On Thu, Oct 13, 2022 at 8:42 PM John Stultz <jstultz@xxxxxxxxxx> wrote: > > On Thu, Oct 13, 2022 at 8:26 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > On Thu, Oct 13, 2022 at 7:39 PM John Stultz <jstultz@xxxxxxxxxx> wrote: > > > 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. > > > > I am not the best person to answer this question (the BPF list is > > CC'd, it's full of more knowledgeable people). > > > > My understanding is that because BPF programs can basically be run in > > any context (because they can attach to almost all functions / > > tracepoints in the kernel), the time accessor needs to be safe in all > > contexts. > > Ah. Ok, the tracepoint connection is indeed likely the case. Thanks > for clarifying. > > > Now that I know that ktime_get_mono_fast_ns() can drift significantly, > > I am wondering why we don't just read sched_clock(). Can the > > difference between sched_clock() on different cpus be even higher than > > the potential drift from ktime_get_mono_fast_ns()? > > sched_clock is also lock free and so I think it's possible to have > inconsistencies. Right, I am just trying to figure out which is worse, ktime_get_mono_fast_ns() or sched_clock(). It appears to me that both can be inconsistent, but at least AFAICT sched_clock() can only be inconsistent if read across different cpus, right? It should also be faster (at least in my experimentation). I am wondering if there is a bound on the inconsistency we might observe from sched_clock() if we read it across different cpus, and if there is, how does it compare to ktime_get_mono_fast_ns() in that regard. > > ktime_get_raw_fast_ns() is possibly closer to what you are looking > for, as it is similarly un-adjusted by NTP. > However that also means the time intervals it measures (especially > long ones) may not be accurate. > > Also I worry that if it's already established as a CLOCK_MONOTONIC > interface, switching it to MONOTONIC_RAW might break some applications > that mix collected timestamps with CLOCK_MONOTONIC. > > thanks > -john
