linux-kernel - [Patch v6 3/7] Add infrastructure to store and update instantaneous thermal pressure

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Date:   Wed, 11 Dec 2019 23:11:44 -0500
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     mingo@...hat.com, peterz@...radead.org, ionela.voinescu@....com,
        vincent.guittot@...aro.org, rui.zhang@...el.com,
        qperret@...gle.com, daniel.lezcano@...aro.org,
        viresh.kumar@...aro.org
Cc:     linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
        javi.merino@...nel.org, amit.kucheria@...durent.com
Subject: [Patch v6 3/7] Add infrastructure to store and update instantaneous thermal pressure

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@...aro.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