Re: Question about ktime_get_mono_fast_ns() non-monotonic behavior

[John Stultz]

On Thu, Oct 13, 2022 at 8:26 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> On Thu, Oct 13, 2022 at 7:39 PM John Stultz <jstultz@xxxxxxxxxx> wrote:
> > On Mon, Sep 26, 2022 at 2:18 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> > >
> > > I have a question about ktime_get_mono_fast_ns(), which is used by the
> > > BPF helper bpf_ktime_get_ns() among other use cases. The comment above
> > > this function specifies that there are cases where the observed clock
> > > would not be monotonic.
> > >
> > > I had 2 beginner questions:
> >
> > Thinking about this a bit more, I have my own "beginner question": Why
> > does bpf_ktime_get_ns() need to use the ktime_get_mono_fast_ns()
> > accessor instead of ktime_get_ns()?
> >
> > I don't know enough about the contexts that bpf logic can run, so it's
> > not clear to me and it's not obviously commented either.
>
> I am not the best person to answer this question (the BPF list is
> CC'd, it's full of more knowledgeable people).
>
> My understanding is that because BPF programs can basically be run in
> any context (because they can attach to almost all functions /
> tracepoints in the kernel), the time accessor needs to be safe in all
> contexts.

Ah. Ok, the tracepoint connection is indeed likely the case. Thanks
for clarifying.

> Now that I know that ktime_get_mono_fast_ns() can drift significantly,
> I am wondering why we don't just read sched_clock(). Can the
> difference between sched_clock() on different cpus be even higher than
> the potential drift from ktime_get_mono_fast_ns()?

sched_clock is also lock free and so I think it's possible to have
inconsistencies.

ktime_get_raw_fast_ns() is possibly closer to what you are looking
for, as it is similarly un-adjusted by NTP.
However that also means the time intervals it measures (especially
long ones) may not be accurate.

Also I worry that if it's already established as a CLOCK_MONOTONIC
interface, switching it to MONOTONIC_RAW might break some applications
that mix collected timestamps with CLOCK_MONOTONIC.

thanks
-john