netdev - Re: [PATCH] bpf: Add bpf_ktime_get_real

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Message-Id: <C50F3QS9W4JM.1OIVL1ZHWEIWI@maharaja>
Date:   Tue, 18 Aug 2020 13:50:48 -0700
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     "Andrii Nakryiko" <andrii.nakryiko@...il.com>,
        <bimmy.pujari@...el.com>
Cc:     "bpf" <bpf@...r.kernel.org>, "Networking" <netdev@...r.kernel.org>,
        <mchehab@...nel.org>, "Alexei Starovoitov" <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        "Martin Lau" <kafai@...com>, <maze@...gle.com>,
        <ashkan.nikravesh@...el.com>
Subject: Re: [PATCH] bpf: Add bpf_ktime_get_real_ns

On Mon Jul 27, 2020 at 10:01 PM PDT, Andrii Nakryiko wrote:
> On Mon, Jul 27, 2020 at 4:35 PM <bimmy.pujari@...el.com> wrote:
> >
> > From: Ashkan Nikravesh <ashkan.nikravesh@...el.com>
> >
> > The existing bpf helper functions to get timestamp return the time
> > elapsed since system boot. This timestamp is not particularly useful
> > where epoch timestamp is required or more than one server is involved
> > and time sync is required. Instead, you want to use CLOCK_REALTIME,
> > which provides epoch timestamp.
> > Hence add bfp_ktime_get_real_ns() based around CLOCK_REALTIME.
> >
>
> This doesn't seem like a good idea. With time-since-boot it's very
> easy to translate timestamp into a real time on the host. 

For bpftrace, we have a need to get millisecond-level precision on
timestamps. bpf has nanosecond level precision via
bpf_ktime_get[_boot]_ns(), but to the best of my knowledge userspace
doesn't have a high precision boot timestamp.

There's /proc/stat's btime, but that's second-level precision. There's
also /proc/uptime which has millisecond-level precision but you need to
make a second call to get current time. Between those two calls there
could be some unknown delta. For millisecond we could probably get away
with calculating a delta and warning on large delta but maybe one day
we'll want microsecond-level precision.

I'll probably put up a patch to add nanoseconds to btime (new field in
/proc/stat) to see how it's received. But either this patch or my patch
would work for bpftrace.

[...]

Thanks,
Daniel