cpuidle: ladder: Better idle duration measurement without using CPUIDLE_FLAG_TIME_INVALID

When the ladder governor sees the CPUIDLE_FLAG_TIME_INVALID flag,
it unconditionally causes a state promotion by setting last_residency
to a number higher than the state's promotion_time:

last_residency = last_state->threshold.promotion_time + 1

It does this for fear that cpuidle_get_last_residency()
will be in-accurate, because cpuidle_enter_state() invoked
a state with CPUIDLE_FLAG_TIME_INVALID.

But the only state with CPUIDLE_FLAG_TIME_INVALID is
acpi_safe_halt(), which may return well after its actual
idle duration because it enables interrupts, so cpuidle_enter_state()
also measures interrupt service time.

So what?  In ladder, a huge invalid last_residency has exactly
the same effect as the current code -- it unconditionally
causes a state promotion.

In the case where the idle residency plus measured interrupt
handling time is less than the state's demotion_time -- we should
use that timestamp to give ladder a chance to demote, rather than
unconditionally promoting.

This can be done by simply ignoring the CPUIDLE_FLAG_TIME_INVALID,
and using the "invalid" time, as it is either equal to what we are
doing today, or better.

Signed-off-by: Len Brown <len.brown@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/drivers/cpuidle/governors/ladder.c b/drivers/cpuidle/governors/ladder.c
index 37263d9a105127079cd71baed298467495000dd6..401c0106ed345eda469a590aa345f88d8ff59eca 100644 (file)
--- a/drivers/cpuidle/governors/ladder.c
+++ b/drivers/cpuidle/governors/ladder.c
@@ -79,12 +79,7 @@ static int ladder_select_state(struct cpuidle_driver *drv,
 
        last_state = &ldev->states[last_idx];
 
-       if (!(drv->states[last_idx].flags & CPUIDLE_FLAG_TIME_INVALID)) {
-               last_residency = cpuidle_get_last_residency(dev) - \
-                                        drv->states[last_idx].exit_latency;
-       }
-       else
-               last_residency = last_state->threshold.promotion_time + 1;
+       last_residency = cpuidle_get_last_residency(dev) - drv->states[last_idx].exit_latency;
 
        /* consider promotion */
        if (last_idx < drv->state_count - 1 &&