@@ -4,7 +4,7 @@ config CPU_IDLE
bool "CPU idle PM support"
default y if ACPI || PPC_PSERIES
select CPU_IDLE_GOV_LADDER if (!NO_HZ && !NO_HZ_IDLE)
- select CPU_IDLE_GOV_MENU if (NO_HZ || NO_HZ_IDLE) && !CPU_IDLE_GOV_TEO
+ select CPU_IDLE_GOV_MENU if (NO_HZ || NO_HZ_IDLE) && !CPU_IDLE_GOV_TEO && !CPU_IDLE_GOV_MBED
help
CPU idle is a generic framework for supporting software-controlled
idle processor power management. It includes modular cross-platform
@@ -32,6 +32,15 @@ config CPU_IDLE_GOV_TEO
Some workloads benefit from using it and it generally should be safe
to use. Say Y here if you are not happy with the alternatives.
+config CPU_IDLE_GOV_MBED
+ bool "Embedded governor"
+ select IRQ_TIMINGS
+ help
+ The embedded governor is based on irq timings measurements and
+ pattern research combined with the next timer. This governor
+ suits very well on embedded systems where the interrupts are
+ grouped on a single core and the power is the priority.
+
config DT_IDLE_STATES
bool
@@ -5,3 +5,4 @@
obj-$(CONFIG_CPU_IDLE_GOV_LADDER) += ladder.o
obj-$(CONFIG_CPU_IDLE_GOV_MENU) += menu.o
obj-$(CONFIG_CPU_IDLE_GOV_TEO) += teo.o
+obj-$(CONFIG_CPU_IDLE_GOV_MBED) += mbed.o
new file mode 100644
@@ -0,0 +1,143 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2019, Linaro Ltd
+ * Author: Daniel Lezcano <daniel.lezcano@linaro.org>
+ */
+#include <linux/cpuidle.h>
+#include <linux/kernel.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/tick.h>
+#include <linux/interrupt.h>
+#include <linux/sched/clock.h>
+
+struct mbed_device {
+ u64 idle_ema_avg;
+ u64 idle_total;
+ unsigned long last_jiffies;
+};
+
+#define EMA_ALPHA_VAL 64
+#define EMA_ALPHA_SHIFT 7
+#define MAX_RESCHED_INTERVAL_MS 100
+
+static DEFINE_PER_CPU(struct mbed_device, mbed_devices);
+
+static int mbed_ema_new(s64 value, s64 ema_old)
+{
+ if (likely(ema_old))
+ return ema_old + (((value - ema_old) * EMA_ALPHA_VAL) >>
+ EMA_ALPHA_SHIFT);
+ return value;
+}
+
+static void mbed_reflect(struct cpuidle_device *dev, int index)
+{
+ struct mbed_device *mbed_dev = this_cpu_ptr(&mbed_devices);
+
+ /*
+ * The idle task was not rescheduled since
+ * MAX_RESCHED_INTERVAL_MS, let's consider the duration is
+ * long enough to clear our stats.
+ */
+ if (time_after(jiffies, mbed_dev->last_jiffies +
+ msecs_to_jiffies(MAX_RESCHED_INTERVAL_MS)))
+ mbed_dev->idle_ema_avg = 0;
+
+ /*
+ * Sum all the residencies in order to compute the total
+ * duration of the idle task.
+ */
+ mbed_dev->idle_total += dev->last_residency;
+
+ /*
+ * We exited the idle state with the need_resched() flag, the
+ * idle task will be rescheduled, so store the duration the
+ * idle task was scheduled in an exponential moving average and
+ * reset the total of the idle duration.
+ */
+ if (need_resched()) {
+ mbed_dev->idle_ema_avg = mbed_ema_new(mbed_dev->idle_total,
+ mbed_dev->idle_ema_avg);
+ mbed_dev->idle_total = 0;
+ mbed_dev->last_jiffies = jiffies;
+ }
+}
+
+static int mbed_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
+ bool *stop_tick)
+{
+ struct mbed_device *mbed_dev = this_cpu_ptr(&mbed_devices);
+ int latency_req = cpuidle_governor_latency_req(dev->cpu);
+ int i, index = 0;
+ ktime_t delta_next;
+ u64 now, irq_length, timer_length;
+ u64 idle_duration_us;
+
+ /*
+ * Get the present time as reference for the next steps
+ */
+ now = local_clock();
+
+ /*
+ * Get the next interrupt event giving the 'now' as a
+ * reference, if the next event appears to have already
+ * expired then we get the 'now' returned which ends up with a
+ * zero duration.
+ */
+ irq_length = irq_timings_next_event(now) - now;
+
+ /*
+ * Get the timer duration before expiration.
+ */
+ timer_length = ktime_to_ns(tick_nohz_get_sleep_length(&delta_next));
+
+ /*
+ * Get the smallest duration between the timer and the irq next event.
+ */
+ idle_duration_us = min_t(u64, irq_length, timer_length) / NSEC_PER_USEC;
+
+ /*
+ * Get the idle task duration average if the information is
+ * available.
+ */
+ if (mbed_dev->idle_ema_avg)
+ idle_duration_us = min_t(u64, idle_duration_us,
+ mbed_dev->idle_ema_avg);
+
+ for (i = 0; i < drv->state_count; i++) {
+ struct cpuidle_state *s = &drv->states[i];
+ struct cpuidle_state_usage *su = &dev->states_usage[i];
+
+ if (s->disabled || su->disable)
+ continue;
+
+ if (s->exit_latency > latency_req)
+ break;
+
+ if (s->target_residency > idle_duration_us)
+ break;
+
+ index = i;
+ }
+
+ if (!index)
+ *stop_tick = false;
+
+ return index;
+}
+
+static struct cpuidle_governor mbed_governor = {
+ .name = "mbed",
+ .rating = 20,
+ .select = mbed_select,
+ .reflect = mbed_reflect,
+};
+
+static int __init init_governor(void)
+{
+ irq_timings_enable();
+ return cpuidle_register_governor(&mbed_governor);
+}
+
+postcore_initcall(init_governor);
The objective is the same for all the governors: save energy, but at the end the governors menu, ladder and teo aim to improve the performances with an acceptable energy drop for some workloads which are identified for servers and desktops (with the help of a firmware). It is very difficult to do changes in these governors for embedded systems without impacting performances on servers/desktops or ruin the optimizations for the workloads on these platforms. The mbed governor is a new governor targeting embedded systems running on battery where the energy saving has a higher priority than servers or desktops. This governor aims to save energy as much as possible but accepting a performance degradation. In this way, we can optimize the governor for specific mobile workload and more generally embedded systems without impacting other platforms. The governor is based on the irq timings where we predict the next interrupt occurrences on the current CPU and the next timer. It is well suited for mobile and more generally embedded systems where the interrupts are usually pinned on one CPU and where the power is more important than the performances. The multimedia applications on the embedded system spawns several threads which are migrated across the different CPUs and waking between them up. In order to catch this situation we have also to track the idle task rescheduling duration with a relative degree of confidence as the scheduler is involved in the task migrations. The resched information is in the scope of the governor via the reflect callback. The governor begins with a clean foundation basing the prediction on the irq behavior returned by the irq timings, the timers and the idle task rescheduling. The advantage of the approach is we have a full view of the wakeup sources as we identify them separately and then we can control the situation without relying on biased heuristics. Despite the naive idle state selection for this first iteration, the governor provides a performance improvement of 30% for Jankbench throughout for the same amount of energy than the menu governor and a similar energy saving for other workloads. There are areas of improvement for the different workloads which will be optimized iteratively with non-regression testing for previous identified workloads on an Android reference platform. Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org> --- drivers/cpuidle/Kconfig | 11 ++- drivers/cpuidle/governors/Makefile | 1 + drivers/cpuidle/governors/mbed.c | 143 +++++++++++++++++++++++++++++ 3 files changed, 154 insertions(+), 1 deletion(-) create mode 100644 drivers/cpuidle/governors/mbed.c