[Patch v10 3/9] drivers/base/arch_topology: Add infrastructure to store and update instantaneous thermal pressure

From: Thara Gopinath <thara.gopinath@linaro.org>
To: mingo@redhat.com, peterz@infradead.org, ionela.voinescu@arm.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
	rui.zhang@intel.com, qperret@google.com,
	daniel.lezcano@linaro.org, viresh.kumar@linaro.org,
	rostedt@goodmis.org, will@kernel.org, catalin.marinas@arm.com,
	sudeep.holla@arm.com, juri.lelli@redhat.com, corbet@lwn.net
Cc: linux-kernel@vger.kernel.org, amit.kachhap@gmail.com,
	javi.merino@kernel.org, amit.kucheria@verdurent.com
Subject: [Patch v10 3/9] drivers/base/arch_topology: Add infrastructure to store and update instantaneous thermal pressure
Date: Fri, 21 Feb 2020 19:52:07 -0500	[thread overview]
Message-ID: <20200222005213.3873-4-thara.gopinath@linaro.org> (raw)
In-Reply-To: <20200222005213.3873-1-thara.gopinath@linaro.org>

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 upto 1 tick period.

Signed-off-by: Thara Gopinath <thara.gopinath@linaro.org>
---
v6->v7:
	- Changed the input argument in arch_set_thermal_pressure from
	  capped capacity to delta capacity(thermal pressure) as per
	  Ionela's review comments.
v9->v10:
	- Split the patch into three thus separating out arch/arm
	  and arch/arm64 specific code into individual patches as
	  suggested by Amit Kucheria.

 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 6119e11a9f95..68dfa49d3b63 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 3015ecbb90b1..88a115e81f27 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;
-- 
2.20.1

