tracing/documentation: Document dynamic ftracer internals

Add more details to the dynamic function tracing design implementation. Signed-off-by: Mike Frysinger <vapier@gentoo.org> LKML-Reference: <1279610015-10250-1-git-send-email-vapier@gentoo.org> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

tracing/documentation: Document dynamic ftracer internals
Add more details to the dynamic function tracing design implementation. Signed-off-by: Mike Frysinger <vapier@gentoo.org> LKML-Reference: <1279610015-10250-1-git-send-email-vapier@gentoo.org> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Mike Frysinger · Steven Rostedt
1 parent ef710e100c
Showing 2 changed files with 153 additions and 5 deletions Side-by-side Diff
Documentation/trace/ftrace-design.txt
include/linux/ftrace.h
@@ -13,7 +13,10 @@
 want more explanation of a feature in terms of common code, review the common
 ftrace.txt file.
  
+Ideally, everyone who wishes to retain performance while supporting tracing in
+their kernel should make it all the way to dynamic ftrace support.
  
+
 Prerequisites
 -------------
  
@@ -215,7 +218,7 @@
 exiting of a function.  On exit, the value is compared and if it does not
 match, then it will panic the kernel.  This is largely a sanity check for bad
 code generation with gcc.  If gcc for your port sanely updates the frame
-pointer under different opitmization levels, then ignore this option.
+pointer under different optimization levels, then ignore this option.
  
 However, adding support for it isn't terribly difficult.  In your assembly code
 that calls prepare_ftrace_return(), pass the frame pointer as the 3rd argument.
@@ -234,7 +237,7 @@
  
  
 HAVE_SYSCALL_TRACEPOINTS
----------------------
+------------------------
  
 You need very few things to get the syscalls tracing in an arch.
  
  
  
  
  
  
@@ -250,13 +253,153 @@
 HAVE_FTRACE_MCOUNT_RECORD
 -------------------------
  
-See scripts/recordmcount.pl for more info.
+See scripts/recordmcount.pl for more info.  Just fill in the arch-specific
+details for how to locate the addresses of mcount call sites via objdump.
+This option doesn't make much sense without also implementing dynamic ftrace.
  
+
+HAVE_DYNAMIC_FTRACE
+-------------------
+
+You will first need HAVE_FTRACE_MCOUNT_RECORD and HAVE_FUNCTION_TRACER, so
+scroll your reader back up if you got over eager.
+
+Once those are out of the way, you will need to implement:
+	- asm/ftrace.h:
+		- MCOUNT_ADDR
+		- ftrace_call_adjust()
+		- struct dyn_arch_ftrace{}
+	- asm code:
+		- mcount() (new stub)
+		- ftrace_caller()
+		- ftrace_call()
+		- ftrace_stub()
+	- C code:
+		- ftrace_dyn_arch_init()
+		- ftrace_make_nop()
+		- ftrace_make_call()
+		- ftrace_update_ftrace_func()
+
+First you will need to fill out some arch details in your asm/ftrace.h.
+
+Define MCOUNT_ADDR as the address of your mcount symbol similar to:
+	#define MCOUNT_ADDR ((unsigned long)mcount)
+Since no one else will have a decl for that function, you will need to:
+	extern void mcount(void);
+
+You will also need the helper function ftrace_call_adjust().  Most people
+will be able to stub it out like so:
+	static inline unsigned long ftrace_call_adjust(unsigned long addr)
+	{
+		return addr;
+	}
 <details to be filled>
  
+Lastly you will need the custom dyn_arch_ftrace structure.  If you need
+some extra state when runtime patching arbitrary call sites, this is the
+place.  For now though, create an empty struct:
+	struct dyn_arch_ftrace {
+		/* No extra data needed */
+	};
  
-HAVE_DYNAMIC_FTRACE
----------------------
+With the header out of the way, we can fill out the assembly code.  While we
+did already create a mcount() function earlier, dynamic ftrace only wants a
+stub function.  This is because the mcount() will only be used during boot
+and then all references to it will be patched out never to return.  Instead,
+the guts of the old mcount() will be used to create a new ftrace_caller()
+function.  Because the two are hard to merge, it will most likely be a lot
+easier to have two separate definitions split up by #ifdefs.  Same goes for
+the ftrace_stub() as that will now be inlined in ftrace_caller().
  
+Before we get confused anymore, let's check out some pseudo code so you can
+implement your own stuff in assembly:
+
+void mcount(void)
+{
+	return;
+}
+
+void ftrace_caller(void)
+{
+	/* implement HAVE_FUNCTION_TRACE_MCOUNT_TEST if you desire */
+
+	/* save all state needed by the ABI (see paragraph above) */
+
+	unsigned long frompc = ...;
+	unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
+
+ftrace_call:
+	ftrace_stub(frompc, selfpc);
+
+	/* restore all state needed by the ABI */
+
+ftrace_stub:
+	return;
+}
+
+This might look a little odd at first, but keep in mind that we will be runtime
+patching multiple things.  First, only functions that we actually want to trace
+will be patched to call ftrace_caller().  Second, since we only have one tracer
+active at a time, we will patch the ftrace_caller() function itself to call the
+specific tracer in question.  That is the point of the ftrace_call label.
+
+With that in mind, let's move on to the C code that will actually be doing the
+runtime patching.  You'll need a little knowledge of your arch's opcodes in
+order to make it through the next section.
+
+Every arch has an init callback function.  If you need to do something early on
+to initialize some state, this is the time to do that.  Otherwise, this simple
+function below should be sufficient for most people:
+
+int __init ftrace_dyn_arch_init(void *data)
+{
+	/* return value is done indirectly via data */
+	*(unsigned long *)data = 0;
+
+	return 0;
+}
+
+There are two functions that are used to do runtime patching of arbitrary
+functions.  The first is used to turn the mcount call site into a nop (which
+is what helps us retain runtime performance when not tracing).  The second is
+used to turn the mcount call site into a call to an arbitrary location (but
+typically that is ftracer_caller()).  See the general function definition in
+linux/ftrace.h for the functions:
+	ftrace_make_nop()
+	ftrace_make_call()
+The rec->ip value is the address of the mcount call site that was collected
+by the scripts/recordmcount.pl during build time.
+
+The last function is used to do runtime patching of the active tracer.  This
+will be modifying the assembly code at the location of the ftrace_call symbol
+inside of the ftrace_caller() function.  So you should have sufficient padding
+at that location to support the new function calls you'll be inserting.  Some
+people will be using a "call" type instruction while others will be using a
+"branch" type instruction.  Specifically, the function is:
+	ftrace_update_ftrace_func()
+
+
+HAVE_DYNAMIC_FTRACE + HAVE_FUNCTION_GRAPH_TRACER
+------------------------------------------------
+
+The function grapher needs a few tweaks in order to work with dynamic ftrace.
+Basically, you will need to:
+	- update:
+		- ftrace_caller()
+		- ftrace_graph_call()
+		- ftrace_graph_caller()
+	- implement:
+		- ftrace_enable_ftrace_graph_caller()
+		- ftrace_disable_ftrace_graph_caller()
+
 <details to be filled>
+Quick notes:
+	- add a nop stub after the ftrace_call location named ftrace_graph_call;
+	  stub needs to be large enough to support a call to ftrace_graph_caller()
+	- update ftrace_graph_caller() to work with being called by the new
+	  ftrace_caller() since some semantics may have changed
+	- ftrace_enable_ftrace_graph_caller() will runtime patch the
+	  ftrace_graph_call location with a call to ftrace_graph_caller()
+	- ftrace_disable_ftrace_graph_caller() will runtime patch the
+	  ftrace_graph_call location with nops
+/*
+ * Ftrace header.  For implementation details beyond the random comments
+ * scattered below, see: Documentation/trace/ftrace-design.txt
+ */
+
 #ifndef _LINUX_FTRACE_H
 #define _LINUX_FTRACE_H
...	...	@@ -13,7 +13,10 @@
13	13	want more explanation of a feature in terms of common code, review the common
14	14	ftrace.txt file.
15	15
	16	+Ideally, everyone who wishes to retain performance while supporting tracing in
	17	+their kernel should make it all the way to dynamic ftrace support.
16	18
	19	+
17	20	Prerequisites
18	21	-------------
19	22
...	...	@@ -215,7 +218,7 @@
215	218	exiting of a function. On exit, the value is compared and if it does not
216	219	match, then it will panic the kernel. This is largely a sanity check for bad
217	220	code generation with gcc. If gcc for your port sanely updates the frame
218		-pointer under different opitmization levels, then ignore this option.
	221	+pointer under different optimization levels, then ignore this option.
219	222
220	223	However, adding support for it isn't terribly difficult. In your assembly code
221	224	that calls prepare_ftrace_return(), pass the frame pointer as the 3rd argument.
...	...	@@ -234,7 +237,7 @@
234	237
235	238
236	239	HAVE_SYSCALL_TRACEPOINTS
237		----------------------
	240	+------------------------
238	241
239	242	You need very few things to get the syscalls tracing in an arch.
240	243
241	244
242	245
243	246
244	247
245	248
...	...	@@ -250,13 +253,153 @@
250	253	HAVE_FTRACE_MCOUNT_RECORD
251	254	-------------------------
252	255
253		-See scripts/recordmcount.pl for more info.
	256	+See scripts/recordmcount.pl for more info. Just fill in the arch-specific
	257	+details for how to locate the addresses of mcount call sites via objdump.
	258	+This option doesn't make much sense without also implementing dynamic ftrace.
254	259
	260	+
	261	+HAVE_DYNAMIC_FTRACE
	262	+-------------------
	263	+
	264	+You will first need HAVE_FTRACE_MCOUNT_RECORD and HAVE_FUNCTION_TRACER, so
	265	+scroll your reader back up if you got over eager.
	266	+
	267	+Once those are out of the way, you will need to implement:
	268	+ - asm/ftrace.h:
	269	+ - MCOUNT_ADDR
	270	+ - ftrace_call_adjust()
	271	+ - struct dyn_arch_ftrace{}
	272	+ - asm code:
	273	+ - mcount() (new stub)
	274	+ - ftrace_caller()
	275	+ - ftrace_call()
	276	+ - ftrace_stub()
	277	+ - C code:
	278	+ - ftrace_dyn_arch_init()
	279	+ - ftrace_make_nop()
	280	+ - ftrace_make_call()
	281	+ - ftrace_update_ftrace_func()
	282	+
	283	+First you will need to fill out some arch details in your asm/ftrace.h.
	284	+
	285	+Define MCOUNT_ADDR as the address of your mcount symbol similar to:
	286	+ #define MCOUNT_ADDR ((unsigned long)mcount)
	287	+Since no one else will have a decl for that function, you will need to:
	288	+ extern void mcount(void);
	289	+
	290	+You will also need the helper function ftrace_call_adjust(). Most people
	291	+will be able to stub it out like so:
	292	+ static inline unsigned long ftrace_call_adjust(unsigned long addr)
	293	+ {
	294	+ return addr;
	295	+ }
255	296	<details to be filled>
256	297
	298	+Lastly you will need the custom dyn_arch_ftrace structure. If you need
	299	+some extra state when runtime patching arbitrary call sites, this is the
	300	+place. For now though, create an empty struct:
	301	+ struct dyn_arch_ftrace {
	302	+ /* No extra data needed */
	303	+ };
257	304
258		-HAVE_DYNAMIC_FTRACE
259		----------------------
	305	+With the header out of the way, we can fill out the assembly code. While we
	306	+did already create a mcount() function earlier, dynamic ftrace only wants a
	307	+stub function. This is because the mcount() will only be used during boot
	308	+and then all references to it will be patched out never to return. Instead,
	309	+the guts of the old mcount() will be used to create a new ftrace_caller()
	310	+function. Because the two are hard to merge, it will most likely be a lot
	311	+easier to have two separate definitions split up by #ifdefs. Same goes for
	312	+the ftrace_stub() as that will now be inlined in ftrace_caller().
260	313
	314	+Before we get confused anymore, let's check out some pseudo code so you can
	315	+implement your own stuff in assembly:
	316	+
	317	+void mcount(void)
	318	+{
	319	+ return;
	320	+}
	321	+
	322	+void ftrace_caller(void)
	323	+{
	324	+ /* implement HAVE_FUNCTION_TRACE_MCOUNT_TEST if you desire */
	325	+
	326	+ /* save all state needed by the ABI (see paragraph above) */
	327	+
	328	+ unsigned long frompc = ...;
	329	+ unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
	330	+
	331	+ftrace_call:
	332	+ ftrace_stub(frompc, selfpc);
	333	+
	334	+ /* restore all state needed by the ABI */
	335	+
	336	+ftrace_stub:
	337	+ return;
	338	+}
	339	+
	340	+This might look a little odd at first, but keep in mind that we will be runtime
	341	+patching multiple things. First, only functions that we actually want to trace
	342	+will be patched to call ftrace_caller(). Second, since we only have one tracer
	343	+active at a time, we will patch the ftrace_caller() function itself to call the
	344	+specific tracer in question. That is the point of the ftrace_call label.
	345	+
	346	+With that in mind, let's move on to the C code that will actually be doing the
	347	+runtime patching. You'll need a little knowledge of your arch's opcodes in
	348	+order to make it through the next section.
	349	+
	350	+Every arch has an init callback function. If you need to do something early on
	351	+to initialize some state, this is the time to do that. Otherwise, this simple
	352	+function below should be sufficient for most people:
	353	+
	354	+int __init ftrace_dyn_arch_init(void *data)
	355	+{
	356	+ /* return value is done indirectly via data */
	357	+ (unsigned long )data = 0;
	358	+
	359	+ return 0;
	360	+}
	361	+
	362	+There are two functions that are used to do runtime patching of arbitrary
	363	+functions. The first is used to turn the mcount call site into a nop (which
	364	+is what helps us retain runtime performance when not tracing). The second is
	365	+used to turn the mcount call site into a call to an arbitrary location (but
	366	+typically that is ftracer_caller()). See the general function definition in
	367	+linux/ftrace.h for the functions:
	368	+ ftrace_make_nop()
	369	+ ftrace_make_call()
	370	+The rec->ip value is the address of the mcount call site that was collected
	371	+by the scripts/recordmcount.pl during build time.
	372	+
	373	+The last function is used to do runtime patching of the active tracer. This
	374	+will be modifying the assembly code at the location of the ftrace_call symbol
	375	+inside of the ftrace_caller() function. So you should have sufficient padding
	376	+at that location to support the new function calls you'll be inserting. Some
	377	+people will be using a "call" type instruction while others will be using a
	378	+"branch" type instruction. Specifically, the function is:
	379	+ ftrace_update_ftrace_func()
	380	+
	381	+
	382	+HAVE_DYNAMIC_FTRACE + HAVE_FUNCTION_GRAPH_TRACER
	383	+------------------------------------------------
	384	+
	385	+The function grapher needs a few tweaks in order to work with dynamic ftrace.
	386	+Basically, you will need to:
	387	+ - update:
	388	+ - ftrace_caller()
	389	+ - ftrace_graph_call()
	390	+ - ftrace_graph_caller()
	391	+ - implement:
	392	+ - ftrace_enable_ftrace_graph_caller()
	393	+ - ftrace_disable_ftrace_graph_caller()
	394	+
261	395	<details to be filled>
	396	+Quick notes:
	397	+ - add a nop stub after the ftrace_call location named ftrace_graph_call;
	398	+ stub needs to be large enough to support a call to ftrace_graph_caller()
	399	+ - update ftrace_graph_caller() to work with being called by the new
	400	+ ftrace_caller() since some semantics may have changed
	401	+ - ftrace_enable_ftrace_graph_caller() will runtime patch the
	402	+ ftrace_graph_call location with a call to ftrace_graph_caller()
	403	+ - ftrace_disable_ftrace_graph_caller() will runtime patch the
	404	+ ftrace_graph_call location with nops
	1	+/*
	2	+ * Ftrace header. For implementation details beyond the random comments
	3	+ * scattered below, see: Documentation/trace/ftrace-design.txt
	4	+ */
	5	+
1	6	#ifndef _LINUX_FTRACE_H
2	7	#define _LINUX_FTRACE_H
3	8