linux-kernel - Re: [PATCH v2 1/3] timekeeping: Introduce fast accessor to clock tai

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Message-ID: <87pml3x7mv.ffs@tglx>
Date:   Wed, 27 Apr 2022 10:17:28 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Kurt Kanzenbach <kurt@...utronix.de>
Cc:     John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Richard Cochran <richardcochran@...il.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] timekeeping: Introduce fast accessor to clock tai

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