linux-kernel - Re: Question about ktime_get_mono_fast

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CANDhNCpHGvw2MxexRFwpx4nAmqgwXw3G3DqkD2s0W3RNXZw2Bg@mail.gmail.com>
Date:   Thu, 13 Oct 2022 19:15:31 -0700
From:   John Stultz <jstultz@...gle.com>
To:     Yosry Ahmed <yosryahmed@...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 6:29 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> 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?

Correct.

> So if HZ is 1000, the error bound is actually ~2 ms, which is very
> large considering that the unit is ns.

Uh, for HZ=1000, I think it's closer to 220us, but yes, for HZ=100 2.2ms.

thanks
-john