Re: [PATCH v2] virtio_net: fix support for small rings

[Jason Wang]

On 2017å03æ30æ 01:42, Michael S. Tsirkin wrote:
> When ring size is small (<32 entries) making buffers smaller means a
> full ring might not be able to hold enough buffers to fit a single large
> packet.
>
> Make sure a ring full of buffers is large enough to allow at least one
> packet of max size.
>
> Fixes: 2613af0ed18a ("virtio_net: migrate mergeable rx buffers to page frag allocators")
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>
> changes from v1:
> 	typo fix
>
>   drivers/net/virtio_net.c | 30 ++++++++++++++++++++++++++----
>   1 file changed, 26 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 9dc31dc..e5f6e34 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -29,6 +29,7 @@
>   #include <linux/slab.h>
>   #include <linux/cpu.h>
>   #include <linux/average.h>
> +#include <net/route.h>

This seems unnecessary.

>   
>   static int napi_weight = NAPI_POLL_WEIGHT;
>   module_param(napi_weight, int, 0444);
> @@ -98,6 +99,9 @@ struct receive_queue {
>   	/* RX: fragments + linear part + virtio header */
>   	struct scatterlist sg[MAX_SKB_FRAGS + 2];
>   
> +	/* Min single buffer size for mergeable buffers case. */
> +	unsigned int min_buf_len;
> +
>   	/* Name of this receive queue: input.$index */
>   	char name[40];
>   };
> @@ -894,13 +898,14 @@ static int add_recvbuf_big(struct virtnet_info *vi, struct receive_queue *rq,
>   	return err;
>   }
>   
> -static unsigned int get_mergeable_buf_len(struct ewma_pkt_len *avg_pkt_len)
> +static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
> +					  struct ewma_pkt_len *avg_pkt_len)
>   {
>   	const size_t hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
>   	unsigned int len;
>   
>   	len = hdr_len + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len),
> -			GOOD_PACKET_LEN, PAGE_SIZE - hdr_len);
> +				rq->min_buf_len - hdr_len, PAGE_SIZE - hdr_len);
>   	return ALIGN(len, L1_CACHE_BYTES);
>   }
>   
> @@ -914,7 +919,7 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
>   	int err;
>   	unsigned int len, hole;
>   
> -	len = get_mergeable_buf_len(&rq->mrg_avg_pkt_len);
> +	len = get_mergeable_buf_len(rq, &rq->mrg_avg_pkt_len);
>   	if (unlikely(!skb_page_frag_refill(len + headroom, alloc_frag, gfp)))
>   		return -ENOMEM;
>   
> @@ -2086,6 +2091,21 @@ static void virtnet_del_vqs(struct virtnet_info *vi)
>   	virtnet_free_queues(vi);
>   }
>   
> +/* How large should a single buffer be so a queue full of these can fit at
> + * least one full packet?
> + * Logic below assumes the mergeable buffer header is used.
> + */
> +static unsigned int mergeable_min_buf_len(struct virtnet_info *vi, struct virtqueue *vq)
> +{
> +	const unsigned int hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> +	unsigned int rq_size = virtqueue_get_vring_size(vq);
> +	unsigned int packet_len = vi->big_packets ? IP_MAX_MTU : vi->dev->max_mtu;
> +	unsigned int buf_len = hdr_len + ETH_HLEN + VLAN_HLEN + packet_len;
> +	unsigned int min_buf_len = DIV_ROUND_UP(buf_len, rq_size);
> +
> +	return max(min_buf_len, hdr_len);
> +}

Consider rq_size may be large, I think this should be max(min_buf_len, 
GOOD_PACKET_LEN)?

> +
>   static int virtnet_find_vqs(struct virtnet_info *vi)
>   {
>   	vq_callback_t **callbacks;
> @@ -2151,6 +2171,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>   
>   	for (i = 0; i < vi->max_queue_pairs; i++) {
>   		vi->rq[i].vq = vqs[rxq2vq(i)];
> +		vi->rq[i].min_buf_len = mergeable_min_buf_len(vi, vi->rq[i].vq);
>   		vi->sq[i].vq = vqs[txq2vq(i)];
>   	}
>   
> @@ -2237,7 +2258,8 @@ static ssize_t mergeable_rx_buffer_size_show(struct netdev_rx_queue *queue,
>   
>   	BUG_ON(queue_index >= vi->max_queue_pairs);
>   	avg = &vi->rq[queue_index].mrg_avg_pkt_len;
> -	return sprintf(buf, "%u\n", get_mergeable_buf_len(avg));
> +	return sprintf(buf, "%u\n",
> +		       get_mergeable_buf_len(&vi->rq[queue_index], avg));
>   }
>   
>   static struct rx_queue_attribute mergeable_rx_buffer_size_attribute =