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Date:   Wed, 13 Feb 2019 10:07:23 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, ast@...nel.org,
        Yonghong Song <yhs@...com>, brakmo@...com,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, shaoyafang@...iglobal.com
Subject: Re: [bpf-next 1/2] tcp: replace SOCK_DEBUG() with tcp_stats()

On Tue, Feb 12, 2019 at 11:07 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 02/12/2019 03:31 AM, Yafang Shao wrote:
> > SOCK_DEBUG is a very ancient debugging interface, and it's not very useful
> > for debugging.
> > So this patch removes the SOCK_DEBUG() and introduce a new function
> > tcp_stats() to trace this kind of events.
> > Some MIBs are added for these events.
> >
> > Regarding the SO_DEBUG in sock_{s,g}etsockopt, I think it is better to
> > keep as-is, because if we return an errno to tell the application that
> > this optname isn't supported for TCP, it may break the application.
> > The application still can use this option but don't take any effect for
> > TCP.
> >
> > Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> > ---
> >  include/uapi/linux/snmp.h |  3 +++
> >  net/ipv4/proc.c           |  3 +++
> >  net/ipv4/tcp_input.c      | 26 +++++++++++---------------
> >  net/ipv6/tcp_ipv6.c       |  2 --
> >  4 files changed, 17 insertions(+), 17 deletions(-)
> >
> > diff --git a/include/uapi/linux/snmp.h b/include/uapi/linux/snmp.h
> > index 86dc24a..fd5c09c 100644
> > --- a/include/uapi/linux/snmp.h
> > +++ b/include/uapi/linux/snmp.h
> > @@ -283,6 +283,9 @@ enum
> >       LINUX_MIB_TCPACKCOMPRESSED,             /* TCPAckCompressed */
> >       LINUX_MIB_TCPZEROWINDOWDROP,            /* TCPZeroWindowDrop */
> >       LINUX_MIB_TCPRCVQDROP,                  /* TCPRcvQDrop */
> > +     LINUX_MIB_TCPINVALIDACK,                /* TCPInvalidAck */
> > +     LINUX_MIB_TCPOLDACK,                    /* TCPOldAck */
> > +     LINUX_MIB_TCPPARTIALPACKET,             /* TCPPartialPacket */
> >       __LINUX_MIB_MAX
> >  };
> >
> > diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
> > index c3610b3..1b0320a 100644
> > --- a/net/ipv4/proc.c
> > +++ b/net/ipv4/proc.c
> > @@ -291,6 +291,9 @@ static int sockstat_seq_show(struct seq_file *seq, void *v)
> >       SNMP_MIB_ITEM("TCPAckCompressed", LINUX_MIB_TCPACKCOMPRESSED),
> >       SNMP_MIB_ITEM("TCPZeroWindowDrop", LINUX_MIB_TCPZEROWINDOWDROP),
> >       SNMP_MIB_ITEM("TCPRcvQDrop", LINUX_MIB_TCPRCVQDROP),
> > +     SNMP_MIB_ITEM("TCPInvalidAck", LINUX_MIB_TCPINVALIDACK),
> > +     SNMP_MIB_ITEM("TCPOldAck", LINUX_MIB_TCPOLDACK),
> > +     SNMP_MIB_ITEM("TCPPartialPacket", LINUX_MIB_TCPPARTIALPACKET),
> >       SNMP_MIB_SENTINEL
> >  };
> >
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index 7a027dec..88deb1f 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -3554,6 +3554,11 @@ static u32 tcp_newly_delivered(struct sock *sk, u32 prior_delivered, int flag)
> >       return delivered;
> >  }
> >
> > +static void tcp_stats(struct sock *sk, int mib_idx)
> > +{
> > +     NET_INC_STATS(sock_net(sk), mib_idx);
> > +}
>
> This is not a very descriptive name.
>
> Why is it static, and in net/ipv4/tcp_input.c ???
>

Because it is only called in net/ipv4/tcp_input.c currently, so I
define it as static in this file,
the reseaon I don't define it as 'static inline' is that I think the
compiler can make a better decision than me.

In the future it may be called in other files, then we can put it into
a more proper file.

> > +
> >  /* This routine deals with incoming acks, but not outgoing ones. */
> >  static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
> >  {
> > @@ -3715,7 +3720,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
> >       return 1;
> >
> >  invalid_ack:
> > -     SOCK_DEBUG(sk, "Ack %u after %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
> > +     tcp_stats(sk, LINUX_MIB_TCPINVALIDACK);
> >       return -1;
> >
> >  old_ack:
> > @@ -3731,7 +3736,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
> >               tcp_xmit_recovery(sk, rexmit);
> >       }
> >
> > -     SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
> > +     tcp_stats(sk, LINUX_MIB_TCPOLDACK);
> >       return 0;
> >  }
> >
>
>
> These counters will add noise to an already crowded MIB space.
>

I have another idea that we can define some tcp bpf events to replace
these MIB counters somehing like,
    #define BPF_EVENT_TCP_OLDACK 1
    #define BPF_EVENT_TCP_PARTIALPACKET 2
    ...
Maybe we could also cleanup some MIBs to make it less crowded.

> What bug do you expect to track and fix with these ?
>

Let me explain the background for you.
I want to track some TCP abnormal  behavior in TCP/IP stack. But I
find there's no good way to do it.
The current MIBs are per net, other than per socket, that makes it not
very powerful.
And the ancient SOCK_DEBUG is not good as well.
So we think why not cleanup this ancient SOCK_DEBUG() and introduce a
more powerful method.

> I see many TCP patches coming adding icache pressure, enabling companies to build their own modified
> TCP stack, but no real meat.
>

Thanks
Yafang

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