Re: [PATCH V3 net-next] net: fec: add XDP_TX feature support

Jesper Dangaard Brouer
2023-08-03T07:40:06Z

On 03/08/2023 05.58, Wei Fang wrote:
>>> } else {
>>> - xdp_return_frame(xdpf);
>>> + xdp_return_frame_rx_napi(xdpf);
>>
>> If you implement Jesper's syncing suggestions, I think you can use
>>
>> page_pool_put_page(pool, page, 0, true);

To Jakub, using 0 here you are trying to bypass the DMA-sync (which is
valid as driver knows XDP_TX have already done the sync).
The code will still call into DMA-sync calls with zero as size, so
wonder if we should detect size zero and skip that call?
(I mean is this something page_pool should support.)

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 7ca456bfab71..778d061e4f2c 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -323,7 +323,8 @@ static void page_pool_dma_sync_for_device(struct
page_pool *pool,
dma_addr_t dma_addr = page_pool_get_dma_addr(page);

dma_sync_size = min(dma_sync_size, pool->p.max_len);
- dma_sync_single_range_for_device(pool->p.dev, dma_addr,
+ if (dma_sync_size)
+ dma_sync_single_range_for_device(pool->p.dev, dma_addr,
pool->p.offset, dma_sync_size,
pool->p.dma_dir);

>>
>> for XDP_TX here to avoid the DMA sync on page recycle.
>
> I tried Jasper's syncing suggestion and used page_pool_put_page() to recycle
> pages, but the results does not seem to improve the performance of XDP_TX,

The optimization will only have effect on those devices which have
dev->dma_coherent=false else DMA function [1] (e.g.
dma_direct_sync_single_for_device) will skip the sync calls.

[1]
https://elixir.bootlin.com/linux/v6.5-rc4/source/kernel/dma/direct.h#L63

(Cc. Andrew Lunn)
Does any of the imx generations have dma-noncoherent memory?

And does any of these use the fec NIC driver?

> it even degrades the speed.

Could be low runs simply be a variation between your test runs?

The specific device (imx8mpevk) this was tested on, clearly have
dma_coherent=true, or else we would have seen a difference.
But the code change should not have any overhead for the
dma_coherent=true case, the only extra overhead is the extra empty DMA
sync call with size zero (as discussed in top).

>
> The result of the current modification.
> root@imx8mpevk:~# ./xdp2 eth0
> proto 17: 260180 pkt/s

These results are *significantly* better than reported in patch-1.
What happened?!?

e.g.
root@imx8mpevk:~# ./xdp2 eth0
proto 17: 135817 pkt/s
proto 17: 142776 pkt/s

> proto 17: 260373 pkt/s
> proto 17: 260363 pkt/s
> proto 17: 259036 pkt/s
> proto 17: 260180 pkt/s
> proto 17: 260048 pkt/s
> proto 17: 260029 pkt/s
> proto 17: 260133 pkt/s
> proto 17: 260021 pkt/s
> proto 17: 260203 pkt/s
> proto 17: 260293 pkt/s
> proto 17: 259418 pkt/s
>
> After using the sync suggestion, the result shows as follow.
> root@imx8mpevk:~# ./xdp2 eth0
> proto 17: 255956 pkt/s
> proto 17: 255841 pkt/s
> proto 17: 255835 pkt/s
> proto 17: 255381 pkt/s
> proto 17: 255736 pkt/s
> proto 17: 255779 pkt/s
> proto 17: 254135 pkt/s
> proto 17: 255584 pkt/s
> proto 17: 255855 pkt/s
> proto 17: 255664 pkt/s
>
> Below are my changes, I don't know what cause it. Based on the results,
> it's better to keep the current modification.
>
> diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
> index d5fda24a4c52..415c0cb83f84 100644
> --- a/drivers/net/ethernet/freescale/fec_main.c
> +++ b/drivers/net/ethernet/freescale/fec_main.c
> @@ -77,7 +77,8 @@
> static void set_multicast_list(struct net_device *ndev);
> static void fec_enet_itr_coal_set(struct net_device *ndev);
> static int fec_enet_xdp_tx_xmit(struct net_device *ndev,
> - struct xdp_buff *xdp);
> + struct xdp_buff *xdp,
> + u32 dma_sync_len);
>
> #define DRIVER_NAME "fec"
>
> @@ -1487,7 +1488,14 @@ fec_enet_tx_queue(struct net_device *ndev, u16 queue_id, int budget)
> /* Free the sk buffer associated with this last transmit */
> dev_kfree_skb_any(skb);
> } else {
> - xdp_return_frame_rx_napi(xdpf);
> + if (txq->tx_buf[index].type == FEC_TXBUF_T_XDP_NDO)
> + xdp_return_frame_rx_napi(xdpf);
> + else {
> + struct page *page;
> +
> + page = virt_to_head_page(xdpf->data);
> + page_pool_put_page(page->pp, page, 0, true);
> + }
>
> txq->tx_buf[index].xdp = NULL;
> /* restore default tx buffer type: FEC_TXBUF_T_SKB */
> @@ -1557,7 +1565,8 @@ fec_enet_run_xdp(struct fec_enet_private *fep, struct bpf_prog *prog,
> act = bpf_prog_run_xdp(prog, xdp);
>
> /* Due xdp_adjust_tail: DMA sync for_device cover max len CPU touch */
> - sync = xdp->data_end - xdp->data_hard_start - FEC_ENET_XDP_HEADROOM;
> + sync = xdp->data_end - xdp->data;
> sync = max(sync, len);
>
> switch (act) {
> @@ -1579,7 +1588,7 @@ fec_enet_run_xdp(struct fec_enet_private *fep, struct bpf_prog *prog,
> break;
>
> case XDP_TX:
> - err = fec_enet_xdp_tx_xmit(fep->netdev, xdp);
> + err = fec_enet_xdp_tx_xmit(fep->netdev, xdp, sync);
> if (unlikely(err)) {
> ret = FEC_ENET_XDP_CONSUMED;
> page = virt_to_head_page(xdp->data);
> @@ -3807,6 +3816,7 @@ fec_enet_xdp_get_tx_queue(struct fec_enet_private *fep, int index)
> static int fec_enet_txq_xmit_frame(struct fec_enet_private *fep,
> struct fec_enet_priv_tx_q *txq,
> struct xdp_frame *frame,
> + u32 dma_sync_len,
> bool ndo_xmit)
> {
> unsigned int index, status, estatus;
> @@ -3840,7 +3850,7 @@ static int fec_enet_txq_xmit_frame(struct fec_enet_private *fep,
> dma_addr = page_pool_get_dma_addr(page) + sizeof(*frame) +
> frame->headroom;
> dma_sync_single_for_device(&fep->pdev->dev, dma_addr,
> - frame->len, DMA_BIDIRECTIONAL);
> + dma_sync_len, DMA_BIDIRECTIONAL);
> txq->tx_buf[index].type = FEC_TXBUF_T_XDP_TX;
> }
>
> @@ -3889,7 +3899,8 @@ static int fec_enet_txq_xmit_frame(struct fec_enet_private *fep,
> }
>
> static int fec_enet_xdp_tx_xmit(struct net_device *ndev,
> - struct xdp_buff *xdp)
> + struct xdp_buff *xdp,
> + u32 dma_sync_len)
> {
> struct xdp_frame *xdpf = xdp_convert_buff_to_frame(xdp);
> struct fec_enet_private *fep = netdev_priv(ndev);
> @@ -3909,7 +3920,7 @@ static int fec_enet_xdp_tx_xmit(struct net_device *ndev,
>
> /* Avoid tx timeout as XDP shares the queue with kernel stack */
> txq_trans_cond_update(nq);
> - ret = fec_enet_txq_xmit_frame(fep, txq, xdpf, false);
> + ret = fec_enet_txq_xmit_frame(fep, txq, xdpf, dma_sync_len, false);
>
> __netif_tx_unlock(nq);
>
> @@ -3938,7 +3949,7 @@ static int fec_enet_xdp_xmit(struct net_device *dev,
> /* Avoid tx timeout as XDP shares the queue with kernel stack */
> txq_trans_cond_update(nq);
> for (i = 0; i < num_frames; i++) {
> - if (fec_enet_txq_xmit_frame(fep, txq, frames[i], true) < 0)
> + if (fec_enet_txq_xmit_frame(fep, txq, frames[i], 0, true) < 0)
> break;
> sent_frames++;
> }
>