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

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Message-ID: <878rrzj4r6.fsf@toke.dk>
Date:   Wed, 20 Apr 2022 14:53:01 +0200
From:   Toke Høiland-Jørgensen <toke@...nel.org>
To:     xiangxia.m.yue@...il.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     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

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?

-Toke