Re: [net-next v1] bpf: add bpf_ktime_get_real_ns helper

["Toke Høiland-Jørgensen" <toke@kernel.org>]

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