On Mon, Sep 26, 2022 at 2:18 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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? :)
