From: "Toke Høiland-Jørgensen" <toke@kernel.org>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
Tonghao Zhang <xiangxia.m.yue@gmail.com>,
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>
Subject: Re: [net-next v1] bpf: add bpf_ktime_get_real_ns helper
Date: Wed, 20 Apr 2022 18:26:57 +0200 [thread overview]
Message-ID: <87wnfjhga6.fsf@toke.dk> (raw)
In-Reply-To: <CAEf4Bzafe3Am5uep7erd7r+-pgdGRc9hsJASYfFH47ty8x9mTA@mail.gmail.com>
Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
> On Wed, Apr 20, 2022 at 5:53 AM Toke Høiland-Jørgensen <toke@kernel.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 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
Uh, neat! Thanks for the link :)
-Toke
next prev parent reply other threads:[~2022-04-20 16:27 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-20 12:23 [net-next v1] bpf: add bpf_ktime_get_real_ns helper xiangxia.m.yue
2022-04-20 12:53 ` Toke Høiland-Jørgensen
2022-04-20 16:17 ` Andrii Nakryiko
2022-04-20 16:26 ` Toke Høiland-Jørgensen [this message]
2022-04-21 2:37 ` Tonghao Zhang
2022-04-21 2:27 ` Tonghao Zhang
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