netdev - Re: [PATCH net-next] tcp: add SNMP counter for the number of packets pruned from ofo queue

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Date:   Thu, 26 Jul 2018 12:31:25 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] tcp: add SNMP counter for the number of packets
 pruned from ofo queue

On Thu, Jul 26, 2018 at 12:06 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
> On 07/25/2018 06:42 PM, Yafang Shao wrote:
>
>>
>> Hi Eric,
>>
>> LINUX_MIB_TCPOFOQUEUE, LINUX_MIB_TCPOFODROP and LINUX_MIB_TCPOFOMERGE
>> are all for the number of SKBs, but only  LINUX_MIB_OFOPRUNED is for
>> the event, that could lead misunderstading.
>> So I think introducing a counter for the number of SKB pruned could be
>> better, that could help us to track the whole behavior of ofo queue.
>> That is why I submit this patch.
>
> Sure, and I said your patch had issues.
> You mixed 'packets' and 'skbs' but apparently you do not get my point.
>

I had noticed that I made this mistake.

> I would rather not add another SNMP counter, and refine the current one,
> trying to track something more meaningful.
>
> The notion of 'skb' is internal to the kernel, and can not be mapped easily
> to 'number of network segments' which probably is more useful for the user.
>
> I will send this instead :
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index d51fa358b2b196d0f9c258b24354813f2128a675..141a062abd0660c8f6d049de1dc7c7ecf7a7230d 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5001,18 +5001,19 @@ static bool tcp_prune_ofo_queue(struct sock *sk)
>  {
>         struct tcp_sock *tp = tcp_sk(sk);
>         struct rb_node *node, *prev;
> +       unsigned int segs = 0;
>         int goal;
>
>         if (RB_EMPTY_ROOT(&tp->out_of_order_queue))
>                 return false;
>
> -       NET_INC_STATS(sock_net(sk), LINUX_MIB_OFOPRUNED);
>         goal = sk->sk_rcvbuf >> 3;
>         node = &tp->ooo_last_skb->rbnode;
>         do {
>                 prev = rb_prev(node);
>                 rb_erase(node, &tp->out_of_order_queue);
>                 goal -= rb_to_skb(node)->truesize;
> +               segs += max_t(u16, 1, skb_shinfo(rb_to_skb(node))->gso_segs);
>                 tcp_drop(sk, rb_to_skb(node));
>                 if (!prev || goal <= 0) {
>                         sk_mem_reclaim(sk);
> @@ -5023,6 +5024,7 @@ static bool tcp_prune_ofo_queue(struct sock *sk)
>                 }
>                 node = prev;
>         } while (node);
> +       NET_ADD_STATS(sock_net(sk), LINUX_MIB_OFOPRUNED, segs);
>         tp->ooo_last_skb = rb_to_skb(prev);
>
>         /* Reset SACK state.  A conforming SACK implementation will
>

You patch will make it more meaningful.
How about the other ones?
I think changing all of them from the number of SKBs to the number of
network segments would be better.

Thanks
Yafang