Commit db50a74d authored Nov 15, 2017 by Prashanth Prakash Committed by Catalin Marinas Jan 02, 2018

cpuidle: Add new macro to enter a retention idle state



If a CPU is entering a low power idle state where it doesn't lose any
context, then there is no need to call cpu_pm_enter()/cpu_pm_exit().
Add a new macro(CPU_PM_CPU_IDLE_ENTER_RETENTION) to be used by cpuidle
drivers when they are entering retention state. By not calling
cpu_pm_enter and cpu_pm_exit we reduce the latency involved in
entering and exiting the retention idle states.

CPU_PM_CPU_IDLE_ENTER_RETENTION assumes that no state is lost and
hence CPU PM notifiers will not be called. We may need a broader
change if we need to support partial retention states effeciently.

On ARM64 based Qualcomm Server Platform we measured below overhead for
for calling cpu_pm_enter and cpu_pm_exit for retention states.

workload: stress --hdd #CPUs --hdd-bytes 32M  -t 30
        Average overhead of cpu_pm_enter - 1.2us
        Average overhead of cpu_pm_exit  - 3.1us

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

parent 1f911c3a

include/linux/cpuidle.h

+24 −16

Original line number	Diff line number	Diff line
		@@ -257,22 +257,30 @@ static inline int cpuidle_register_governor(struct cpuidle_governor *gov)
		{return 0;}
		#endif

		#define CPU_PM_CPU_IDLE_ENTER(low_level_idle_enter, idx) \
		#define __CPU_PM_CPU_IDLE_ENTER(low_level_idle_enter, idx, is_retention) \
		({ \
		int __ret; \
		int __ret = 0; \
		\
		if (!idx) { \
		cpu_do_idle(); \
		return idx; \
		} \
		\
		if (!is_retention) \
		__ret = cpu_pm_enter(); \
		if (!__ret) { \
		__ret = low_level_idle_enter(idx); \
		if (!is_retention) \
		cpu_pm_exit(); \
		} \
		\
		__ret ? -1 : idx; \
		})

		#define CPU_PM_CPU_IDLE_ENTER(low_level_idle_enter, idx) \
		__CPU_PM_CPU_IDLE_ENTER(low_level_idle_enter, idx, 0)

		#define CPU_PM_CPU_IDLE_ENTER_RETENTION(low_level_idle_enter, idx) \
		__CPU_PM_CPU_IDLE_ENTER(low_level_idle_enter, idx, 1)

		#endif /* _LINUX_CPUIDLE_H */

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