tracing: Add sample code for custom trace events

Add sample code to show how to create custom trace events in the tracefs
directory that can be enabled and modified like any event in tracefs
(including triggers, histograms, synthetic events and event probes).

The example is creating a custom sched_switch and a sched_waking to limit
what is recorded:

If the custom sched switch only records the prev_prio, next_prio and
next_pid, it can bring the size from 64 bytes per event, down to just 16
bytes!

If sched_waking only records the prio and pid of the woken event, it will
bring the size down from 36 bytes to 12 bytes per event.

This will allow for a much smaller footprint into the ring buffer and keep
more events from dropping.

Link: https://lkml.kernel.org/r/20220303220625.369226746@goodmis.org

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Tom Zanussi <zanussi@kernel.org>
Suggested-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

samples/Kconfig

@@ -14,7 +14,13 @@ config SAMPLE_TRACE_EVENTS
 	tristate "Build trace_events examples -- loadable modules only"
 	depends on EVENT_TRACING && m
 	help
-	  This build trace event example modules.
+	  This builds the trace event example module.
+
+config SAMPLE_TRACE_CUSTOM_EVENTS
+	tristate "Build custom trace event example -- loadable modules only"
+	depends on EVENT_TRACING && m
+	help
+	  This builds the custom trace event example module.
 
 config SAMPLE_TRACE_PRINTK
         tristate "Build trace_printk module - tests various trace_printk formats"

samples/Makefile

@@ -20,6 +20,7 @@ obj-$(CONFIG_SAMPLE_RPMSG_CLIENT)	+= rpmsg/
 subdir-$(CONFIG_SAMPLE_SECCOMP)		+= seccomp
 subdir-$(CONFIG_SAMPLE_TIMER)		+= timers
 obj-$(CONFIG_SAMPLE_TRACE_EVENTS)	+= trace_events/
+obj-$(CONFIG_SAMPLE_TRACE_CUSTOM_EVENTS) += trace_events/
 obj-$(CONFIG_SAMPLE_TRACE_PRINTK)	+= trace_printk/
 obj-$(CONFIG_SAMPLE_FTRACE_DIRECT)	+= ftrace/
 obj-$(CONFIG_SAMPLE_FTRACE_DIRECT_MULTI) += ftrace/

samples/trace_events/Makefile

@@ -13,3 +13,5 @@
 CFLAGS_trace-events-sample.o := -I$(src)
 
 obj-$(CONFIG_SAMPLE_TRACE_EVENTS) += trace-events-sample.o
+
+obj-$(CONFIG_SAMPLE_TRACE_CUSTOM_EVENTS) += trace_custom_sched.o

samples/trace_events/trace_custom_sched.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * event tracer
+ *
+ * Copyright (C) 2022 Google Inc, Steven Rostedt <rostedt@goodmis.org>
+ */
+
+#define pr_fmt(fmt) fmt
+
+#include <linux/trace_events.h>
+#include <linux/version.h>
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <trace/events/sched.h>
+
+#define THIS_SYSTEM "custom_sched"
+
+#define SCHED_PRINT_FMT							\
+	C("prev_prio=%d next_pid=%d next_prio=%d", REC->prev_prio, REC->next_pid, \
+	  REC->next_prio)
+
+#define SCHED_WAKING_FMT				\
+	C("pid=%d prio=%d", REC->pid, REC->prio)
+
+#undef C
+#define C(a, b...) a, b
+
+static struct trace_event_fields sched_switch_fields[] = {
+	{
+		.type = "unsigned short",
+		.name = "prev_prio",
+		.size = sizeof(short),
+		.align = __alignof__(short),
+		.is_signed = 0,
+		.filter_type = FILTER_OTHER,
+	},
+	{
+		.type = "unsigned short",
+		.name = "next_prio",
+		.size = sizeof(short),
+		.align = __alignof__(short),
+		.is_signed = 0,
+		.filter_type = FILTER_OTHER,
+	},
+	{
+		.type = "unsigned int",
+		.name = "next_prio",
+		.size = sizeof(int),
+		.align = __alignof__(int),
+		.is_signed = 0,
+		.filter_type = FILTER_OTHER,
+	},
+	{}
+};
+
+struct sched_event {
+	struct trace_entry	ent;
+	unsigned short		prev_prio;
+	unsigned short		next_prio;
+	unsigned int		next_pid;
+};
+
+static struct trace_event_fields sched_waking_fields[] = {
+	{
+		.type = "unsigned int",
+		.name = "pid",
+		.size = sizeof(int),
+		.align = __alignof__(int),
+		.is_signed = 0,
+		.filter_type = FILTER_OTHER,
+	},
+	{
+		.type = "unsigned short",
+		.name = "prio",
+		.size = sizeof(short),
+		.align = __alignof__(short),
+		.is_signed = 0,
+		.filter_type = FILTER_OTHER,
+	},
+	{}
+};
+
+struct wake_event {
+	struct trace_entry	ent;
+	unsigned int		pid;
+	unsigned short		prio;
+};
+
+static void sched_switch_probe(void *data, bool preempt, struct task_struct *prev,
+			       struct task_struct *next)
+{
+	struct trace_event_file *trace_file = data;
+	struct trace_event_buffer fbuffer;
+	struct sched_event *entry;
+
+	if (trace_trigger_soft_disabled(trace_file))
+		return;
+
+	entry = trace_event_buffer_reserve(&fbuffer, trace_file,
+					   sizeof(*entry));
+
+	if (!entry)
+		return;
+
+	entry->prev_prio = prev->prio;
+	entry->next_prio = next->prio;
+	entry->next_pid = next->pid;
+
+	trace_event_buffer_commit(&fbuffer);
+}
+
+static struct trace_event_class sched_switch_class = {
+	.system			= THIS_SYSTEM,
+	.reg			= trace_event_reg,
+	.fields_array		= sched_switch_fields,
+	.fields			= LIST_HEAD_INIT(sched_switch_class.fields),
+	.probe			= sched_switch_probe,
+};
+
+static void sched_waking_probe(void *data, struct task_struct *t)
+{
+	struct trace_event_file *trace_file = data;
+	struct trace_event_buffer fbuffer;
+	struct wake_event *entry;
+
+	if (trace_trigger_soft_disabled(trace_file))
+		return;
+
+	entry = trace_event_buffer_reserve(&fbuffer, trace_file,
+					   sizeof(*entry));
+
+	if (!entry)
+		return;
+
+	entry->prio = t->prio;
+	entry->pid = t->pid;
+
+	trace_event_buffer_commit(&fbuffer);
+}
+
+static struct trace_event_class sched_waking_class = {
+	.system			= THIS_SYSTEM,
+	.reg			= trace_event_reg,
+	.fields_array		= sched_waking_fields,
+	.fields			= LIST_HEAD_INIT(sched_waking_class.fields),
+	.probe			= sched_waking_probe,
+};
+
+static enum print_line_t sched_switch_output(struct trace_iterator *iter, int flags,
+					     struct trace_event *trace_event)
+{
+	struct trace_seq *s = &iter->seq;
+	struct sched_event *REC = (struct sched_event *)iter->ent;
+	int ret;
+
+	ret = trace_raw_output_prep(iter, trace_event);
+	if (ret != TRACE_TYPE_HANDLED)
+		return ret;
+
+	trace_seq_printf(s, SCHED_PRINT_FMT);
+	trace_seq_putc(s, '\n');
+
+	return trace_handle_return(s);
+}
+
+static struct trace_event_functions sched_switch_funcs = {
+	.trace			= sched_switch_output,
+};
+
+static enum print_line_t sched_waking_output(struct trace_iterator *iter, int flags,
+					     struct trace_event *trace_event)
+{
+	struct trace_seq *s = &iter->seq;
+	struct wake_event *REC = (struct wake_event *)iter->ent;
+	int ret;
+
+	ret = trace_raw_output_prep(iter, trace_event);
+	if (ret != TRACE_TYPE_HANDLED)
+		return ret;
+
+	trace_seq_printf(s, SCHED_WAKING_FMT);
+	trace_seq_putc(s, '\n');
+
+	return trace_handle_return(s);
+}
+
+static struct trace_event_functions sched_waking_funcs = {
+	.trace			= sched_waking_output,
+};
+
+#undef C
+#define C(a, b...) #a "," __stringify(b)
+
+static struct trace_event_call sched_switch_call = {
+	.class			= &sched_switch_class,
+	.event			= {
+		.funcs			= &sched_switch_funcs,
+	},
+	.print_fmt		= SCHED_PRINT_FMT,
+	.module			= THIS_MODULE,
+	.flags			= TRACE_EVENT_FL_TRACEPOINT,
+};
+
+static struct trace_event_call sched_waking_call = {
+	.class			= &sched_waking_class,
+	.event			= {
+		.funcs			= &sched_waking_funcs,
+	},
+	.print_fmt		= SCHED_WAKING_FMT,
+	.module			= THIS_MODULE,
+	.flags			= TRACE_EVENT_FL_TRACEPOINT,
+};
+
+static void fct(struct tracepoint *tp, void *priv)
+{
+	if (tp->name && strcmp(tp->name, "sched_switch") == 0)
+		sched_switch_call.tp = tp;
+	else if (tp->name && strcmp(tp->name, "sched_waking") == 0)
+		sched_waking_call.tp = tp;
+}
+
+static int add_event(struct trace_event_call *call)
+{
+	int ret;
+
+	ret = register_trace_event(&call->event);
+	if (WARN_ON(!ret))
+		return -ENODEV;
+
+	ret = trace_add_event_call(call);
+	if (WARN_ON(ret))
+		unregister_trace_event(&call->event);
+
+	return ret;
+}
+
+static int __init trace_sched_init(void)
+{
+	int ret;
+
+	check_trace_callback_type_sched_switch(sched_switch_probe);
+	check_trace_callback_type_sched_waking(sched_waking_probe);
+
+	for_each_kernel_tracepoint(fct, NULL);
+
+	ret = add_event(&sched_switch_call);
+	if (ret)
+		return ret;
+
+	ret = add_event(&sched_waking_call);
+	if (ret)
+		trace_remove_event_call(&sched_switch_call);
+
+	return ret;
+}
+
+static void __exit trace_sched_exit(void)
+{
+	trace_set_clr_event(THIS_SYSTEM, "sched_switch", 0);
+	trace_set_clr_event(THIS_SYSTEM, "sched_waking", 0);
+
+	trace_remove_event_call(&sched_switch_call);
+	trace_remove_event_call(&sched_waking_call);
+}
+
+module_init(trace_sched_init);
+module_exit(trace_sched_exit);
+
+MODULE_AUTHOR("Steven Rostedt");
+MODULE_DESCRIPTION("Custom scheduling events");
+MODULE_LICENSE("GPL");