Re: [PATCH v3 1/5] tracing: add ref_trace_final_put tracepoint

[Steven Rostedt <rostedt@goodmis.org>]

On Sat, 11 Jul 2026 12:40:19 +0800
Eugene Mavick <m@mavick.dev> wrote:

> Add ref_trace_final_put tracepoint and related core infrastructure
> 
> ref_trace_final_put fires when a reference
> count reaches zero and the object enters its final release path.
> 
> The tracepoint records three fields:
> - caller: function that called the refcounting
>   function(refcount_sub_and_test, percpu_ref_put_many)
> - fn: refcounting function(eg refcount_sub_and_test)

Why save the string just to figure out which function is used. It's quite
expensive in both recording the string and saving the name all over the
place.

> - obj: refcount object(struct percpu_ref, refcount_t)
> 
> Signed-off-by: Eugene Mavick <m@mavick.dev>
> ---
>  include/linux/ref_trace.h        | 26 +++++++++++++++++++++++
>  include/trace/events/ref_trace.h | 46 ++++++++++++++++++++++++++++++++++++++++
>  lib/Makefile                     |  2 ++
>  lib/ref_trace.c                  | 13 ++++++++++++
>  4 files changed, 87 insertions(+)
> 
> diff --git a/include/linux/ref_trace.h b/include/linux/ref_trace.h
> new file mode 100644
> index 000000000000..6c6600938a47
> --- /dev/null
> +++ b/include/linux/ref_trace.h
> @@ -0,0 +1,26 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_REF_TRACE_H
> +#define _LINUX_REF_TRACE_H
> +
> +#include <linux/tracepoint-defs.h>
> +#include <linux/instruction_pointer.h>
> +
> +/* Declare the tracepoint so tracepoint_enabled() can be used */
> +DECLARE_TRACEPOINT(ref_trace_final_put);
> +
> +#ifdef CONFIG_TRACEPOINTS
> +/* Wrapper function implemented in lib/ref_trace.c */
> +extern void do_ref_trace_final_put(unsigned long caller, const char *fn, const void *obj);
> +
> +#define do_trace_ref_final_put(obj)						\
> +	do {									\
> +		if (tracepoint_enabled(ref_trace_final_put))			\
> +			do_ref_trace_final_put(_RET_IP_, __func__, obj);	\

Why save __func__ and and not just __THIS_IP__?

The name is quite expensive to save and even to store in memory, as it will
*always* be there.

> +	} while (0)
> +
> +#else /* !CONFIG_TRACEPOINTS */
> +static inline void do_ref_trace_final_put(unsigned long caller, const char *fn, const void *obj) { }
> +#define do_trace_ref_final_put(obj) do { } while (0)
> +#endif
> +
> +#endif /* _LINUX_REF_TRACE_H */
> diff --git a/include/trace/events/ref_trace.h b/include/trace/events/ref_trace.h
> new file mode 100644
> index 000000000000..3bf1b1d390b8
> --- /dev/null
> +++ b/include/trace/events/ref_trace.h
> @@ -0,0 +1,46 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#undef TRACE_SYSTEM
> +#define TRACE_SYSTEM ref_trace
> +
> +#if !defined(_TRACE_REF_TRACE_H) || defined(TRACE_HEADER_MULTI_READ)
> +#define _TRACE_REF_TRACE_H
> +
> +#include <linux/tracepoint.h>
> +
> +/**
> + * ref_trace_final_put - trace when a reference count reaches zero
> + * @caller: return address of refcount
> + * function(refcount_sub_and_test, percpu_ref_put_many)
> + * @fn: refcounting function(eg refcount_sub_and_test)
> + * @obj: refcount object(struct percpu_ref, refcount_t)
> + *
> + * Tracepoint instrumentation can be added using the do_ref_trace_final_put
> + * macro defined in include/linux/ref_trace.h
> + * which uses _RET_IP_ and __func__ for caller and fn arguments respectively,
> + * thus only requiring obj arg to be supplied
> + */
> +TRACE_EVENT(ref_trace_final_put,
> +
> +	TP_PROTO(unsigned long caller, const char *fn, const void *obj),
> +
> +	TP_ARGS(caller, fn, obj),
> +
> +	TP_STRUCT__entry(
> +	__field(unsigned long, caller)
> +	__string(fn, fn)
> +	__field(const void *, obj)

The TP_STRUCT__entry is to be in structure format:

	TP_STRUCT__entry(
		__field(	unsigned long,		caller	)
		__field(	unsigned long,		ip	)
		__field(	const void *,		obj	)
> +	),

See how much better that looks and easier to read. Just like writing:

struct entry {
	unsigned long	caller;
	unsigned long	ip;
	const void	*obj;
};

	
> +
> +	TP_fast_assign(
> +	__entry->caller = caller;
> +	__assign_str(fn);
> +	__entry->obj = obj;

The code should be indented:

	TP_fast_assign(
		__entry->caller = caller;
		__entry->ip = ip;
		__entry->obj = obj;
> +	),

> +
> +	TP_printk("caller=%pS fn=%s obj=%p", (void *)__entry->caller, __get_str(fn), __entry->obj)

	TP_printk("caller=%pS fn=%pS obj=%p", ...

-- Steve

> +);
> +
> +#endif /* _TRACE_REF_TRACE_H */
> +
> +/* This part must be outside protection */
> +#include <trace/define_trace.h>
> diff --git a/lib/Makefile b/lib/Makefile
> index f33a24bf1c19..41737090a95d 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -335,3 +335,5 @@ CONTEXT_ANALYSIS_test_context-analysis.o := y
>  obj-$(CONFIG_CONTEXT_ANALYSIS_TEST) += test_context-analysis.o
>  
>  subdir-$(CONFIG_FORTIFY_SOURCE) += test_fortify
> +
> +obj-$(CONFIG_TRACEPOINTS) += ref_trace.o
> diff --git a/lib/ref_trace.c b/lib/ref_trace.c
> new file mode 100644
> index 000000000000..466647772ce1
> --- /dev/null
> +++ b/lib/ref_trace.c
> @@ -0,0 +1,13 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#define CREATE_TRACE_POINTS
> +#include <trace/events/ref_trace.h>
> +#include <linux/ref_trace.h>
> +
> +//Wrapper function for functions defined entirely in header files
> +void do_ref_trace_final_put(unsigned long caller, const char *fn, const void *obj)
> +{
> +	trace_call__ref_trace_final_put(caller, fn, obj);
> +}
> +EXPORT_SYMBOL_GPL(do_ref_trace_final_put);
> +
> +EXPORT_TRACEPOINT_SYMBOL_GPL(ref_trace_final_put);
>