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Subject[RFC/PATCH] ftrace: Reduce size of function graph entries
Date
Currently ftrace_graph_ent{,_entry} and ftrace_graph_ret{,_entry} struct
can have padding bytes at the end due to alignment in 64-bit data type.
As these data are recorded so frequently, those paddings waste
non-negligible space. As some archs can have efficient unaligned
accesses, reducing the alignment can save ~10% of data size:

ftrace_graph_ent_entry: 24 -> 20
ftrace_graph_ret_entry: 48 -> 44

Also I moved the 'overrun' field in struct ftrace_graph_ret to minimize
the padding. Tested on x86_64 only.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
include/linux/ftrace.h | 16 ++++++++++++----
kernel/trace/trace.h | 11 +++++++++++
kernel/trace/trace_entries.h | 4 ++--
3 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index dea12a6e413b..35c523ba5c59 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -751,25 +751,33 @@ extern void ftrace_init(void);
static inline void ftrace_init(void) { }
#endif

+#ifndef CONFIG_HAVE_64BIT_ALIGNED_ACCESS
+# define FTRACE_ALIGNMENT 4
+#else
+# define FTRACE_ALIGNMENT 8
+#endif
+
+#define FTRACE_ALIGN_DATA __attribute__((packed, aligned(FTRACE_ALIGNMENT)))
+
/*
* Structure that defines an entry function trace.
*/
struct ftrace_graph_ent {
unsigned long func; /* Current function */
int depth;
-};
+} FTRACE_ALIGN_DATA;

/*
* Structure that defines a return function trace.
*/
struct ftrace_graph_ret {
unsigned long func; /* Current function */
- unsigned long long calltime;
- unsigned long long rettime;
/* Number of functions that overran the depth limit for current task */
unsigned long overrun;
+ unsigned long long calltime;
+ unsigned long long rettime;
int depth;
-};
+} FTRACE_ALIGN_DATA;

/* Type of the callback handlers for tracing function graph*/
typedef void (*trace_func_graph_ret_t)(struct ftrace_graph_ret *); /* return */
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 5167c366d6b7..d2dd49ca55ee 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -80,6 +80,12 @@ enum trace_type {
FTRACE_ENTRY(name, struct_name, id, PARAMS(tstruct), PARAMS(print), \
filter)

+#undef FTRACE_ENTRY_PACKED
+#define FTRACE_ENTRY_PACKED(name, struct_name, id, tstruct, print, \
+ filter) \
+ FTRACE_ENTRY(name, struct_name, id, PARAMS(tstruct), PARAMS(print), \
+ filter) FTRACE_ALIGN_DATA
+
#include "trace_entries.h"

/*
@@ -1600,6 +1606,11 @@ int set_tracer_flag(struct trace_array *tr, unsigned int mask, int enabled);
#define FTRACE_ENTRY_DUP(call, struct_name, id, tstruct, print, filter) \
FTRACE_ENTRY(call, struct_name, id, PARAMS(tstruct), PARAMS(print), \
filter)
+#undef FTRACE_ENTRY_PACKED
+#define FTRACE_ENTRY_PACKED(call, struct_name, id, tstruct, print, filter) \
+ FTRACE_ENTRY(call, struct_name, id, PARAMS(tstruct), PARAMS(print), \
+ filter)
+
#include "trace_entries.h"

#if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_FUNCTION_TRACER)
diff --git a/kernel/trace/trace_entries.h b/kernel/trace/trace_entries.h
index ee7b94a4810a..5c30efcda5e6 100644
--- a/kernel/trace/trace_entries.h
+++ b/kernel/trace/trace_entries.h
@@ -72,7 +72,7 @@ FTRACE_ENTRY_REG(function, ftrace_entry,
);

/* Function call entry */
-FTRACE_ENTRY(funcgraph_entry, ftrace_graph_ent_entry,
+FTRACE_ENTRY_PACKED(funcgraph_entry, ftrace_graph_ent_entry,

TRACE_GRAPH_ENT,

@@ -88,7 +88,7 @@ FTRACE_ENTRY(funcgraph_entry, ftrace_graph_ent_entry,
);

/* Function return entry */
-FTRACE_ENTRY(funcgraph_exit, ftrace_graph_ret_entry,
+FTRACE_ENTRY_PACKED(funcgraph_exit, ftrace_graph_ret_entry,

TRACE_GRAPH_RET,

--
2.8.0
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