wang wrote:
> >>>> This patch introduces virtio_enable_cb_avail() to publish avail idx
> >>>> and used event. This could be used by batched buffer submitting to
> >>>> reduce the number of tx interrupts.
> >>>>
> >>>> Cc: Rusty Russell <[email-masked]>
> >>>> Cc: Michael S. Tsirkin <[email-masked]>
> >>>> Signed-off-by: Jason Wang <[email-masked]>
> >>>> ---
> >>>>  drivers/virtio/virtio_ring.c |   22 ++++++++++++++++++++--
> >>>>  include/linux/virtio.h       |    2 ++
> >>>>  2 files changed, 22 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> >>>> index 1b3929f..d67fbf8 100644
> >>>> --- a/drivers/virtio/virtio_ring.c
> >>>> +++ b/drivers/virtio/virtio_ring.c
> >>>> @@ -567,14 +567,32 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> >>>>  	 * entry. Always do both to keep code simple. */
> >>>>  	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> >>>>  	/* Make sure used event never go backwards */
> >>>> -	if (!vring_need_event(vring_used_event(&vq->vring),
> >>>> -			      vq->vring.avail->idx, last_used_idx))
> >>>> +	if (vq->vring.avail->idx != vring_used_event(&vq->vring) &&
> >>>> +	    !vring_need_event(vring_used_event(&vq->vring),
> >>>> +			      vq->vring.avail->idx, last_used_idx)) {
> >>>>  		vring_used_event(&vq->vring) = last_used_idx;
> >>>> +	}
> >>>>  	END_USE(vq);
> >>>>  	return last_used_idx;
> >>>>  }
> >>>>  EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
> >>>>
> >>> I see you are also changing virtqueue_enable_cb_prepare, why?
> >> This is also used to prevent it from moving the used event backwards.
> >> This may happens when we handle tx napi after we publish avail idx as
> >> used event (virtqueue_enable_cb_avail() was called).
> > So it's wrong exactly in the same way.
> >
> > But also, please document this stuff, don't put
> > unrelated changes in a patch called "introduce
> > virtqueue_enable_cb_avail".
> >
> >
> >>>> +bool virtqueue_enable_cb_avail(struct virtqueue *_vq)
> >>>> +{
> >>>> +	struct vring_virtqueue *vq = to_vvq(_vq);
> >>>> +	bool ret;
> >>>> +
> >>>> +	START_USE(vq);
> >>>> +	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> >>>> +	vring_used_event(&vq->vring) = vq->vring.avail->idx;
> >>>> +	ret = vring_need_event(vq->vring.avail->idx,
> >>>> +			       vq->last_used_idx, vq->vring.used->idx);
> >>>> +	END_USE(vq);
> >>>> +
> >>>> +	return ret;
> >>>> +}
> >>>> +EXPORT_SYMBOL_GPL(virtqueue_enable_cb_avail);
> >>>> +
> >>>>  /**
> >>>>   * virtqueue_poll - query pending used buffers
> >>>>   * @vq: the struct virtqueue we're talking about.
> >>> Could not figure out what this does.
> >>> Please add documentation.
> >>>
> >> Sure, does something like below explain what does this function do?
> >>
> >> /**                                                                            
> >>
> >>  * virtqueue_enable_cb_avail - restart callbacks after
> >> disable_cb.           
> >>  * @vq: the struct virtqueue we're talking
> >> about.                              
> >>  *                                                                             
> >>
> >>  * This re-enables callbacks but hints to the other side to
> >> delay              
> >>  * interrupts all of the available buffers have been processed;         
> >
> > So this is like virtqueue_enable_cb_delayed but even more
> > aggressive?
> > I think it's too agressive: it's better to wake up guest
> > after you are through most of buffers, but not all,
> > so guest and host can work in parallel.
> 
> Note that:
> 
> - it was only used when there are still few of free slots (which is
> greater than 2 + MAX_SKB_FRAGS)
> - my patch keeps the free_old_xmit_skbs() in the beginning of
> start_xmit(), so the tx skb reclaiming does not depends totally on tx
> interrupt. If more packets comes, we'd expect some of them were freed in
> ndo_start_xmit(). If not, finally we may trigger the
> virtqueue_enable_cb_delayed().
> 
> So probably not as aggressive as it looks. I will do benchmark on this.

Mine too:
        } else if (virtqueue_enable_cb_delayed(sq->vq)) {
                free_old_xmit_skbs(txq, sq, qsize);
        }

> >
> >
> >>  * it returns "false" if there are at least one pending buffer in the
> >> queue,          
> >>  * to detect a possible race between the driver checking for more
> >> work,        
> >>  * and enabling
> >> callbacks.                                                     
> >>  *                                                                             
> >>
> >>  * Caller must ensure we don't call this with other
> >> virtqueue                  
> >>  * operations at the same time (except where
> >> noted).                           
> >>  */
> >>
> >>>> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> >>>> index b46671e..bfaf058 100644
> >>>> --- a/include/linux/virtio.h
> >>>> +++ b/include/linux/virtio.h
> >>>> @@ -65,6 +65,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
> >>>>  
> >>>>  unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
> >>>>  
> >>>> +bool virtqueue_enable_cb_avail(struct virtqueue *vq);
> >>>> +
> >>>>  bool virtqueue_poll(struct virtqueue *vq, unsigned);
> >>>>  
> >>>>  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
> >>>> -- 
> >>>> 1.7.1
> >>> --
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