netdev - Re: [PATCHv4 bpf-next 0/2] xdp: add dev map multicast support

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Date:   Wed, 27 May 2020 20:38:58 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Jiri Benc <jbenc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
Subject: Re: [PATCHv4 bpf-next 0/2] xdp: add dev map multicast support

On Wed, May 27, 2020 at 12:21:54PM +0200, Toke Høiland-Jørgensen wrote:
> > The example in patch 2 is functional, but not a lot of effort
> > has been made on performance optimisation. I did a simple test(pkt size 64)
> > with pktgen. Here is the test result with BPF_MAP_TYPE_DEVMAP_HASH
> > arrays:
> >
> > bpf_redirect_map() with 1 ingress, 1 egress:
> > generic path: ~1600k pps
> > native path: ~980k pps
> >
> > bpf_redirect_map_multi() with 1 ingress, 3 egress:
> > generic path: ~600k pps
> > native path: ~480k pps
> >
> > bpf_redirect_map_multi() with 1 ingress, 9 egress:
> > generic path: ~125k pps
> > native path: ~100k pps
> >
> > The bpf_redirect_map_multi() is slower than bpf_redirect_map() as we loop
> > the arrays and do clone skb/xdpf. The native path is slower than generic
> > path as we send skbs by pktgen. So the result looks reasonable.
> 
> How are you running these tests? Still on virtual devices? We really

I run it with the test topology in patch 2/2. The test is run on physical
machines, but I use veth interface. Do you mean use a physical NIC driver
for testing?


BTW, when using pktgen, I got an panic because the skb don't have enough
header room. The code path looks like

do_xdp_generic()
  - netif_receive_generic_xdp()
    - skb_headroom(skb) < XDP_PACKET_HEADROOM
      - pskb_expand_head()
        - BUG_ON(skb_shared(skb))

So I added a draft patch for pktgen, not sure if it has any influence.

index 08e2811b5274..fee17310c178 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -170,6 +170,7 @@
 #include <linux/uaccess.h>
 #include <asm/dma.h>
 #include <asm/div64.h>         /* do_div */
+#include <linux/bpf.h>

 #define VERSION        "2.75"
 #define IP_NAME_SZ 32
@@ -2692,7 +2693,7 @@ static void pktgen_finalize_skb(struct pktgen_dev *pkt_dev, struct sk_buff *skb,
 static struct sk_buff *pktgen_alloc_skb(struct net_device *dev,
                                        struct pktgen_dev *pkt_dev)
 {
-       unsigned int extralen = LL_RESERVED_SPACE(dev);
+       unsigned int extralen = LL_RESERVED_SPACE(dev) + XDP_PACKET_HEADROOM;
        struct sk_buff *skb = NULL;
        unsigned int size;

> need results from a physical setup in native mode to assess the impact
> on the native-XDP fast path. The numbers above don't tell much in this
> regard. I'd also like to see a before/after patch for straight
> bpf_redirect_map(), since you're messing with the fast path, and we want
> to make sure it's not causing a performance regression for regular
> redirect.

OK, I will write a test with 1 ingress + 1 egress for bpf_redirect_map_multi.
Just as Eelco said.
> 
> Finally, since the overhead seems to be quite substantial: A comparison
> with a regular network stack bridge might make sense? After all we also
> want to make sure it's a performance win over that :)

OK, Will do it.

Thanks
Hangbin