Re: [bpf-next 1/2] tcp: replace SOCK_DEBUG() with tcp_stats()

[Eric Dumazet <eric.dumazet@gmail.com>]

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@gmail.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 ???

> +
>  /* 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.

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

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