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References: <20220420122307.5290-1-xiangxia.m.yue@gmail.com> <878rrzj4r6.fsf@toke.dk>
In-Reply-To: <878rrzj4r6.fsf@toke.dk>
From:   Tonghao Zhang <xiangxia.m.yue@gmail.com>
Date:   Thu, 21 Apr 2022 10:27:05 +0800
Message-ID: <CAMDZJNVo1dwUA3gkUc8UKEthDtM8m2yif-4CE7aGxtJyANVgpw@mail.gmail.com>
Subject: Re: [net-next v1] bpf: add bpf_ktime_get_real_ns helper
To:     =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@kernel.org>
Cc:     Linux Kernel Network Developers <netdev@vger.kernel.org>,
        bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Jiri Olsa <jolsa@kernel.org>,
        Dave Marchevsky <davemarchevsky@fb.com>,
        Kuniyuki Iwashima <kuniyu@amazon.co.jp>,
        Joanne Koong <joannekoong@fb.com>,
        Geliang Tang <geliang.tang@suse.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Eric Dumazet <edumazet@google.com>
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On Wed, Apr 20, 2022 at 8:53 PM Toke H=C3=B8iland-J=C3=B8rgensen <toke@kern=
el.org> wrote:
>
> xiangxia.m.yue@gmail.com writes:
>
> > From: Tonghao Zhang <xiangxia.m.yue@gmail.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 helpe=
r.
> >   Then we can inspect how long time elapsed since HW/SW.
> > * If inserting BPF kprobe tcp_update_recv_tstamps invoked by tcp_recvms=
g,
> >   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?
Yes, That also looks good to me.
> -Toke



--=20
Best regards, Tonghao