On Mon, Sep 26, 2022 at 2:18 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > 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: Thinking about this a bit more, I have my own "beginner question": Why does bpf_ktime_get_ns() need to use the ktime_get_mono_fast_ns() accessor instead of ktime_get_ns()? I don't know enough about the contexts that bpf logic can run, so it's not clear to me and it's not obviously commented either. Looking at some of the uses of ktime_get_mono_fast_ns() spread around the kernel, some are clearly necessary (trying to get timestamps in suspend paths after timekeeping might be shutdown, etc). But there's also a few cases where the need isn't clear and I'm worried the reasoning is because it says "fast" in its name. "Why stop with ktime_get_mono_fast_ns() when you could instead use ktime_get_real_fast()! It's *real fast*!" :) thanks -john
