On Tue, Apr 26 2022 at 17:53, Steven Rostedt wrote:
>> +/**
>> + * ktime_get_tai_fast_ns - NMI safe and fast access to tai clock.
>> + *
>> + * The same limitations as described for ktime_get_boot_fast_ns() apply. The
>> + * mono time and the TAI offset are not read atomically which may yield wrong
>> + * readouts. However, an update of the TAI offset is an rare event e.g., caused
>> + * by settime or adjtimex with an offset. The user of this function has to deal
>> + * with the possibility of wrong timestamps in post processing.
>> + */
>> +u64 notrace ktime_get_tai_fast_ns(void)
>> +{
>> + struct timekeeper *tk = &tk_core.timekeeper;
>> +
>> + return (ktime_get_mono_fast_ns() + ktime_to_ns(data_race(tk->offs_tai)));
>
> As you are using this for tracing, can you open code the
> ktime_get_mono_fast_ns(), otherwise we need to mark that function as
> notrace.
which we should do anyway....
kernel/trace/trace.c: { ktime_get_mono_fast_ns, "mono", 1 },
> Not to mention, this is a fast path and using the noinline of
> __ktime_get_fast_ns() should be less overhead.
>
> That said, I hit this too:
>
> less-5071 [000] d.h2. 498087876.351330: do_raw_spin_trylock <-_raw_spin_lock
> less-5071 [000] d.h4. 498087876.351334: ktime_get_mono_fast_ns <-ktime_get_tai_fast_ns
> less-5071 [000] d.h5. 498087876.351334: ktime_get_mono_fast_ns <-ktime_get_tai_fast_ns
> less-5071 [000] d.h3. 498087876.351334: rcu_read_lock_sched_held <-lock_acquired
> less-5071 [000] d.h5. 498087876.351337: ktime_get_mono_fast_ns <-ktime_get_tai_fast_ns
> kworker/u8:1-45 [003] d.h7. 1651009380.982749: ktime_get_mono_fast_ns <-ktime_get_tai_fast_ns
> kworker/u8:1-45 [003] d.h7. 1651009380.982749: ktime_get_mono_fast_ns <-ktime_get_tai_fast_ns
> kworker/u8:1-45 [003] d.h5. 1651009380.982749: rcu_read_lock_held_common <-rcu_read_lock_sched_held
> kworker/u8:1-45 [003] d.h7. 498087876.375905: ktime_get_mono_fast_ns <-ktime_get_tai_fast_ns
> kworker/u8:1-45 [003] d.h7. 498087876.375905: ktime_get_mono_fast_ns <-ktime_get_tai_fast_ns
> kworker/u8:1-45 [003] d.h5. 498087876.375905: update_cfs_group <-task_tick_fair
> kworker/u8:1-45 [003] d.h7. 498087876.375909: ktime_get_mono_fast_ns <-ktime_get_tai_fast_ns
>
> The clock seems to be toggling between 1651009380 and 498087876 causing the
> ftrace ring buffer to shutdown (it doesn't allow for time to go backwards).
That does not make sense. offs_tai changes only when clock is set via
adjtimex() or settimeofday().
Thanks,
tglx
