netdev - Re: [PATCH v6 net-next 10/13] net: Postpone skb_clear_delivery_time() until knowing the skb is delivered locally

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Message-ID: <CANn89i+ZLB8EK2CUC7dnERvcawSAOhpzHpeKSvL0dVfK-fusXg@mail.gmail.com>
Date:   Wed, 2 Mar 2022 15:41:59 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        kernel-team <kernel-team@...com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v6 net-next 10/13] net: Postpone skb_clear_delivery_time()
 until knowing the skb is delivered locally

On Wed, Mar 2, 2022 at 2:34 PM Martin KaFai Lau <kafai@...com> wrote:
>
> On Wed, Mar 02, 2022 at 12:30:14PM -0800, Eric Dumazet wrote:
> > On Wed, Mar 2, 2022 at 11:56 AM Martin KaFai Lau <kafai@...com> wrote:
> > >
> > > The previous patches handled the delivery_time in the ingress path
> > > before the routing decision is made.  This patch can postpone clearing
> > > delivery_time in a skb until knowing it is delivered locally and also
> > > set the (rcv) timestamp if needed.  This patch moves the
> > > skb_clear_delivery_time() from dev.c to ip_local_deliver_finish()
> > > and ip6_input_finish().
> > >
> > > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > > ---
> > >  net/core/dev.c       | 8 ++------
> > >  net/ipv4/ip_input.c  | 1 +
> > >  net/ipv6/ip6_input.c | 1 +
> > >  3 files changed, 4 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index 0fc02cf32476..3ff686cc8c84 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -5193,10 +5193,8 @@ static int __netif_receive_skb_core(struct sk_buff **pskb, bool pfmemalloc,
> > >                         goto out;
> > >         }
> > >
> > > -       if (skb_skip_tc_classify(skb)) {
> > > -               skb_clear_delivery_time(skb);
> > > +       if (skb_skip_tc_classify(skb))
> > >                 goto skip_classify;
> > > -       }
> > >
> > >         if (pfmemalloc)
> > >                 goto skip_taps;
> > > @@ -5225,14 +5223,12 @@ static int __netif_receive_skb_core(struct sk_buff **pskb, bool pfmemalloc,
> > >                         goto another_round;
> > >                 if (!skb)
> > >                         goto out;
> > > -               skb_clear_delivery_time(skb);
> > >
> > >                 nf_skip_egress(skb, false);
> > >                 if (nf_ingress(skb, &pt_prev, &ret, orig_dev) < 0)
> > >                         goto out;
> > > -       } else
> > > +       }
> > >  #endif
> > > -               skb_clear_delivery_time(skb);
> > >         skb_reset_redirect(skb);
> > >  skip_classify:
> > >         if (pfmemalloc && !skb_pfmemalloc_protocol(skb))
> > > diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
> > > index d94f9f7e60c3..95f7bb052784 100644
> > > --- a/net/ipv4/ip_input.c
> > > +++ b/net/ipv4/ip_input.c
> > > @@ -226,6 +226,7 @@ void ip_protocol_deliver_rcu(struct net *net, struct sk_buff *skb, int protocol)
> > >
> > >  static int ip_local_deliver_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
> > >  {
> > > +       skb_clear_delivery_time(skb);
> > >         __skb_pull(skb, skb_network_header_len(skb));
> > >
> > >         rcu_read_lock();
> > > diff --git a/net/ipv6/ip6_input.c b/net/ipv6/ip6_input.c
> > > index d4b1e2c5aa76..5b5ea35635f9 100644
> > > --- a/net/ipv6/ip6_input.c
> > > +++ b/net/ipv6/ip6_input.c
> > > @@ -459,6 +459,7 @@ void ip6_protocol_deliver_rcu(struct net *net, struct sk_buff *skb, int nexthdr,
> > >
> > >  static int ip6_input_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
> > >  {
> > > +       skb_clear_delivery_time(skb);
> > >         rcu_read_lock();
> > >         ip6_protocol_deliver_rcu(net, skb, 0, false);
> > >         rcu_read_unlock();
> > > --
> > > 2.30.2
> > >
> >
> > It is not clear to me why we need to clear tstamp if packet is locally
> > delivered ?
> It does not clear the rx tstamp in skb->tstamp.
>
> It only clears the EDT in skb->tstamp when the skb
> is transmitted out of a local tcp_sock and then loop back from egress
> to ingress through virtual interface like veth.
>
> >
> > TCP stack is using tstamp for incoming packets (look for
> > TCP_SKB_CB(skb)->has_rxtstamp)
> skb_clear_delivery_time() will put ktime_get_real() back to skb->tstamp
> so that the receiving tcp_sock can use it.


Oh... I had to look at

+static inline void skb_clear_delivery_time(struct sk_buff *skb)
+{
+       if (skb->mono_delivery_time) {
+               skb->mono_delivery_time = 0;
+               if (static_branch_unlikely(&netstamp_needed_key))
+                       skb->tstamp = ktime_get_real();
+               else
+                       skb->tstamp = 0;
+       }
+}

Name was a bit confusing :)

And it seems you have a big opportunity to not call ktime_get_real()
when skb->sk is known at this point (early demux)
because few sockets actually enable timestamping ?