On Thu, Oct 13, 2022 at 8:47 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > 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. Again, I think ktime_get_raw_fast_ns() (so CLOCK_MONOTONIC_RAW) is likely to be closer to sched_clock() as neither of them are NTP adjusted. (Which also likely makes them unusable for the case where timestamps are compared with userland CLOCK_MONOTONIC timestamps). So folks might need a new bpf interface for that. Also I think folks would want to avoid exporting sched_clock timestamps out to userland as they aren't connected to a well defined clockid, and may have odd behavior around suspend/resume, etc. thanks -john
