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Mike Frysinger555f3862009-09-14 20:10:15 -04001 function tracer guts
2 ====================
Mike Frysinger03688972010-01-22 08:12:47 -05003 By Mike Frysinger
Mike Frysinger555f3862009-09-14 20:10:15 -04004
5Introduction
6------------
7
8Here we will cover the architecture pieces that the common function tracing
9code relies on for proper functioning. Things are broken down into increasing
10complexity so that you can start simple and at least get basic functionality.
11
12Note that this focuses on architecture implementation details only. If you
13want more explanation of a feature in terms of common code, review the common
14ftrace.txt file.
15
Mike Frysinger9849ed42010-07-20 03:13:35 -040016Ideally, everyone who wishes to retain performance while supporting tracing in
17their kernel should make it all the way to dynamic ftrace support.
18
Mike Frysinger555f3862009-09-14 20:10:15 -040019
20Prerequisites
21-------------
22
23Ftrace relies on these features being implemented:
24 STACKTRACE_SUPPORT - implement save_stack_trace()
25 TRACE_IRQFLAGS_SUPPORT - implement include/asm/irqflags.h
26
27
28HAVE_FUNCTION_TRACER
29--------------------
30
31You will need to implement the mcount and the ftrace_stub functions.
32
33The exact mcount symbol name will depend on your toolchain. Some call it
34"mcount", "_mcount", or even "__mcount". You can probably figure it out by
35running something like:
36 $ echo 'main(){}' | gcc -x c -S -o - - -pg | grep mcount
37 call mcount
38We'll make the assumption below that the symbol is "mcount" just to keep things
39nice and simple in the examples.
40
41Keep in mind that the ABI that is in effect inside of the mcount function is
42*highly* architecture/toolchain specific. We cannot help you in this regard,
43sorry. Dig up some old documentation and/or find someone more familiar than
44you to bang ideas off of. Typically, register usage (argument/scratch/etc...)
45is a major issue at this point, especially in relation to the location of the
46mcount call (before/after function prologue). You might also want to look at
47how glibc has implemented the mcount function for your architecture. It might
48be (semi-)relevant.
49
50The mcount function should check the function pointer ftrace_trace_function
51to see if it is set to ftrace_stub. If it is, there is nothing for you to do,
52so return immediately. If it isn't, then call that function in the same way
53the mcount function normally calls __mcount_internal -- the first argument is
54the "frompc" while the second argument is the "selfpc" (adjusted to remove the
55size of the mcount call that is embedded in the function).
56
57For example, if the function foo() calls bar(), when the bar() function calls
58mcount(), the arguments mcount() will pass to the tracer are:
59 "frompc" - the address bar() will use to return to foo()
Randy Dunlap7e25f442009-12-18 15:17:12 -080060 "selfpc" - the address bar() (with mcount() size adjustment)
Mike Frysinger555f3862009-09-14 20:10:15 -040061
62Also keep in mind that this mcount function will be called *a lot*, so
63optimizing for the default case of no tracer will help the smooth running of
64your system when tracing is disabled. So the start of the mcount function is
Randy Dunlap7e25f442009-12-18 15:17:12 -080065typically the bare minimum with checking things before returning. That also
66means the code flow should usually be kept linear (i.e. no branching in the nop
67case). This is of course an optimization and not a hard requirement.
Mike Frysinger555f3862009-09-14 20:10:15 -040068
69Here is some pseudo code that should help (these functions should actually be
70implemented in assembly):
71
72void ftrace_stub(void)
73{
74 return;
75}
76
77void mcount(void)
78{
79 /* save any bare state needed in order to do initial checking */
80
81 extern void (*ftrace_trace_function)(unsigned long, unsigned long);
82 if (ftrace_trace_function != ftrace_stub)
83 goto do_trace;
84
85 /* restore any bare state */
86
87 return;
88
89do_trace:
90
91 /* save all state needed by the ABI (see paragraph above) */
92
93 unsigned long frompc = ...;
94 unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
95 ftrace_trace_function(frompc, selfpc);
96
97 /* restore all state needed by the ABI */
98}
99
100Don't forget to export mcount for modules !
101extern void mcount(void);
102EXPORT_SYMBOL(mcount);
103
104
105HAVE_FUNCTION_TRACE_MCOUNT_TEST
106-------------------------------
107
108This is an optional optimization for the normal case when tracing is turned off
109in the system. If you do not enable this Kconfig option, the common ftrace
110code will take care of doing the checking for you.
111
112To support this feature, you only need to check the function_trace_stop
113variable in the mcount function. If it is non-zero, there is no tracing to be
114done at all, so you can return.
115
116This additional pseudo code would simply be:
117void mcount(void)
118{
119 /* save any bare state needed in order to do initial checking */
120
121+ if (function_trace_stop)
122+ return;
123
124 extern void (*ftrace_trace_function)(unsigned long, unsigned long);
125 if (ftrace_trace_function != ftrace_stub)
126...
127
128
129HAVE_FUNCTION_GRAPH_TRACER
130--------------------------
131
132Deep breath ... time to do some real work. Here you will need to update the
133mcount function to check ftrace graph function pointers, as well as implement
134some functions to save (hijack) and restore the return address.
135
136The mcount function should check the function pointers ftrace_graph_return
137(compare to ftrace_stub) and ftrace_graph_entry (compare to
Randy Dunlap7e25f442009-12-18 15:17:12 -0800138ftrace_graph_entry_stub). If either of those is not set to the relevant stub
Mike Frysinger555f3862009-09-14 20:10:15 -0400139function, call the arch-specific function ftrace_graph_caller which in turn
140calls the arch-specific function prepare_ftrace_return. Neither of these
Randy Dunlap7e25f442009-12-18 15:17:12 -0800141function names is strictly required, but you should use them anyway to stay
Mike Frysinger555f3862009-09-14 20:10:15 -0400142consistent across the architecture ports -- easier to compare & contrast
143things.
144
145The arguments to prepare_ftrace_return are slightly different than what are
146passed to ftrace_trace_function. The second argument "selfpc" is the same,
147but the first argument should be a pointer to the "frompc". Typically this is
148located on the stack. This allows the function to hijack the return address
149temporarily to have it point to the arch-specific function return_to_handler.
150That function will simply call the common ftrace_return_to_handler function and
Randy Dunlap7e25f442009-12-18 15:17:12 -0800151that will return the original return address with which you can return to the
Mike Frysinger555f3862009-09-14 20:10:15 -0400152original call site.
153
154Here is the updated mcount pseudo code:
155void mcount(void)
156{
157...
158 if (ftrace_trace_function != ftrace_stub)
159 goto do_trace;
160
161+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
162+ extern void (*ftrace_graph_return)(...);
163+ extern void (*ftrace_graph_entry)(...);
164+ if (ftrace_graph_return != ftrace_stub ||
165+ ftrace_graph_entry != ftrace_graph_entry_stub)
166+ ftrace_graph_caller();
167+#endif
168
169 /* restore any bare state */
170...
171
172Here is the pseudo code for the new ftrace_graph_caller assembly function:
173#ifdef CONFIG_FUNCTION_GRAPH_TRACER
174void ftrace_graph_caller(void)
175{
176 /* save all state needed by the ABI */
177
178 unsigned long *frompc = &...;
179 unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
Mike Frysinger03688972010-01-22 08:12:47 -0500180 /* passing frame pointer up is optional -- see below */
181 prepare_ftrace_return(frompc, selfpc, frame_pointer);
Mike Frysinger555f3862009-09-14 20:10:15 -0400182
183 /* restore all state needed by the ABI */
184}
185#endif
186
Mike Frysinger03688972010-01-22 08:12:47 -0500187For information on how to implement prepare_ftrace_return(), simply look at the
188x86 version (the frame pointer passing is optional; see the next section for
189more information). The only architecture-specific piece in it is the setup of
Mike Frysinger555f3862009-09-14 20:10:15 -0400190the fault recovery table (the asm(...) code). The rest should be the same
191across architectures.
192
193Here is the pseudo code for the new return_to_handler assembly function. Note
194that the ABI that applies here is different from what applies to the mcount
195code. Since you are returning from a function (after the epilogue), you might
196be able to skimp on things saved/restored (usually just registers used to pass
197return values).
198
199#ifdef CONFIG_FUNCTION_GRAPH_TRACER
200void return_to_handler(void)
201{
202 /* save all state needed by the ABI (see paragraph above) */
203
204 void (*original_return_point)(void) = ftrace_return_to_handler();
205
206 /* restore all state needed by the ABI */
207
208 /* this is usually either a return or a jump */
209 original_return_point();
210}
211#endif
212
213
Mike Frysinger03688972010-01-22 08:12:47 -0500214HAVE_FUNCTION_GRAPH_FP_TEST
215---------------------------
216
217An arch may pass in a unique value (frame pointer) to both the entering and
218exiting of a function. On exit, the value is compared and if it does not
219match, then it will panic the kernel. This is largely a sanity check for bad
220code generation with gcc. If gcc for your port sanely updates the frame
Mike Frysinger9849ed42010-07-20 03:13:35 -0400221pointer under different optimization levels, then ignore this option.
Mike Frysinger03688972010-01-22 08:12:47 -0500222
223However, adding support for it isn't terribly difficult. In your assembly code
224that calls prepare_ftrace_return(), pass the frame pointer as the 3rd argument.
225Then in the C version of that function, do what the x86 port does and pass it
226along to ftrace_push_return_trace() instead of a stub value of 0.
227
228Similarly, when you call ftrace_return_to_handler(), pass it the frame pointer.
229
230
Mike Frysinger555f3862009-09-14 20:10:15 -0400231HAVE_FTRACE_NMI_ENTER
232---------------------
233
234If you can't trace NMI functions, then skip this option.
235
236<details to be filled>
237
238
Frederic Weisbecker459c6d12009-09-19 07:14:15 +0200239HAVE_SYSCALL_TRACEPOINTS
Mike Frysinger9849ed42010-07-20 03:13:35 -0400240------------------------
Mike Frysinger555f3862009-09-14 20:10:15 -0400241
Frederic Weisbecker459c6d12009-09-19 07:14:15 +0200242You need very few things to get the syscalls tracing in an arch.
243
Mike Frysingere7b8e672010-01-26 04:40:03 -0500244- Support HAVE_ARCH_TRACEHOOK (see arch/Kconfig).
Frederic Weisbecker459c6d12009-09-19 07:14:15 +0200245- Have a NR_syscalls variable in <asm/unistd.h> that provides the number
246 of syscalls supported by the arch.
Mike Frysingere7b8e672010-01-26 04:40:03 -0500247- Support the TIF_SYSCALL_TRACEPOINT thread flags.
Frederic Weisbecker459c6d12009-09-19 07:14:15 +0200248- Put the trace_sys_enter() and trace_sys_exit() tracepoints calls from ptrace
249 in the ptrace syscalls tracing path.
Ian Munsiec763ba02011-02-03 14:27:22 +1100250- If the system call table on this arch is more complicated than a simple array
251 of addresses of the system calls, implement an arch_syscall_addr to return
252 the address of a given system call.
Ian Munsieb2d55492011-02-03 14:27:23 +1100253- If the symbol names of the system calls do not match the function names on
254 this arch, define ARCH_HAS_SYSCALL_MATCH_SYM_NAME in asm/ftrace.h and
255 implement arch_syscall_match_sym_name with the appropriate logic to return
256 true if the function name corresponds with the symbol name.
Frederic Weisbecker459c6d12009-09-19 07:14:15 +0200257- Tag this arch as HAVE_SYSCALL_TRACEPOINTS.
Mike Frysinger555f3862009-09-14 20:10:15 -0400258
259
260HAVE_FTRACE_MCOUNT_RECORD
261-------------------------
262
Mike Frysinger9849ed42010-07-20 03:13:35 -0400263See scripts/recordmcount.pl for more info. Just fill in the arch-specific
264details for how to locate the addresses of mcount call sites via objdump.
265This option doesn't make much sense without also implementing dynamic ftrace.
Mike Frysinger555f3862009-09-14 20:10:15 -0400266
267
268HAVE_DYNAMIC_FTRACE
Mike Frysinger9849ed42010-07-20 03:13:35 -0400269-------------------
270
271You will first need HAVE_FTRACE_MCOUNT_RECORD and HAVE_FUNCTION_TRACER, so
272scroll your reader back up if you got over eager.
273
274Once those are out of the way, you will need to implement:
275 - asm/ftrace.h:
276 - MCOUNT_ADDR
277 - ftrace_call_adjust()
278 - struct dyn_arch_ftrace{}
279 - asm code:
280 - mcount() (new stub)
281 - ftrace_caller()
282 - ftrace_call()
283 - ftrace_stub()
284 - C code:
285 - ftrace_dyn_arch_init()
286 - ftrace_make_nop()
287 - ftrace_make_call()
288 - ftrace_update_ftrace_func()
289
290First you will need to fill out some arch details in your asm/ftrace.h.
291
292Define MCOUNT_ADDR as the address of your mcount symbol similar to:
293 #define MCOUNT_ADDR ((unsigned long)mcount)
294Since no one else will have a decl for that function, you will need to:
295 extern void mcount(void);
296
297You will also need the helper function ftrace_call_adjust(). Most people
298will be able to stub it out like so:
299 static inline unsigned long ftrace_call_adjust(unsigned long addr)
300 {
301 return addr;
302 }
303<details to be filled>
304
305Lastly you will need the custom dyn_arch_ftrace structure. If you need
306some extra state when runtime patching arbitrary call sites, this is the
307place. For now though, create an empty struct:
308 struct dyn_arch_ftrace {
309 /* No extra data needed */
310 };
311
312With the header out of the way, we can fill out the assembly code. While we
313did already create a mcount() function earlier, dynamic ftrace only wants a
314stub function. This is because the mcount() will only be used during boot
315and then all references to it will be patched out never to return. Instead,
316the guts of the old mcount() will be used to create a new ftrace_caller()
317function. Because the two are hard to merge, it will most likely be a lot
318easier to have two separate definitions split up by #ifdefs. Same goes for
319the ftrace_stub() as that will now be inlined in ftrace_caller().
320
321Before we get confused anymore, let's check out some pseudo code so you can
322implement your own stuff in assembly:
323
324void mcount(void)
325{
326 return;
327}
328
329void ftrace_caller(void)
330{
331 /* implement HAVE_FUNCTION_TRACE_MCOUNT_TEST if you desire */
332
333 /* save all state needed by the ABI (see paragraph above) */
334
335 unsigned long frompc = ...;
336 unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
337
338ftrace_call:
339 ftrace_stub(frompc, selfpc);
340
341 /* restore all state needed by the ABI */
342
343ftrace_stub:
344 return;
345}
346
347This might look a little odd at first, but keep in mind that we will be runtime
348patching multiple things. First, only functions that we actually want to trace
349will be patched to call ftrace_caller(). Second, since we only have one tracer
350active at a time, we will patch the ftrace_caller() function itself to call the
351specific tracer in question. That is the point of the ftrace_call label.
352
353With that in mind, let's move on to the C code that will actually be doing the
354runtime patching. You'll need a little knowledge of your arch's opcodes in
355order to make it through the next section.
356
357Every arch has an init callback function. If you need to do something early on
358to initialize some state, this is the time to do that. Otherwise, this simple
359function below should be sufficient for most people:
360
361int __init ftrace_dyn_arch_init(void *data)
362{
363 /* return value is done indirectly via data */
364 *(unsigned long *)data = 0;
365
366 return 0;
367}
368
369There are two functions that are used to do runtime patching of arbitrary
370functions. The first is used to turn the mcount call site into a nop (which
371is what helps us retain runtime performance when not tracing). The second is
372used to turn the mcount call site into a call to an arbitrary location (but
373typically that is ftracer_caller()). See the general function definition in
374linux/ftrace.h for the functions:
375 ftrace_make_nop()
376 ftrace_make_call()
377The rec->ip value is the address of the mcount call site that was collected
378by the scripts/recordmcount.pl during build time.
379
380The last function is used to do runtime patching of the active tracer. This
381will be modifying the assembly code at the location of the ftrace_call symbol
382inside of the ftrace_caller() function. So you should have sufficient padding
383at that location to support the new function calls you'll be inserting. Some
384people will be using a "call" type instruction while others will be using a
385"branch" type instruction. Specifically, the function is:
386 ftrace_update_ftrace_func()
387
388
389HAVE_DYNAMIC_FTRACE + HAVE_FUNCTION_GRAPH_TRACER
390------------------------------------------------
391
392The function grapher needs a few tweaks in order to work with dynamic ftrace.
393Basically, you will need to:
394 - update:
395 - ftrace_caller()
396 - ftrace_graph_call()
397 - ftrace_graph_caller()
398 - implement:
399 - ftrace_enable_ftrace_graph_caller()
400 - ftrace_disable_ftrace_graph_caller()
Mike Frysinger555f3862009-09-14 20:10:15 -0400401
402<details to be filled>
Mike Frysinger9849ed42010-07-20 03:13:35 -0400403Quick notes:
404 - add a nop stub after the ftrace_call location named ftrace_graph_call;
405 stub needs to be large enough to support a call to ftrace_graph_caller()
406 - update ftrace_graph_caller() to work with being called by the new
407 ftrace_caller() since some semantics may have changed
408 - ftrace_enable_ftrace_graph_caller() will runtime patch the
409 ftrace_graph_call location with a call to ftrace_graph_caller()
410 - ftrace_disable_ftrace_graph_caller() will runtime patch the
411 ftrace_graph_call location with nops