[RFC/PATCH v2] ftrace: Reduce size of function graph entries

[Namhyung Kim]

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.  I think the FTRACE_ALIGNMENT still needs to have proper
alignment (even if ring buffer handles the alignment after all) since
the ftrace_graph_ent/ret struct is located on stack before copying to
the ring buffer.

Tested on x86_64 only.

Cc: linux-arch@xxxxxxxxxxxxxxx
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 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..a86cdf167419 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
 
+#ifdef CONFIG_HAVE_64BIT_ALIGNED_ACCESS
+# define FTRACE_ALIGNMENT	8
+#else
+# define FTRACE_ALIGNMENT	4
+#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