linux-kernel - [PATCH v2] arch_topology: Trace the update thermal pressure

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Date:   Mon, 25 Apr 2022 14:53:17 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     linux-kernel@...r.kernel.org
Cc:     sudeep.holla@....com, dietmar.eggemann@....com,
        vincent.guittot@...aro.org, gregkh@...uxfoundation.org,
        rafael@...nel.org, rostedt@...dmis.org, mingo@...hat.com,
        lukasz.luba@....com
Subject: [PATCH v2] arch_topology: Trace the update thermal pressure

Add trace event to capture the moment of the call for updating the thermal
pressure value. It's helpful to investigate how often those events occurs
in a system dealing with throttling. This trace event is needed since the
old 'cdev_update' might not be used by some drivers. Also, the new trace
event shows capacity value, not a cooling state.

We already have a trace point 'pelt_thermal_tp' which after a change to
trace event can be paired with this new 'thermal_pressure_update' and
derive more insight what is going on in the system under pressure
(and why).

Reported-by: kernel test robot <lkp@...el.com>
Signed-off-by: Lukasz Luba <lukasz.luba@....com>
---
Hi all,

This v2 solves the issue reported by build robot.
After experiments and checks I decided to not add the
EXPORT_TRACEPOINT_SYMBOL_GPL(thermal_pressure_update)
The trace event shouldn't be used by modules, since they use
the exported update function:
EXPORT_SYMBOL_GPL(topology_update_thermal_pressure)
which calls that trace event internally.
The code duplication for updating thermal pressure in a modules
is not recommended - that was the goal of introducing
topology_update_thermal_pressure().

Changes in v2:
- as suggested by Steve, I have swapped the fields
- a new trace header, so no conflicts with thermal.h
- added CREATE_TRACE_POINTS before header

The v1 and discussion can be found here at:
https://lore.kernel.org/lkml/20220419164801.29078-1-lukasz.luba@arm.com/

Regards,
Lukasz

 drivers/base/arch_topology.c            |  5 +++++
 include/trace/events/thermal_pressure.h | 29 +++++++++++++++++++++++++
 2 files changed, 34 insertions(+)
 create mode 100644 include/trace/events/thermal_pressure.h

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 1d6636ebaac5..20bdad18dccd 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -19,6 +19,9 @@
 #include <linux/rcupdate.h>
 #include <linux/sched.h>
 
+#define CREATE_TRACE_POINTS
+#include <trace/events/thermal_pressure.h>
+
 static DEFINE_PER_CPU(struct scale_freq_data __rcu *, sft_data);
 static struct cpumask scale_freq_counters_mask;
 static bool scale_freq_invariant;
@@ -195,6 +198,8 @@ void topology_update_thermal_pressure(const struct cpumask *cpus,
 
 	th_pressure = max_capacity - capacity;
 
+	trace_thermal_pressure_update(cpu, th_pressure);
+
 	for_each_cpu(cpu, cpus)
 		WRITE_ONCE(per_cpu(thermal_pressure, cpu), th_pressure);
 }
diff --git a/include/trace/events/thermal_pressure.h b/include/trace/events/thermal_pressure.h
new file mode 100644
index 000000000000..b68680201360
--- /dev/null
+++ b/include/trace/events/thermal_pressure.h
@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM thermal_pressure
+
+#if !defined(_TRACE_THERMAL_PRESSURE_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_THERMAL_PRESSURE_H
+
+#include <linux/tracepoint.h>
+
+TRACE_EVENT(thermal_pressure_update,
+	TP_PROTO(int cpu, unsigned long thermal_pressure),
+	TP_ARGS(cpu, thermal_pressure),
+
+	TP_STRUCT__entry(
+		__field(unsigned long, thermal_pressure)
+		__field(int, cpu)
+	),
+
+	TP_fast_assign(
+		__entry->thermal_pressure = thermal_pressure;
+		__entry->cpu = cpu;
+	),
+
+	TP_printk("cpu=%d thermal_pressure=%lu", __entry->cpu, __entry->thermal_pressure)
+);
+#endif /* _TRACE_THERMAL_PRESSURE_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>
-- 
2.17.1