Messages in this thread | | | From | Nicolai Stange <> | Subject | Re: [RFC v3 1/3] kernel/time/clockevents: initial support for mono to raw time conversion | Date | Thu, 21 Jul 2016 21:11:41 +0200 |
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John Stultz <john.stultz@linaro.org> writes:
> On Wed, Jul 13, 2016 at 6:00 AM, Nicolai Stange <nicstange@gmail.com> wrote: >> With NOHZ_FULL and one single well-isolated, CPU consumptive task, one >> would expect approximately one clockevent interrupt per second. However, on >> my Intel Haswell where the monotonic clock is the TSC monotonic clock and >> the clockevent device is the TSC deadline device, it turns out that every >> second, there are two such interrupts: the first one arrives always >> approximately ~50us before the scheduled deadline as programmed by >> tick_nohz_stop_sched_tick() through the hrtimer API. The >> __hrtimer_run_queues() called in this interrupt detects that the queued >> tick_sched_timer hasn't expired yet and simply does nothing except >> reprogramming the clock event device to fire shortly after again. >> >> These too early programmed deadlines are explained as follows: >> clockevents_program_event() programs the clockevent device to fire >> after >> f_event * delta_t_progr >> clockevent device cycles where f_event is the clockevent device's hardware >> frequency and delta_t_progr is the requested time interval. After that many >> clockevent device cycles have elapsed, the device underlying the monotonic >> clock, that is the monotonic raw clock has seen f_raw / f_event as many >> cycles. >> The ktime_get() called from __hrtimer_run_queues() interprets those >> cycles to run at the frequency of the monotonic clock. Summarizing: >> delta_t_perc = 1/f_mono * f_raw/f_event * f_event * delta_t_progr >> = f_raw / f_mono * delta_t_progr >> with f_mono being the monotonic clock's frequency and delta_t_perc being >> the elapsed time interval as perceived by __hrtimer_run_queues(). >> >> Now, f_mono is not a constant, but is dynamically adjusted in >> timekeeping_adjust() in order to compensate for the NTP error. With the >> large values of delta_t_progr of 10^9ns with NOHZ_FULL, the error made >> becomes significant and results in the double timer interrupts described >> above. >> >> Compensate for this error by multiplying the clockevent device's f_event >> by f_mono/f_raw. >> >> Namely: >> - Introduce a ->mult_mono member to the struct clock_event_device. It's >> value is supposed to be equal to ->mult * f_mono/f_raw. >> - Introduce the timekeeping_get_mono_mult() helper which provides >> the clockevent core with access to the timekeeping's current f_mono >> and f_raw. >> - Introduce the helper __clockevents_adjust_freq() which >> sets a clockevent device's ->mult_mono member as appropriate. It is >> implemented with the help of the new __clockevents_calc_adjust_freq(). >> - Call __clockevents_adjust_freq() at clockevent device registration time >> as well as at frequency updates through clockevents_update_freq(). >> - Finally, use the ->mult_mono rather than ->mult in the ns to cycle >> conversion made in clockevents_program_event(). >> >> Note that future adjustments of the monotonic clock are not taken into >> account yet. Furthemore, this patch assumes that after a clockevent >> device's registration, its ->mult changes only through calls to >> clockevents_update_freq(). > > Sorry for being a little slow to review here. Been swamped.
No need for any hurry here, I don't expect this to make it into 4.8 anyway.
> I was about to queue this but had a few nits that need addressing.
There are even more "known issues" listed in the cover letter. Given the huge amount of patches potentially required to get this into a good shape, the question whether you want to have this at all came up (c.f. http://lkml.kernel.org/g/alpine.DEB.2.11.1607121651580.4083@nanos).
So, once you give it a go, ideally accompanied with some comments on which of the known issues to address and which ones to ignore, I'll send another RFC series consisting of way more (mostly trivial) patches.
> >> Signed-off-by: Nicolai Stange <nicstange@gmail.com> >> --- >> include/linux/clockchips.h | 1 + >> kernel/time/clockevents.c | 49 ++++++++++++++++++++++++++++++++++++++++++++- >> kernel/time/tick-internal.h | 1 + >> kernel/time/timekeeping.c | 8 ++++++++ >> 4 files changed, 58 insertions(+), 1 deletion(-) >> >> diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h >> index 0d442e3..2863742 100644 >> --- a/include/linux/clockchips.h >> +++ b/include/linux/clockchips.h >> @@ -104,6 +104,7 @@ struct clock_event_device { >> u64 max_delta_ns; >> u64 min_delta_ns; >> u32 mult; >> + u32 mult_mono; > > So in this context(for me at least), mult and mult_mono are a bit > confusing. I tend to think of it as mult and mult_raw, but in this > case mult is the "raw" unmodified value and mult_mono is the adjusted > one. > > I'd probably suggest mult_adjusted or some other name to make it more > clear how it differs from the clockevent mult. >
Totally agreed. I didn't want to rename ->mult in order to avoid touching clockevents driver initizalization code all over the tree. But "mult_adjusted" is really more intuitive.
>> >> +void timekeeping_get_mono_mult(u32 *mult_cs_mono, u32 *mult_cs_raw) >> +{ >> + struct tk_read_base *tkr_mono = &tk_core.timekeeper.tkr_mono; >> + >> + *mult_cs_mono = tkr_mono->mult; >> + *mult_cs_raw = tkr_mono->clock->mult; >> +} > > So.. you probably should have some locking here. Or at least a big > comment making it clear why locking isn't necessary.
Agreed. To my taste, this timekeeping_get_mono_mult is an ugly hack anyway and I'd like to make tk_core non-static instead (c.f. cover letter). I'm lacking the needed guts though.
Thanks,
Nicolai
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