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

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

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++;
>          }
>