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

[Ville Syrjälä]

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@xxxxxxxxxxxxxxx wrote:
> > From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxxxx>

> 
> ---
>  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