cpuidle: Fix last_residency division

Snooze is a poll idle state in powernv and pseries platforms. Snooze
has a timeout so that if a CPU stays in snooze for more than target
residency of the next available idle state, then it would exit
thereby giving chance to the cpuidle governor to re-evaluate and
promote the CPU to a deeper idle state. Therefore whenever snooze
exits due to this timeout, its last_residency will be target_residency
of the next deeper state.

Commit e93e59ce5b85 "cpuidle: Replace ktime_get() with local_clock()"
changed the math around last_residency calculation. Specifically,
while converting last_residency value from nano- to microseconds, it
carries out right shift by 10. Because of that, in snooze timeout
exit scenarios last_residency calculated is roughly 2.3% less than
target_residency of the next available state. This pattern is picked
up by get_typical_interval() in the menu governor and therefore
expected_interval in menu_select() is frequently less than the
target_residency of any state other than snooze.

Due to this we are entering snooze at a higher rate, thereby
affecting the single thread performance.

Fix this by using more precise division via ktime_us_delta().

Fixes: e93e59ce5b85 "cpuidle: Replace ktime_get() with local_clock()"
Reported-by: Anton Blanchard <anton@samba.org>
Bisected-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index a4d0059..c73207a 100644 (file)
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -173,7 +173,7 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
 
        struct cpuidle_state *target_state = &drv->states[index];
        bool broadcast = !!(target_state->flags & CPUIDLE_FLAG_TIMER_STOP);
-       u64 time_start, time_end;
+       ktime_t time_start, time_end;
        s64 diff;
 
        /*
@@ -195,13 +195,13 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
        sched_idle_set_state(target_state);
 
        trace_cpu_idle_rcuidle(index, dev->cpu);
-       time_start = local_clock();
+       time_start = ns_to_ktime(local_clock());
 
        stop_critical_timings();
        entered_state = target_state->enter(dev, drv, index);
        start_critical_timings();
 
-       time_end = local_clock();
+       time_end = ns_to_ktime(local_clock());
        trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
 
        /* The cpu is no longer idle or about to enter idle. */
@@ -217,11 +217,7 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
        if (!cpuidle_state_is_coupled(drv, index))
                local_irq_enable();
 
-       /*
-        * local_clock() returns the time in nanosecond, let's shift
-        * by 10 (divide by 1024) to have microsecond based time.
-        */
-       diff = (time_end - time_start) >> 10;
+       diff = ktime_us_delta(time_end, time_start);
        if (diff > INT_MAX)
                diff = INT_MAX;