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Commit dbd1b8ea authored by Shreyas B. Prabhu's avatar Shreyas B. Prabhu Committed by Rafael J. Wysocki
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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 e93e59ce "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: e93e59ce "cpuidle: Replace ktime_get() with local_clock()"
Reported-by: default avatarAnton Blanchard <anton@samba.org>
Bisected-by: default avatarShilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: default avatarShreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Acked-by: default avatarDaniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: default avatarBalbir Singh <bsingharora@gmail.com>
Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
parent 4c2e07c6

drivers/cpuidle/cpuidle.c

+4 −8
Original line number Diff line number Diff line
@@ -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;