linux-kernel - Re: Question about ktime_get_mono_fast

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Message-ID: <CAJD7tkb=FSoRETXDMBs+ZUO1BhT7X1aG7wziYNtFg8XqmXH-Zw@mail.gmail.com>
Date:   Thu, 13 Oct 2022 17:03:14 -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 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.

>
> thanks
> -john