Re: [dpdk-dev] [PATCH v1] vhost: fix mbuf allocation failures

[Flavio Leitner <fbl@redhat.com>]

On Wed, Apr 29, 2020 at 10:43:01AM +0200, Maxime Coquelin wrote:
> Hi Sivaprasad,
> 
> On 4/28/20 11:52 AM, Sivaprasad Tummala wrote:
> > vhost buffer allocation is successful for packets that fit
> > into a linear buffer. If it fails, vhost library is expected
> > to drop the current buffer descriptor and skip to the next.
> > 
> > The patch fixes the error scenario by skipping to next descriptor.
> > Note: Drop counters are not currently supported.

In that case shouldn't we continue to process the ring?

Also, don't we have the same issue with copy_desc_to_mbuf() 
and get_zmbuf()?

fbl

> Fixes tag is missing here, and stable@dpdk.org should be cc'ed if
> necessary.
> 
> > Signed-off-by: Sivaprasad Tummala <Sivaprasad.Tummala@intel.com>
> > ---
> >  lib/librte_vhost/virtio_net.c | 16 ++++++++++++++--
> >  1 file changed, 14 insertions(+), 2 deletions(-)
> > 
> > diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> > index 37c47c7dc..b0d3a85c2 100644
> > --- a/lib/librte_vhost/virtio_net.c
> > +++ b/lib/librte_vhost/virtio_net.c
> > @@ -1688,6 +1688,7 @@ virtio_dev_tx_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
> >  {
> 
> You only fix split ring path, but not packed ring.
> 
> >  	uint16_t i;
> >  	uint16_t free_entries;
> > +	uint16_t dropped = 0;
> >  
> >  	if (unlikely(dev->dequeue_zero_copy)) {
> >  		struct zcopy_mbuf *zmbuf, *next;
> > @@ -1751,8 +1752,19 @@ virtio_dev_tx_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
> >  			update_shadow_used_ring_split(vq, head_idx, 0);
> >  
> >  		pkts[i] = virtio_dev_pktmbuf_alloc(dev, mbuf_pool, buf_len);
> > -		if (unlikely(pkts[i] == NULL))
> > +		if (unlikely(pkts[i] == NULL)) {
> > +			/*
> > +			 * mbuf allocation fails for jumbo packets with
> > +			 * linear buffer flag set. Drop this packet and
> > +			 * proceed with the next available descriptor to
> > +			 * avoid HOL blocking
> > +			 */
> > +			VHOST_LOG_DATA(WARNING,
> > +				"Failed to allocate memory for mbuf. Packet dropped!\n");
> 
> I think we need a better logging, otherwise it is going to flood the log
> file quite rapidly if issue happens. Either some rate-limited logging or
> warn-once would be better.
> 
> The warning message could be also improved, because when using linear
> buffers, one would expect that the size of the mbufs could handle a
> jumbo frame. So it should differentiate two
> 
> > +			dropped += 1;
> > +			i++;
> >  			break;
> > +		}
> >  
> >  		err = copy_desc_to_mbuf(dev, vq, buf_vec, nr_vec, pkts[i],
> >  				mbuf_pool);
> > @@ -1796,7 +1808,7 @@ virtio_dev_tx_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
> >  		}
> >  	}
> >  
> > -	return i;
> > +	return (i - dropped);
> >  }
> >  
> >  static __rte_always_inline int
> > 
> 

-- 
fbl