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

Peter Zijlstra
2017-04-20T13:08:13Z

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.

> 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?

---
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);