Re: [PATCH] bpf: Add bpf_ktime_get_real_ns

["Daniel Xu" <dxu@dxuuu.xyz>]

Hi Maciej,

On Tue Aug 18, 2020 at 2:19 PM PDT, Maciej Żenczykowski wrote:
> 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.

I tried out the triple vdso sandwich and I got pretty good results
(~30ns ballpark). Thanks for the tip.

Daniel