netdev - Re: [PATCH] tcp: Add TCP_INFO counter for packets received out-of-order

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Date:   Tue, 10 Sep 2019 19:59:53 +0000
From:   Thomas Higdon <tph@...com>
To:     Eric Dumazet <edumazet@...gle.com>
CC:     netdev <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Dave Jones <dsj@...com>
Subject: Re: [PATCH] tcp: Add TCP_INFO counter for packets received
 out-of-order

On Mon, Sep 09, 2019 at 05:01:46PM +0200, Eric Dumazet wrote:
> On Mon, Sep 9, 2019 at 4:30 PM Thomas Higdon <tph@...com> wrote:
> > diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> > index b3564f85a762..20237987ccc8 100644
> > --- a/include/uapi/linux/tcp.h
> > +++ b/include/uapi/linux/tcp.h
> > @@ -270,6 +270,8 @@ struct tcp_info {
> >         __u64   tcpi_bytes_retrans;  /* RFC4898 tcpEStatsPerfOctetsRetrans */
> >         __u32   tcpi_dsack_dups;     /* RFC4898 tcpEStatsStackDSACKDups */
> >         __u32   tcpi_reord_seen;     /* reordering events seen */
> > +
> > +       __u32   tcpi_rcv_ooopack;    /* Out-of-order packets received */
> 
> This is problematic : you create a 32bit hole in this structure that
> we will never be able to fill.
> 
> We need to add another metric here so that the whole 64bit space is used.

I don't have another metric to add currently. Perhaps I could first place
a '__u32 _reserved' member so that someone else may replace it with
a 32-bit member in the future. Unless there is a canonical way to do
this? I couldn't find any prior examples.

> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index 706cbb3b2986..2774680c5d05 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -4555,6 +4555,7 @@ static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
> >         tp->pred_flags = 0;
> >         inet_csk_schedule_ack(sk);
> >
> > +       tp->rcv_ooopack++;
> 
> We count skbs or we count segments ?
> 
> (GRO might have aggregated multiple segments)

Let's count segments -- I will copy the technique of tcp_segs_in(), which
checks the maximum of 1 and gso_segs from the lower layer. Interestingly,
on my development machine, which uses the virtio-net driver, when LRO is
enabled, gso_segs is always zero, even when an aggregated segment is
passed up the stack. I guess this may be a problem with virtio-net? It
still seems that using gso_segs here is the correct solution.