netdev - Re: [net-next v1] bpf: add bpf_ktime_get_real

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Message-ID: <CAEf4Bzafe3Am5uep7erd7r+-pgdGRc9hsJASYfFH47ty8x9mTA@mail.gmail.com>
Date:   Wed, 20 Apr 2022 09:17:48 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...nel.org>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Tonghao Zhang <xiangxia.m.yue@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Dave Marchevsky <davemarchevsky@...com>,
        Kuniyuki Iwashima <kuniyu@...zon.co.jp>,
        Joanne Koong <joannekoong@...com>,
        Geliang Tang <geliang.tang@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [net-next v1] bpf: add bpf_ktime_get_real_ns helper

On Wed, Apr 20, 2022 at 5:53 AM Toke Høiland-Jørgensen <toke@...nel.org> wrote:
>
> xiangxia.m.yue@...il.com writes:
>
> > From: Tonghao Zhang <xiangxia.m.yue@...il.com>
> >
> > This patch introduce a new bpf_ktime_get_real_ns helper, which may
> > help us to measure the skb latency in the ingress/forwarding path:
> >
> > HW/SW[1] -> ip_rcv/tcp_rcv_established -> tcp_recvmsg_locked/tcp_update_recv_tstamps
> >
> > * Insert BPF kprobe into ip_rcv/tcp_rcv_established invoking this helper.
> >   Then we can inspect how long time elapsed since HW/SW.
> > * If inserting BPF kprobe tcp_update_recv_tstamps invoked by tcp_recvmsg,
> >   we can measure how much latency skb in tcp receive queue. The reason for
> >   this can be application fetch the TCP messages too late.
>
> Why not just use one of the existing ktime helpers and also add a BPF
> probe to set the initial timestamp instead of relying on skb->tstamp?
>

You don't even need a BPF probe for this. See [0] for how retsnoop is
converting bpf_ktime_get_ns() into real time.

  [0] https://github.com/anakryiko/retsnoop/blob/master/src/retsnoop.c#L649-L668

> -Toke