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Subject[Patch v6 3/7] Add infrastructure to store and update instantaneous thermal pressure
Date
Add architecture specific APIs to update and track thermal pressure on a
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.
capacity and capped capacity due to a thermal event.

topology_get_thermal_pressure can be hooked into the scheduler specified
arch_scale_thermal_capacity to retrieve instantaneius thermal pressure of
a cpu.

arch_set_thermal_presure can be used to update the thermal pressure by
providing a capped maximum capacity.

Considering topology_get_thermal_pressure reads thermal_pressure and
arch_set_thermal_pressure writes into thermal_pressure, one can argue for
some sort of locking mechanism to avoid a stale value. But considering
topology_get_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.
v5->v6:
- As per review comments moved all the infrastructure to track
and retrieve instantaneous thermal pressure out of scheduler
to topology files.

arch/arm/include/asm/topology.h | 3 +++
arch/arm64/include/asm/topology.h | 3 +++
drivers/base/arch_topology.c | 13 +++++++++++++
include/linux/arch_topology.h | 11 +++++++++++
4 files changed, 30 insertions(+)

diff --git a/arch/arm/include/asm/topology.h b/arch/arm/include/asm/topology.h
index 8a0fae9..90b18c3 100644
--- a/arch/arm/include/asm/topology.h
+++ b/arch/arm/include/asm/topology.h
@@ -16,6 +16,9 @@
/* Enable topology flag updates */
#define arch_update_cpu_topology topology_update_cpu_topology

+/* Replace task scheduler's defalut thermal pressure retrieve API */
+#define arch_scale_thermal_capacity topology_get_thermal_pressure
+
#else

static inline void init_cpu_topology(void) { }
diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
index a4d945d..ccb277b 100644
--- a/arch/arm64/include/asm/topology.h
+++ b/arch/arm64/include/asm/topology.h
@@ -25,6 +25,9 @@ int pcibus_to_node(struct pci_bus *bus);
/* Enable topology flag updates */
#define arch_update_cpu_topology topology_update_cpu_topology

+/* Replace task scheduler's defalut thermal pressure retrieve API */
+#define arch_scale_thermal_capacity topology_get_thermal_pressure
+
#include <asm-generic/topology.h>

#endif /* _ASM_ARM_TOPOLOGY_H */
diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 1eb81f11..3a91379 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -42,6 +42,19 @@ void topology_set_cpu_scale(unsigned int cpu, unsigned long capacity)
per_cpu(cpu_scale, cpu) = capacity;
}

+DEFINE_PER_CPU(unsigned long, thermal_pressure);
+
+void arch_set_thermal_pressure(struct cpumask *cpus,
+ unsigned long capped_capacity)
+{
+ int cpu;
+ unsigned long delta = arch_scale_cpu_capacity(cpumask_first(cpus)) -
+ capped_capacity;
+
+ for_each_cpu(cpu, cpus)
+ WRITE_ONCE(per_cpu(thermal_pressure, cpu), delta);
+}
+
static ssize_t cpu_capacity_show(struct device *dev,
struct device_attribute *attr,
char *buf)
diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
index 3015ecb..7a04364 100644
--- a/include/linux/arch_topology.h
+++ b/include/linux/arch_topology.h
@@ -33,6 +33,17 @@ unsigned long topology_get_freq_scale(int cpu)
return per_cpu(freq_scale, cpu);
}

+DECLARE_PER_CPU(unsigned long, thermal_pressure);
+
+static inline
+unsigned long topology_get_thermal_pressure(int cpu)
+{
+ return per_cpu(thermal_pressure, cpu);
+}
+
+void arch_set_thermal_pressure(struct cpumask *cpus,
+ unsigned long capped_capacity);
+
struct cpu_topology {
int thread_id;
int core_id;
--
2.1.4
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