forked from Minki/linux
ftrace: document function and function graph implementation
While implementing function tracer and function tracer graph support, I found the exact arch implementation details to be a bit lacking (and my x86 foo ain't great). So after pounding out support for the Blackfin arch, start documenting the requirements/details. Signed-off-by: Mike Frysinger <vapier@gentoo.org> LKML-Reference: <1252973415-21264-1-git-send-email-vapier@gentoo.org> Acked-by: Frederic Weisbecker <fweisbec@gmail.com> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
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Mike Frysinger
Steven Rostedt
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Documentation/trace/ftrace-design.txt
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233
Documentation/trace/ftrace-design.txt
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function tracer guts
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====================
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Introduction
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------------
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Here we will cover the architecture pieces that the common function tracing
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code relies on for proper functioning. Things are broken down into increasing
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complexity so that you can start simple and at least get basic functionality.
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Note that this focuses on architecture implementation details only. If you
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want more explanation of a feature in terms of common code, review the common
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ftrace.txt file.
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Prerequisites
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-------------
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Ftrace relies on these features being implemented:
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STACKTRACE_SUPPORT - implement save_stack_trace()
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TRACE_IRQFLAGS_SUPPORT - implement include/asm/irqflags.h
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HAVE_FUNCTION_TRACER
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--------------------
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You will need to implement the mcount and the ftrace_stub functions.
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The exact mcount symbol name will depend on your toolchain. Some call it
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"mcount", "_mcount", or even "__mcount". You can probably figure it out by
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running something like:
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$ echo 'main(){}' | gcc -x c -S -o - - -pg | grep mcount
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call mcount
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We'll make the assumption below that the symbol is "mcount" just to keep things
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nice and simple in the examples.
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Keep in mind that the ABI that is in effect inside of the mcount function is
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*highly* architecture/toolchain specific. We cannot help you in this regard,
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sorry. Dig up some old documentation and/or find someone more familiar than
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you to bang ideas off of. Typically, register usage (argument/scratch/etc...)
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is a major issue at this point, especially in relation to the location of the
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mcount call (before/after function prologue). You might also want to look at
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how glibc has implemented the mcount function for your architecture. It might
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be (semi-)relevant.
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The mcount function should check the function pointer ftrace_trace_function
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to see if it is set to ftrace_stub. If it is, there is nothing for you to do,
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so return immediately. If it isn't, then call that function in the same way
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the mcount function normally calls __mcount_internal -- the first argument is
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the "frompc" while the second argument is the "selfpc" (adjusted to remove the
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size of the mcount call that is embedded in the function).
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For example, if the function foo() calls bar(), when the bar() function calls
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mcount(), the arguments mcount() will pass to the tracer are:
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"frompc" - the address bar() will use to return to foo()
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"selfpc" - the address bar() (with _mcount() size adjustment)
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Also keep in mind that this mcount function will be called *a lot*, so
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optimizing for the default case of no tracer will help the smooth running of
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your system when tracing is disabled. So the start of the mcount function is
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typically the bare min with checking things before returning. That also means
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the code flow should usually kept linear (i.e. no branching in the nop case).
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This is of course an optimization and not a hard requirement.
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Here is some pseudo code that should help (these functions should actually be
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implemented in assembly):
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void ftrace_stub(void)
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{
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return;
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}
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void mcount(void)
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{
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/* save any bare state needed in order to do initial checking */
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extern void (*ftrace_trace_function)(unsigned long, unsigned long);
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if (ftrace_trace_function != ftrace_stub)
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goto do_trace;
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/* restore any bare state */
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return;
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do_trace:
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/* save all state needed by the ABI (see paragraph above) */
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unsigned long frompc = ...;
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unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
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ftrace_trace_function(frompc, selfpc);
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/* restore all state needed by the ABI */
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}
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Don't forget to export mcount for modules !
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extern void mcount(void);
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EXPORT_SYMBOL(mcount);
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HAVE_FUNCTION_TRACE_MCOUNT_TEST
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-------------------------------
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This is an optional optimization for the normal case when tracing is turned off
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in the system. If you do not enable this Kconfig option, the common ftrace
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code will take care of doing the checking for you.
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To support this feature, you only need to check the function_trace_stop
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variable in the mcount function. If it is non-zero, there is no tracing to be
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done at all, so you can return.
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This additional pseudo code would simply be:
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void mcount(void)
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{
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/* save any bare state needed in order to do initial checking */
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+ if (function_trace_stop)
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+ return;
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extern void (*ftrace_trace_function)(unsigned long, unsigned long);
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if (ftrace_trace_function != ftrace_stub)
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...
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HAVE_FUNCTION_GRAPH_TRACER
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--------------------------
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Deep breath ... time to do some real work. Here you will need to update the
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mcount function to check ftrace graph function pointers, as well as implement
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some functions to save (hijack) and restore the return address.
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The mcount function should check the function pointers ftrace_graph_return
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(compare to ftrace_stub) and ftrace_graph_entry (compare to
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ftrace_graph_entry_stub). If either of those are not set to the relevant stub
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function, call the arch-specific function ftrace_graph_caller which in turn
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calls the arch-specific function prepare_ftrace_return. Neither of these
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function names are strictly required, but you should use them anyways to stay
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consistent across the architecture ports -- easier to compare & contrast
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things.
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The arguments to prepare_ftrace_return are slightly different than what are
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passed to ftrace_trace_function. The second argument "selfpc" is the same,
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but the first argument should be a pointer to the "frompc". Typically this is
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located on the stack. This allows the function to hijack the return address
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temporarily to have it point to the arch-specific function return_to_handler.
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That function will simply call the common ftrace_return_to_handler function and
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that will return the original return address with which, you can return to the
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original call site.
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Here is the updated mcount pseudo code:
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void mcount(void)
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{
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...
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if (ftrace_trace_function != ftrace_stub)
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goto do_trace;
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+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
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+ extern void (*ftrace_graph_return)(...);
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+ extern void (*ftrace_graph_entry)(...);
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+ if (ftrace_graph_return != ftrace_stub ||
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+ ftrace_graph_entry != ftrace_graph_entry_stub)
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+ ftrace_graph_caller();
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+#endif
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/* restore any bare state */
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...
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Here is the pseudo code for the new ftrace_graph_caller assembly function:
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#ifdef CONFIG_FUNCTION_GRAPH_TRACER
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void ftrace_graph_caller(void)
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{
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/* save all state needed by the ABI */
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unsigned long *frompc = &...;
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unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
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prepare_ftrace_return(frompc, selfpc);
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/* restore all state needed by the ABI */
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}
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#endif
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For information on how to implement prepare_ftrace_return(), simply look at
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the x86 version. The only architecture-specific piece in it is the setup of
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the fault recovery table (the asm(...) code). The rest should be the same
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across architectures.
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Here is the pseudo code for the new return_to_handler assembly function. Note
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that the ABI that applies here is different from what applies to the mcount
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code. Since you are returning from a function (after the epilogue), you might
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be able to skimp on things saved/restored (usually just registers used to pass
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return values).
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#ifdef CONFIG_FUNCTION_GRAPH_TRACER
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void return_to_handler(void)
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{
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/* save all state needed by the ABI (see paragraph above) */
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void (*original_return_point)(void) = ftrace_return_to_handler();
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/* restore all state needed by the ABI */
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/* this is usually either a return or a jump */
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original_return_point();
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}
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#endif
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HAVE_FTRACE_NMI_ENTER
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---------------------
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If you can't trace NMI functions, then skip this option.
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<details to be filled>
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HAVE_FTRACE_SYSCALLS
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---------------------
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<details to be filled>
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HAVE_FTRACE_MCOUNT_RECORD
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-------------------------
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See scripts/recordmcount.pl for more info.
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<details to be filled>
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HAVE_DYNAMIC_FTRACE
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---------------------
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<details to be filled>
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@ -26,6 +26,12 @@ disabled, and more (ftrace allows for tracer plugins, which
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means that the list of tracers can always grow).
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Implementation Details
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----------------------
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See ftrace-design.txt for details for arch porters and such.
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The File System
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---------------
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@ -11,12 +11,18 @@ config NOP_TRACER
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config HAVE_FTRACE_NMI_ENTER
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bool
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help
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See Documentation/trace/ftrace-implementation.txt
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config HAVE_FUNCTION_TRACER
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bool
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help
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See Documentation/trace/ftrace-implementation.txt
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config HAVE_FUNCTION_GRAPH_TRACER
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bool
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help
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See Documentation/trace/ftrace-implementation.txt
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config HAVE_FUNCTION_GRAPH_FP_TEST
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bool
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@ -28,21 +34,25 @@ config HAVE_FUNCTION_GRAPH_FP_TEST
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config HAVE_FUNCTION_TRACE_MCOUNT_TEST
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bool
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help
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This gets selected when the arch tests the function_trace_stop
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variable at the mcount call site. Otherwise, this variable
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is tested by the called function.
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See Documentation/trace/ftrace-implementation.txt
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config HAVE_DYNAMIC_FTRACE
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bool
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help
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See Documentation/trace/ftrace-implementation.txt
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config HAVE_FTRACE_MCOUNT_RECORD
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bool
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help
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See Documentation/trace/ftrace-implementation.txt
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config HAVE_HW_BRANCH_TRACER
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bool
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config HAVE_SYSCALL_TRACEPOINTS
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bool
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help
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See Documentation/trace/ftrace-implementation.txt
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config TRACER_MAX_TRACE
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bool
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