[net-next V8 PATCH 15/16] xdp: transition into using xdp_frame for return API

Jesper Dangaard Brouer
2018-03-31T12:07:07Z

Changing API xdp_return_frame() to take struct xdp_frame as argument,
seems like a natural choice. But there are some subtle performance
details here that needs extra care, which is a deliberate choice.

When de-referencing xdp_frame on a remote CPU during DMA-TX
completion, result in the cache-line is change to "Shared"
state. Later when the page is reused for RX, then this xdp_frame
cache-line is written, which change the state to "Modified".

This situation already happens (naturally) for, virtio_net, tun and
cpumap as the xdp_frame pointer is the queued object. In tun and
cpumap, the ptr_ring is used for efficiently transferring cache-lines
(with pointers) between CPUs. Thus, the only option is to
de-referencing xdp_frame.

It is only the ixgbe driver that had an optimization, in which it can
avoid doing the de-reference of xdp_frame. The driver already have
TX-ring queue, which (in case of remote DMA-TX completion) have to be
transferred between CPUs anyhow. In this data area, we stored a
struct xdp_mem_info and a data pointer, which allowed us to avoid
de-referencing xdp_frame.

To compensate for this, a prefetchw is used for telling the cache
coherency protocol about our access pattern. My benchmarks show that
this prefetchw is enough to compensate the ixgbe driver.

V7: Adjust for commit d9314c474d4f ("i40e: add support for XDP_REDIRECT")
V8: Adjust for commit bd658dda4237 ("net/mlx5e: Separate dma base address
and offset in dma_sync call")

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
drivers/net/ethernet/intel/i40e/i40e_txrx.c | 5 ++---
drivers/net/ethernet/intel/ixgbe/ixgbe.h | 4 +---
drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 17 +++++++++++------
drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 1 +
drivers/net/tun.c | 4 ++--
drivers/net/virtio_net.c | 2 +-
include/net/xdp.h | 2 +-
kernel/bpf/cpumap.c | 6 +++---
net/core/xdp.c | 4 +++-
9 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.c b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
index 96c54cbfb1f9..c8bf4d35fdea 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_txrx.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
@@ -638,8 +638,7 @@ static void i40e_unmap_and_free_tx_resource(struct i40e_ring *ring,
if (tx_buffer->tx_flags & I40E_TX_FLAGS_FD_SB)
kfree(tx_buffer->raw_buf);
else if (ring_is_xdp(ring))
- xdp_return_frame(tx_buffer->xdpf->data,
- &tx_buffer->xdpf->mem);
+ xdp_return_frame(tx_buffer->xdpf);
else
dev_kfree_skb_any(tx_buffer->skb);
if (dma_unmap_len(tx_buffer, len))
@@ -842,7 +841,7 @@ static bool i40e_clean_tx_irq(struct i40e_vsi *vsi,

/* free the skb/XDP data */
if (ring_is_xdp(tx_ring))
- xdp_return_frame(tx_buf->xdpf->data, &tx_buf->xdpf->mem);
+ xdp_return_frame(tx_buf->xdpf);
else
napi_consume_skb(tx_buf->skb, napi_budget);

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe.h b/drivers/net/ethernet/intel/ixgbe/ixgbe.h
index abb5248e917e..7dd5038cfcc4 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe.h
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe.h
@@ -241,8 +241,7 @@ struct ixgbe_tx_buffer {
unsigned long time_stamp;
union {
struct sk_buff *skb;
- /* XDP uses address ptr on irq_clean */
- void *data;
+ struct xdp_frame *xdpf;
};
unsigned int bytecount;
unsigned short gso_segs;
@@ -250,7 +249,6 @@ struct ixgbe_tx_buffer {
DEFINE_DMA_UNMAP_ADDR(dma);
DEFINE_DMA_UNMAP_LEN(len);
u32 tx_flags;
- struct xdp_mem_info xdp_mem;
};

struct ixgbe_rx_buffer {
diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index f10904ec2172..4f2864165723 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -1216,7 +1216,7 @@ static bool ixgbe_clean_tx_irq(struct ixgbe_q_vector *q_vector,

/* free the skb */
if (ring_is_xdp(tx_ring))
- xdp_return_frame(tx_buffer->data, &tx_buffer->xdp_mem);
+ xdp_return_frame(tx_buffer->xdpf);
else
napi_consume_skb(tx_buffer->skb, napi_budget);

@@ -2386,6 +2386,7 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
xdp.data_hard_start = xdp.data -
ixgbe_rx_offset(rx_ring);
xdp.data_end = xdp.data + size;
+ prefetchw(xdp.data_hard_start); /* xdp_frame write */

skb = ixgbe_run_xdp(adapter, rx_ring, &xdp);
}
@@ -5797,7 +5798,7 @@ static void ixgbe_clean_tx_ring(struct ixgbe_ring *tx_ring)

/* Free all the Tx ring sk_buffs */
if (ring_is_xdp(tx_ring))
- xdp_return_frame(tx_buffer->data, &tx_buffer->xdp_mem);
+ xdp_return_frame(tx_buffer->xdpf);
else
dev_kfree_skb_any(tx_buffer->skb);

@@ -8348,16 +8349,21 @@ static int ixgbe_xmit_xdp_ring(struct ixgbe_adapter *adapter,
struct ixgbe_ring *ring = adapter->xdp_ring[smp_processor_id()];
struct ixgbe_tx_buffer *tx_buffer;
union ixgbe_adv_tx_desc *tx_desc;
+ struct xdp_frame *xdpf;
u32 len, cmd_type;
dma_addr_t dma;
u16 i;

- len = xdp->data_end - xdp->data;
+ xdpf = convert_to_xdp_frame(xdp);
+ if (unlikely(!xdpf))
+ return -EOVERFLOW;
+
+ len = xdpf->len;

if (unlikely(!ixgbe_desc_unused(ring)))
return IXGBE_XDP_CONSUMED;

- dma = dma_map_single(ring->dev, xdp->data, len, DMA_TO_DEVICE);
+ dma = dma_map_single(ring->dev, xdpf->data, len, DMA_TO_DEVICE);
if (dma_mapping_error(ring->dev, dma))
return IXGBE_XDP_CONSUMED;

@@ -8372,8 +8378,7 @@ static int ixgbe_xmit_xdp_ring(struct ixgbe_adapter *adapter,

dma_unmap_len_set(tx_buffer, len, len);
dma_unmap_addr_set(tx_buffer, dma, dma);
- tx_buffer->data = xdp->data;
- tx_buffer->xdp_mem = xdp->rxq->mem;
+ tx_buffer->xdpf = xdpf;

tx_desc->read.buffer_addr = cpu_to_le64(dma);

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
index 3f21e4badfc3..33f3cd207f3b 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
@@ -893,6 +893,7 @@ struct sk_buff *skb_from_cqe(struct mlx5e_rq *rq, struct mlx5_cqe64 *cqe,

dma_sync_single_range_for_cpu(rq->pdev, di->addr, wi->offset,
frag_size, DMA_FROM_DEVICE);
+ prefetchw(va); /* xdp_frame data area */
prefetch(data);
wi->offset += frag_size;

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index b52d69801b2d..a6a74e896430 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -663,7 +663,7 @@ void tun_ptr_free(void *ptr)
if (tun_is_xdp_frame(ptr)) {
struct xdp_frame *xdpf = tun_ptr_to_xdp(ptr);

- xdp_return_frame(xdpf->data, &xdpf->mem);
+ xdp_return_frame(xdpf);
} else {
__skb_array_destroy_skb(ptr);
}
@@ -2189,7 +2189,7 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
struct xdp_frame *xdpf = tun_ptr_to_xdp(ptr);

ret = tun_put_user_xdp(tun, tfile, xdpf, to);
- xdp_return_frame(xdpf->data, &xdpf->mem);
+ xdp_return_frame(xdpf);
} else {
struct sk_buff *skb = ptr;

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index aeef92456419..026443ae945f 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -430,7 +430,7 @@ static bool __virtnet_xdp_xmit(struct virtnet_info *vi,

/* Free up any pending old buffers before queueing new ones. */
while ((xdpf_sent = virtqueue_get_buf(sq->vq, &len)) != NULL)
- xdp_return_frame(xdpf_sent->data, &xdpf_sent->mem);
+ xdp_return_frame(xdpf_sent);

xdpf = convert_to_xdp_frame(xdp);
if (unlikely(!xdpf))
diff --git a/include/net/xdp.h b/include/net/xdp.h
index d0ee437753dc..137ad5f9f40f 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -103,7 +103,7 @@ struct xdp_frame *convert_to_xdp_frame(struct xdp_buff *xdp)
return xdp_frame;
}

-void xdp_return_frame(void *data, struct xdp_mem_info *mem);
+void xdp_return_frame(struct xdp_frame *xdpf);

int xdp_rxq_info_reg(struct xdp_rxq_info *xdp_rxq,
struct net_device *dev, u32 queue_index);
diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
index bcdc4dea5ce7..c95b04ec103e 100644
--- a/kernel/bpf/cpumap.c
+++ b/kernel/bpf/cpumap.c
@@ -219,7 +219,7 @@ static void __cpu_map_ring_cleanup(struct ptr_ring *ring)

while ((xdpf = ptr_ring_consume(ring)))
if (WARN_ON_ONCE(xdpf))
- xdp_return_frame(xdpf->data, &xdpf->mem);
+ xdp_return_frame(xdpf);
}

static void put_cpu_map_entry(struct bpf_cpu_map_entry *rcpu)
@@ -275,7 +275,7 @@ static int cpu_map_kthread_run(void *data)

skb = cpu_map_build_skb(rcpu, xdpf);
if (!skb) {
- xdp_return_frame(xdpf->data, &xdpf->mem);
+ xdp_return_frame(xdpf);
continue;
}

@@ -578,7 +578,7 @@ static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu,
err = __ptr_ring_produce(q, xdpf);
if (err) {
drops++;
- xdp_return_frame(xdpf->data, &xdpf->mem);
+ xdp_return_frame(xdpf);
}
processed++;
}
diff --git a/net/core/xdp.c b/net/core/xdp.c
index d8115903e454..6247aa8a04fc 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -308,9 +308,11 @@ int xdp_rxq_info_reg_mem_model(struct xdp_rxq_info *xdp_rxq,
}
EXPORT_SYMBOL_GPL(xdp_rxq_info_reg_mem_model);

-void xdp_return_frame(void *data, struct xdp_mem_info *mem)
+void xdp_return_frame(struct xdp_frame *xdpf)
{
+ struct xdp_mem_info *mem = &xdpf->mem;
struct xdp_mem_allocator *xa;
+ void *data = xdpf->data;
struct page *page;

switch (mem->type) {