[tip: sched/core] drivers/base/arch_topology: Add infrastructure to store and update instantaneous thermal pressure

[tip-bot2 for Thara Gopinath]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     ad58cc5cc50ca8423cf630778594bd38252a0a58
Gitweb:        https://git.kernel.org/tip/ad58cc5cc50ca8423cf630778594bd38252a0a58
Author:        Thara Gopinath <thara.gopinath@xxxxxxxxxx>
AuthorDate:    Fri, 21 Feb 2020 19:52:07 -05:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 06 Mar 2020 12:57:18 +01:00

drivers/base/arch_topology: 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.

topology_get_thermal_pressure can be hooked into the scheduler specified
arch_scale_thermal_pressure to retrieve instantaneous thermal pressure of
a CPU.

arch_set_thermal_pressure can be used to update the thermal pressure.

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 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 up to 1 tick period.

Signed-off-by: Thara Gopinath <thara.gopinath@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200222005213.3873-4-thara.gopinath@xxxxxxxxxx
---
 drivers/base/arch_topology.c  | 11 +++++++++++
 include/linux/arch_topology.h | 10 ++++++++++
 2 files changed, 21 insertions(+)

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 6119e11..68dfa49 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -42,6 +42,17 @@ 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 th_pressure)
+{
+	int cpu;
+
+	for_each_cpu(cpu, cpus)
+		WRITE_ONCE(per_cpu(thermal_pressure, cpu), th_pressure);
+}
+
 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..88a115e 100644
--- a/include/linux/arch_topology.h
+++ b/include/linux/arch_topology.h
@@ -33,6 +33,16 @@ 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 th_pressure);
+
 struct cpu_topology {
 	int thread_id;
 	int core_id;