Re: [PATCH v3 1/5] tracing: add ref_trace_final_put tracepoint
From: Steven Rostedt <rostedt@goodmis.org>
Date: 2026-07-27 20:20:59
Also in:
linux-mm, lkml
On Sat, 11 Jul 2026 12:40:19 +0800 Eugene Mavick [off-list ref] 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.
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- obj: refcount object(struct percpu_ref, refcount_t) Signed-off-by: Eugene Mavick <redacted> --- 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.
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+ } 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
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+); + +#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.odiff --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);