linux-kernel - Re: Question about ktime_get_mono_fast

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Message-ID: <CAJD7tkYUoW3YU-R1wNBADdUDHVprq6CP3axD9md3gJzW=yhiFw@mail.gmail.com>
Date:   Thu, 13 Oct 2022 18:28:53 -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 5:03 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Wed, Oct 12, 2022 at 8:07 PM John Stultz <jstultz@...gle.com> wrote:
> >
> > On Wed, Oct 12, 2022 at 8:02 PM John Stultz <jstultz@...gle.com> wrote:
> > > On Mon, Sep 26, 2022 at 2:18 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > > So I think it reasonable to say its bounded by approximately  2 *
> > > NSEC_PER_SEC/HZ +/- 11%.
> >
> > Sorry, this should be 2*NSEC_PER_SEC/HZ * 0.11
>
> Thanks so much for the detailed response :)
>
> IIUC this error bound is in ns. So on a 2 GHz cpu the bound is 0.11 ns
> (essentially 0)? I feel like I miscalculated, this error bound is too
> good to be true.

Never mind, I thought HZ is the cpu speed for some reason. It's the
number of jiffies per second, right?
So if HZ is 1000, the error bound is actually ~2 ms, which is very
large considering that the unit is ns.

Right?

>
> >
> > thanks
> > -john