linux-kernel - Re: Question about ktime_get_mono_fast

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Message-ID: <CAJD7tkZ6dmbFS4wba8bcYaHWyMJCi+M1PPEc_WbuaHtvMY4HaA@mail.gmail.com>
Date:   Thu, 13 Oct 2022 20:46:44 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     John Stultz <jstultz@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Hao Luo <haoluo@...gle.com>,
        Stanislav Fomichev <sdf@...gle.com>
Subject: Re: Question about ktime_get_mono_fast_ns() non-monotonic behavior

On Thu, Oct 13, 2022 at 8:42 PM John Stultz <jstultz@...gle.com> wrote:
>
> On Thu, Oct 13, 2022 at 8:26 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > On Thu, Oct 13, 2022 at 7:39 PM John Stultz <jstultz@...gle.com> wrote:
> > > On Mon, Sep 26, 2022 at 2:18 PM Yosry Ahmed <yosryahmed@...gle.com> 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.

Right, I am just trying to figure out which is worse,
ktime_get_mono_fast_ns() or sched_clock(). It appears to me that both
can be inconsistent, but at least AFAICT sched_clock() can only be
inconsistent if read across different cpus, right? It should also be
faster (at least in my experimentation).

I am wondering if there is a bound on the inconsistency we might
observe from sched_clock() if we read it across different cpus, and if
there is, how does it compare to ktime_get_mono_fast_ns() in that
regard.

>
> 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