[PATCH net-next v3 5/5] virtio-net: initial rx sysfs support, export mergeable rx buffer size

[Michael Dalton <mwdalton@xxxxxxxxxx>]

Add initial support for per-rx queue sysfs attributes to virtio-net. If
mergeable packet buffers are enabled, adds a read-only mergeable packet
buffer size sysfs attribute for each RX queue.

Signed-off-by: Michael Dalton <mwdalton@xxxxxxxxxx>
---
 drivers/net/virtio_net.c | 66 +++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 62 insertions(+), 4 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 3e82311..f315cbb 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -27,6 +27,7 @@
 #include <linux/slab.h>
 #include <linux/cpu.h>
 #include <linux/average.h>
+#include <linux/seqlock.h>
 
 static int napi_weight = NAPI_POLL_WEIGHT;
 module_param(napi_weight, int, 0444);
@@ -89,6 +90,12 @@ struct receive_queue {
 	/* Average packet length for mergeable receive buffers. */
 	struct ewma mrg_avg_pkt_len;
 
+	/* Sequence counter to allow sysfs readers to safely access stats.
+	 * Assumes a single virtio-net writer, which is enforced by virtio-net
+	 * and NAPI.
+	 */
+	seqcount_t sysfs_seq;
+
 	/* Page frag for packet buffer allocation. */
 	struct page_frag alloc_frag;
 
@@ -416,7 +423,9 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 		}
 	}
 
+	write_seqcount_begin(&rq->sysfs_seq);
 	ewma_add(&rq->mrg_avg_pkt_len, head_skb->len);
+	write_seqcount_end(&rq->sysfs_seq);
 	return head_skb;
 
 err_skb:
@@ -604,18 +613,29 @@ static int add_recvbuf_big(struct receive_queue *rq, gfp_t gfp)
 	return err;
 }
 
-static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
+static unsigned int get_mergeable_buf_len(struct ewma *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_read(avg_pkt_len),
+			GOOD_PACKET_LEN, PAGE_SIZE - hdr_len);
+	return ALIGN(len, MERGEABLE_BUFFER_ALIGN);
+}
+
+static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
+{
 	struct page_frag *alloc_frag = &rq->alloc_frag;
 	char *buf;
 	unsigned long ctx;
 	int err;
 	unsigned int len, hole;
 
-	len = hdr_len + clamp_t(unsigned int, ewma_read(&rq->mrg_avg_pkt_len),
-				GOOD_PACKET_LEN, PAGE_SIZE - hdr_len);
-	len = ALIGN(len, MERGEABLE_BUFFER_ALIGN);
+	/* avg_pkt_len is written only in NAPI rx softirq context. We may
+	 * read avg_pkt_len without using the sysfs_seq seqcount, as this code
+	 * is called only in NAPI rx softirq context or when NAPI is disabled.
+	 */
+	len = get_mergeable_buf_len(&rq->mrg_avg_pkt_len);
 	if (unlikely(!skb_page_frag_refill(len, alloc_frag, gfp)))
 		return -ENOMEM;
 
@@ -1557,6 +1577,7 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
 			       napi_weight);
 
 		sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
+		seqcount_init(&vi->rq[i].sysfs_seq);
 		ewma_init(&vi->rq[i].mrg_avg_pkt_len, 1, RECEIVE_AVG_WEIGHT);
 		sg_init_table(vi->sq[i].sg, ARRAY_SIZE(vi->sq[i].sg));
 	}
@@ -1594,6 +1615,39 @@ err:
 	return ret;
 }
 
+#ifdef CONFIG_SYSFS
+static ssize_t mergeable_rx_buffer_size_show(struct netdev_rx_queue *queue,
+		struct rx_queue_attribute *attribute, char *buf)
+{
+	struct virtnet_info *vi = netdev_priv(queue->dev);
+	unsigned int queue_index = get_netdev_rx_queue_index(queue);
+	struct receive_queue *rq;
+	struct ewma avg;
+	unsigned int start;
+
+	BUG_ON(queue_index >= vi->max_queue_pairs);
+	rq = &vi->rq[queue_index];
+	do {
+		start = read_seqcount_begin(&rq->sysfs_seq);
+		avg = rq->mrg_avg_pkt_len;
+	} while (read_seqcount_retry(&rq->sysfs_seq, start));
+	return sprintf(buf, "%u\n", get_mergeable_buf_len(&avg));
+}
+
+static struct rx_queue_attribute mergeable_rx_buffer_size_attribute =
+	__ATTR_RO(mergeable_rx_buffer_size);
+
+static struct attribute *virtio_net_mrg_rx_attrs[] = {
+	&mergeable_rx_buffer_size_attribute.attr,
+	NULL
+};
+
+static const struct attribute_group virtio_net_mrg_rx_group = {
+	.name = "virtio_net",
+	.attrs = virtio_net_mrg_rx_attrs
+};
+#endif
+
 static int virtnet_probe(struct virtio_device *vdev)
 {
 	int i, err;
@@ -1708,6 +1762,10 @@ static int virtnet_probe(struct virtio_device *vdev)
 	if (err)
 		goto free_stats;
 
+#ifdef CONFIG_SYSFS
+	if (vi->mergeable_rx_bufs)
+		dev->sysfs_rx_queue_group = &virtio_net_mrg_rx_group;
+#endif
 	netif_set_real_num_tx_queues(dev, vi->curr_queue_pairs);
 	netif_set_real_num_rx_queues(dev, vi->curr_queue_pairs);
 
-- 
1.8.5.2

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