[Patch v5 2/6] sched/fair: Add infrastructure to store and update instantaneous thermal pressure

[Thara Gopinath <thara.gopinath@linaro.org>]

Add interface APIs to initialize, update/average, track, accumulate
and decay thermal pressure per cpu basis. A per cpu variable
thermal_pressure is introduced to keep track of instantaneous per
cpu thermal pressure. Thermal pressure is the delta between maximum
capacity and capped capacity due to a thermal event.
API trigger_thermal_pressure_average is called for periodic accumulate
and decay of the thermal pressure.This API passes on the instantaneous
thermal pressure of a cpu to update_thermal_load_avg to do the necessary
accumulate, decay and average.
API update_thermal_pressure is for the system to update the thermal
pressure by providing a capped maximum capacity.
Considering, trigger_thermal_pressure_average reads thermal_pressure and
update_thermal_pressure writes into thermal_pressure, one can argue for
some sort of locking mechanism to avoid a stale value.
But considering trigger_thermal_pressure_average can be called from a
system critical path like scheduler tick function, a locking mechanism
is not ideal. This means that it is possible the thermal_pressure value
used to calculate average thermal pressure for a cpu can be
stale for upto 1 tick period.

Signed-off-by: Thara Gopinath <thara.gopinath@linaro.org>
---

v3->v4:
        - Dropped per cpu max_capacity_info struct and instead added a per
          delta_capacity variable to store the delta between maximum
          capacity and capped capacity. The delta is now calculated when
          thermal pressure is updated and not every tick.
        - Dropped populate_max_capacity_info api as only per cpu delta
          capacity is stored.
        - Renamed update_periodic_maxcap to
          trigger_thermal_pressure_average and update_maxcap_capacity to
          update_thermal_pressure.
v4->v5:
	- As per Peter's review comments folded thermal.c into fair.c.
	- As per Ionela's review comments revamped update_thermal_pressure
	  to take maximum available capacity as input instead of maximum
	  capped frequency ration.

---
 include/linux/sched.h |  9 +++++++++
 kernel/sched/fair.c   | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 263cf08..3c31084 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1993,6 +1993,15 @@ static inline void rseq_syscall(struct pt_regs *regs)
 
 #endif
 
+#ifdef CONFIG_SMP
+void update_thermal_pressure(int cpu, unsigned long capped_capacity);
+#else
+static inline void
+update_thermal_pressure(int cpu, unsigned long capped_capacity)
+{
+}
+#endif
+
 const struct sched_avg *sched_trace_cfs_rq_avg(struct cfs_rq *cfs_rq);
 char *sched_trace_cfs_rq_path(struct cfs_rq *cfs_rq, char *str, int len);
 int sched_trace_cfs_rq_cpu(struct cfs_rq *cfs_rq);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 682a754..2e907cc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -86,6 +86,12 @@ static unsigned int normalized_sysctl_sched_wakeup_granularity	= 1000000UL;
 
 const_debug unsigned int sysctl_sched_migration_cost	= 500000UL;
 
+/*
+ * Per-cpu instantaneous delta between maximum capacity
+ * and maximum available capacity due to thermal events.
+ */
+static DEFINE_PER_CPU(unsigned long, thermal_pressure);
+
 #ifdef CONFIG_SMP
 /*
  * For asym packing, by default the lower numbered CPU has higher priority.
@@ -10401,6 +10407,37 @@ static unsigned int get_rr_interval_fair(struct rq *rq, struct task_struct *task
 	return rr_interval;
 }
 
+#ifdef CONFIG_SMP
+/**
+ * update_thermal_pressure: Update thermal pressure
+ * @cpu: the cpu for which thermal pressure is to be updated for
+ * @capped_capacity: maximum capacity of the cpu after the capping
+ *		     due to thermal event.
+ *
+ * Delta between the arch_scale_cpu_capacity and capped max capacity is
+ * stored in per cpu thermal_pressure variable.
+ */
+void update_thermal_pressure(int cpu, unsigned long capped_capacity)
+{
+	unsigned long delta;
+
+	delta = arch_scale_cpu_capacity(cpu) - capped_capacity;
+	per_cpu(thermal_pressure, cpu) = delta;
+}
+#endif
+
+/**
+ * trigger_thermal_pressure_average: Trigger the thermal pressure accumulate
+ *				     and average algorithm
+ */
+static void trigger_thermal_pressure_average(struct rq *rq)
+{
+#ifdef CONFIG_SMP
+	update_thermal_load_avg(rq_clock_task(rq), rq,
+				per_cpu(thermal_pressure, cpu_of(rq)));
+#endif
+}
+
 /*
  * All the scheduling class methods:
  */
-- 
2.1.4