netdev - Re: [bpf-next PATCH v2 5/5] selftest/bpf: Use global variables instead of maps for test_tcpbpf

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Date:   Tue, 3 Nov 2020 07:42:46 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Kernel Team <kernel-team@...com>,
        Netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Lawrence Brakmo <brakmo@...com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        alexanderduyck@...com
Subject: Re: [bpf-next PATCH v2 5/5] selftest/bpf: Use global variables
 instead of maps for test_tcpbpf_kern

On Mon, Nov 2, 2020 at 5:26 PM Martin KaFai Lau <kafai@...com> wrote:
>
> On Sat, Oct 31, 2020 at 11:52:37AM -0700, Alexander Duyck wrote:
> [ ... ]
>
> > +struct tcpbpf_globals global = { 0 };
> >  int _version SEC("version") = 1;
> >
> >  SEC("sockops")
> > @@ -105,29 +72,15 @@ int bpf_testcb(struct bpf_sock_ops *skops)
> >
> >       op = (int) skops->op;
> >
> > -     update_event_map(op);
> > +     global.event_map |= (1 << op);
> >
> >       switch (op) {
> >       case BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB:
> >               /* Test failure to set largest cb flag (assumes not defined) */
> > -             bad_call_rv = bpf_sock_ops_cb_flags_set(skops, 0x80);
> > +             global.bad_cb_test_rv = bpf_sock_ops_cb_flags_set(skops, 0x80);
> >               /* Set callback */
> > -             good_call_rv = bpf_sock_ops_cb_flags_set(skops,
> > +             global.good_cb_test_rv = bpf_sock_ops_cb_flags_set(skops,
> >                                                BPF_SOCK_OPS_STATE_CB_FLAG);
> > -             /* Update results */
> > -             {
> > -                     __u32 key = 0;
> > -                     struct tcpbpf_globals g, *gp;
> > -
> > -                     gp = bpf_map_lookup_elem(&global_map, &key);
> > -                     if (!gp)
> > -                             break;
> > -                     g = *gp;
> > -                     g.bad_cb_test_rv = bad_call_rv;
> > -                     g.good_cb_test_rv = good_call_rv;
> > -                     bpf_map_update_elem(&global_map, &key, &g,
> > -                                         BPF_ANY);
> > -             }
> >               break;
> >       case BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB:
> >               skops->sk_txhash = 0x12345f;
> > @@ -143,10 +96,8 @@ int bpf_testcb(struct bpf_sock_ops *skops)
> >
> >                               thdr = (struct tcphdr *)(header + offset);
> >                               v = thdr->syn;
> > -                             __u32 key = 1;
> >
> > -                             bpf_map_update_elem(&sockopt_results, &key, &v,
> > -                                                 BPF_ANY);
> > +                             global.tcp_saved_syn = v;
> >                       }
> >               }
> >               break;
> > @@ -156,25 +107,16 @@ int bpf_testcb(struct bpf_sock_ops *skops)
> >               break;
> >       case BPF_SOCK_OPS_STATE_CB:
> >               if (skops->args[1] == BPF_TCP_CLOSE) {
> > -                     __u32 key = 0;
> > -                     struct tcpbpf_globals g, *gp;
> > -
> > -                     gp = bpf_map_lookup_elem(&global_map, &key);
> > -                     if (!gp)
> > -                             break;
> > -                     g = *gp;
> >                       if (skops->args[0] == BPF_TCP_LISTEN) {
> > -                             g.num_listen++;
> > +                             global.num_listen++;
> >                       } else {
> > -                             g.total_retrans = skops->total_retrans;
> > -                             g.data_segs_in = skops->data_segs_in;
> > -                             g.data_segs_out = skops->data_segs_out;
> > -                             g.bytes_received = skops->bytes_received;
> > -                             g.bytes_acked = skops->bytes_acked;
> > +                             global.total_retrans = skops->total_retrans;
> > +                             global.data_segs_in = skops->data_segs_in;
> > +                             global.data_segs_out = skops->data_segs_out;
> > +                             global.bytes_received = skops->bytes_received;
> > +                             global.bytes_acked = skops->bytes_acked;
> >                       }
> > -                     g.num_close_events++;
> > -                     bpf_map_update_elem(&global_map, &key, &g,
> > -                                         BPF_ANY);
> It is interesting that there is no race in the original "g.num_close_events++"
> followed by the bpf_map_update_elem().  It seems quite fragile though.

How would it race with the current code though? At this point we are
controlling the sockets in a single thread. As such the close events
should already be serialized shouldn't they? This may have been a
problem with the old code, but even then it was only two sockets so I
don't think there was much risk of them racing against each other
since the two sockets were linked anyway.

> > +                     global.num_close_events++;
> There is __sync_fetch_and_add().
>
> not sure about the global.event_map though, may be use an individual
> variable for each _CB.  Thoughts?

I think this may be overkill for what we actually need. Since we are
closing the sockets in a single threaded application there isn't much
risk of the sockets all racing against each other in the close is
there?