netdev - Re: [PATCH v6 04/12] net/mlx4_en: add support for fast rx drop bpf program

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Date:	Mon, 11 Jul 2016 14:48:17 +0300
From:	Saeed Mahameed <saeedm@....mellanox.co.il>
To:	Brenden Blanco <bblanco@...mgrid.com>
Cc:	Tariq Toukan <ttoukan.linux@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Martin KaFai Lau <kafai@...com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Ari Saha <as754m@....com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Or Gerlitz <gerlitz.or@...il.com>,
	john fastabend <john.fastabend@...il.com>,
	hannes@...essinduktion.org, Thomas Graf <tgraf@...g.ch>,
	Tom Herbert <tom@...bertland.com>,
	Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH v6 04/12] net/mlx4_en: add support for fast rx drop bpf program

On Sun, Jul 10, 2016 at 7:05 PM, Brenden Blanco <bblanco@...mgrid.com> wrote:
> On Sun, Jul 10, 2016 at 06:25:40PM +0300, Tariq Toukan wrote:
>>
>> On 09/07/2016 10:58 PM, Saeed Mahameed wrote:
>> >On Fri, Jul 8, 2016 at 5:15 AM, Brenden Blanco <bblanco@...mgrid.com> wrote:
>> >>+               /* A bpf program gets first chance to drop the packet. It may
>> >>+                * read bytes but not past the end of the frag.
>> >>+                */
>> >>+               if (prog) {
>> >>+                       struct xdp_buff xdp;
>> >>+                       dma_addr_t dma;
>> >>+                       u32 act;
>> >>+
>> >>+                       dma = be64_to_cpu(rx_desc->data[0].addr);
>> >>+                       dma_sync_single_for_cpu(priv->ddev, dma,
>> >>+                                               priv->frag_info[0].frag_size,
>> >>+                                               DMA_FROM_DEVICE);
>> >In case of XDP_PASS we will dma_sync again in the normal path, this
>> >can be improved by doing the dma_sync as soon as we can and once and
>> >for all, regardless of the path the packet is going to take
>> >(XDP_DROP/mlx4_en_complete_rx_desc/mlx4_en_rx_skb).
>> I agree with Saeed, dma_sync is a heavy operation that is now done
>> twice for all packets with XDP_PASS.
>> We should try our best to avoid performance degradation in the flow
>> of unfiltered packets.
> Makes sense, do folks here see a way to do this cleanly?

yes, we need something like:

+static inline void
+mlx4_en_sync_dma(struct mlx4_en_priv *priv,
+                struct mlx4_en_rx_desc *rx_desc,
+                int length)
+{
+       dma_addr_t dma;
+
+       /* Sync dma addresses from HW descriptor */
+       for (nr = 0; nr < priv->num_frags; nr++) {
+               struct mlx4_en_frag_info *frag_info = &priv->frag_info[nr];
+
+               if (length <= frag_info->frag_prefix_size)
+                       break;
+
+               dma = be64_to_cpu(rx_desc->data[nr].addr);
+               dma_sync_single_for_cpu(priv->ddev, dma, frag_info->frag_size,
+                                       DMA_FROM_DEVICE);
+       }
+}


@@ -790,6 +808,10 @@ int mlx4_en_process_rx_cq(struct net_device *dev,
struct mlx4_en_cq *cq, int bud
                        goto next;
                }

+               length = be32_to_cpu(cqe->byte_cnt);
+               length -= ring->fcs_del;
+
+               mlx4_en_sync_dma(priv,rx_desc, length);
                 /* data is available continue processing the packet */

and make sure to remove all explicit dma_sync_single_for_cpu calls.