Re: [PATCH] bpf: Add bpf_ktime_get_real_ns

["Maciej Żenczykowski" <maze@google.com>]

On Tue, Aug 18, 2020 at 2:11 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> On Mon Jul 27, 2020 at 10:01 PM PDT, Andrii Nakryiko wrote:
> > On Mon, Jul 27, 2020 at 4:35 PM <bimmy.pujari@intel.com> wrote:
> > >
> > > From: Ashkan Nikravesh <ashkan.nikravesh@intel.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

Not sure what problem you're trying to solve and thus what exactly you
need... but you can probably get something very very close with 1 or 2
of clock_gettime(CLOCK_{BOOTTIME,MONOTONIC,REALTIME}) possibly in a
triple vdso call sandwich and iterated a few times and picking the one
with smallest delta between 1st and 3rd calls.  And then assuming the
avg of 1st and 3rd matches the 2nd.
ie.

5 times do:
t1[i] = clock_gettime(REALTIME)
t2[i] = clock_gettime(BOOTTIME)
t3[i] = clock_gettime(REALTIME)

pick i so t3[i] - t1[i] is smallest

t2[i] is near equivalent to (t1[i] + t3[i]) / 2

which basically gives you a REAL to BOOT offset.