Re: [PATCH v2 0/5] tracepoints: Add warnings for unused tracepoints and trace events

From: Steven Rostedt
Date: Fri Jun 13 2025 - 10:30:32 EST

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On Thu, 12 Jun 2025 19:58:27 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> Every trace event can take up to 5K of memory in text and meta data regardless
> if they are used or not. Trace events should not be created if they are not
> used. Currently there's over a hundred events in the kernel that are defined
> but unused, either because their callers were removed without removing the
> trace event with it, or a config hides the trace event caller but not the
> trace event itself. And in some cases, trace events were simply added but were
> never called for whatever reason. The number of unused trace events continues
> to grow.

Now it's been a while since I looked at the actual sizes, so I decided
to see what they are again.

So I created a trace header with 10 events (attached file), that had this:

TRACE_EVENT(size_event_1,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);

And I created 9 more by just renaming the event name (size_event_2, etc).

I also looked at how well DEFINE_EVENT() works (note a TRACE_EVENT()
macro is just a DECLARE_EVENT_CLASS() followed by a DEFINE_EVENT() with
the same name as the class, so I could use the first TRACE_EVENT as a
class and the first event).

DEFINE_EVENT(size_event_1, size_event_2,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B));

The module is simply:

echo '#include <linux/module.h>

#define CREATE_TRACE_POINTS
#include "size_events.h"

static __init int size_init(void)
{
return 0;
}

static __exit void size_exit(void)
{
}

module_init(size_init);
module_exit(size_exit);

MODULE_AUTHOR("Steven Rostedt");
MODULE_DESCRIPTION("Test the size of trace event");
MODULE_LICENSE("GPL");' > event-mod.c

The results are (renaming the module to what they did):

text data bss dec hex filename
629 1440 0 2069 815 no-events.ko
44837 15424 0 60261 eb65 trace-events.ko
11495 8064 0 19559 4c67 define-events.ko

With no events, the size is 2069.
With full trace events it jumped to 60261
With One DECLARE_EVENT_CLASS() and 9 DEFINE_EVENT(), it changed to 19559

That means each TRACE_EVENT() is approximately 5819 bytes.
(60261 - 2069) / 10

And each DEFINE_EVENT() is approximately 1296 bytes.
((19559 - 2069) - 5819) / 9

Now I do have a bit of debugging options enabled which could cause this
to bloat even more. But yeah, trace events do take up a bit of memory.

-- Steve

/* SPDX-License-Identifier: GPL-2.0 */
#undef TRACE_SYSTEM
#define TRACE_SYSTEM event-sizes

#undef TRACE_SYSTEM_VAR
#define TRACE_SYSTEM_VAR event_sizes

#if !defined(_SIZE_EVENT_H) || defined(TRACE_HEADER_MULTI_READ)
#define _SIZE_EVENT_H

#include <linux/tracepoint.h>

#ifndef SIZE_EVENT_DEFINED
#define SIZE_EVENT_DEFINED
struct size_event_struct {
unsigned long a;
unsigned long b;
};
#endif

#define DEFINE_EVENT_SIZES 1
#define DEFINE_FULL_EVENTS 0

#if DEFINE_EVENT_SIZES

TRACE_EVENT(size_event_1,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);

#if DEFINE_FULL_EVENTS
TRACE_EVENT(size_event_2,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);
TRACE_EVENT(size_event_3,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);
TRACE_EVENT(size_event_4,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);
TRACE_EVENT(size_event_5,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);
TRACE_EVENT(size_event_6,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);
TRACE_EVENT(size_event_7,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);
TRACE_EVENT(size_event_8,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);
TRACE_EVENT(size_event_9,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);
TRACE_EVENT(size_event_10,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B),
TP_STRUCT__entry(
__field( unsigned long, Aa)
__field( unsigned long, Ab)
__field( unsigned long, Ba)
__field( unsigned long, Bb)
),
TP_fast_assign(
__entry->Aa = A->a;
__entry->Ab = A->b;
__entry->Ba = B->a;
__entry->Bb = B->b;
),
TP_printk("Aa=%ld Ab=%ld Ba=%ld Bb=%ld",
__entry->Aa, __entry->Ab, __entry->Ba, __entry->Bb)
);
#else /* !DEFINE_FULL_EVENTS */
DEFINE_EVENT(size_event_1, size_event_2,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B));

DEFINE_EVENT(size_event_1, size_event_3,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B));

DEFINE_EVENT(size_event_1, size_event_4,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B));

DEFINE_EVENT(size_event_1, size_event_5,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B));

DEFINE_EVENT(size_event_1, size_event_6,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B));

DEFINE_EVENT(size_event_1, size_event_7,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B));

DEFINE_EVENT(size_event_1, size_event_8,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B));

DEFINE_EVENT(size_event_1, size_event_9,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B));

DEFINE_EVENT(size_event_1, size_event_10,
TP_PROTO(struct size_event_struct *A, struct size_event_struct *B),
TP_ARGS(A, B));

#endif /* !DEFINE_FULL_EVENTS */
#endif /* DEFINE_EVENT_SIZES */

#endif

/***** NOTICE! The #if protection ends here. *****/

#undef TRACE_INCLUDE_PATH
#undef TRACE_INCLUDE_FILE
#define TRACE_INCLUDE_PATH .
#define TRACE_INCLUDE_FILE size_events
#include <trace/define_trace.h>

Next message: Matthew Gerlach: "Re: [PATCH] fpga: altera-cvp: Add FPGA Configuration Framework dependency"
Previous message: Lorenzo Stoakes: "Re: [PATCH v2 0/2] fix MADV_COLLAPSE issue if THP settings are disabled"
In reply to: Steven Rostedt: "Re: [PATCH v2 0/5] tracepoints: Add warnings for unused tracepoints and trace events"
Next in thread: Steven Rostedt: "Re: [PATCH v2 0/5] tracepoints: Add warnings for unused tracepoints and trace events"
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