linux-kernel - Re: Question about ktime_get_mono_fast

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Date:   Tue, 11 Oct 2022 10:33:58 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     John Stultz <jstultz@...gle.com>, tglx@...utronix.de,
        sboyd@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     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 Mon, Sep 26, 2022 at 2:18 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> Hey everyone,
>
> 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:
>
> 1) Is there a (rough) bound as to how much the clock can go backwards?
> My understanding is that it is bounded by (slope update * delta), but
> I don't know what's the bound of either of those (if any).
>
> 2) The comment specifies that for a single cpu, the only way for this
> behavior to happen is when observing the time in the context of an NMI
> that happens during an update.
> For observations across different cpus, are the scenarios where the
> non-monotonic behavior happens also tied to observing time within NMI
> contexts? or is it something that can happen outside of NMI contexts
> as well?
>
> Thanks in advance! (and please excuse any dumb/obvious questions :) )

Anyone can help me with this? :)