Re: [PATCH] Revert "cpuidle: Replace ktime_get() with local_clock()"

Ville Syrjälä
2017-04-20T13:43:45Z

On Thu, Apr 20, 2017 at 03:08:13PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 20, 2017 at 03:44:47PM +0300, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >
> > This reverts commit e93e59ce5b85e6c2b444f09fd1f707274ec066dc.
> >
> > The TSC stops in deeper C states,
>
> On some old hardware (Core2 era and before) only. You've forgotten to
> mention what hardware you've observed problems with.

Yeah, Core2 is what I used when I finally decided to bisect this. I've
been plagued by the bogus powertop numbers on many machines, most
likely all of them were of some older vintage.

>
> > so using local_clock() in cpuidle
>
> But on said hardware, local_clock() isn't an immediate TSC user.
>
> > to track the C state residency seems like a bad idea. With local_clock()
> > powertop is reporting mostly 0% residency for C states here. Presumably
> > the core is still spending most of its time in some deep C-state since
> > the totals typically add up to only 5% or so, so perhaps the governor
> > isn't getting totally confused by these bogus numbers. But let's go
> > back to using ktime_get() as that at least works correctly across the
> > board.
>
> Does this cure it?

It does indeed.

Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

>
> ---
> drivers/cpuidle/cpuidle.c | 2 ++
> kernel/sched/clock.c | 7 +++----
> 2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 548b90be7685..e0d4ad108887 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -219,6 +219,8 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
> entered_state = target_state->enter(dev, drv, index);
> start_critical_timings();
>
> + sched_clock_idle_wakeup_event(0);
> +
> time_end = ns_to_ktime(local_clock());
> trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
>
> diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
> index 00a45c45beca..15e848706be4 100644
> --- a/kernel/sched/clock.c
> +++ b/kernel/sched/clock.c
> @@ -347,6 +347,9 @@ void sched_clock_tick(void)
> {
> struct sched_clock_data *scd;
>
> + if (timekeeping_suspended)
> + return;
> +
> WARN_ON_ONCE(!irqs_disabled());
>
> /*
> @@ -378,11 +381,7 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
> */
> void sched_clock_idle_wakeup_event(u64 delta_ns)
> {
> - if (timekeeping_suspended)
> - return;
> -
> sched_clock_tick();
> - touch_softlockup_watchdog_sched();
> }
> EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
>

--
Ville Syrjälä
Intel OTC