diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/trace/events.txt | 184 | ||||
| -rw-r--r-- | Documentation/trace/ftrace-design.txt | 233 | ||||
| -rw-r--r-- | Documentation/trace/ftrace.txt | 6 |
3 files changed, 422 insertions, 1 deletions
diff --git a/Documentation/trace/events.txt b/Documentation/trace/events.txt index 90e8b3383ba..78c45a87be5 100644 --- a/Documentation/trace/events.txt +++ b/Documentation/trace/events.txt @@ -1,7 +1,7 @@ Event Tracing Documentation written by Theodore Ts'o - Updated by Li Zefan + Updated by Li Zefan and Tom Zanussi 1. Introduction =============== @@ -97,3 +97,185 @@ The format of this boot option is the same as described in section 2.1. See The example provided in samples/trace_events +4. Event formats +================ + +Each trace event has a 'format' file associated with it that contains +a description of each field in a logged event. This information can +be used to parse the binary trace stream, and is also the place to +find the field names that can be used in event filters (see section 5). + +It also displays the format string that will be used to print the +event in text mode, along with the event name and ID used for +profiling. + +Every event has a set of 'common' fields associated with it; these are +the fields prefixed with 'common_'. The other fields vary between +events and correspond to the fields defined in the TRACE_EVENT +definition for that event. + +Each field in the format has the form: + + field:field-type field-name; offset:N; size:N; + +where offset is the offset of the field in the trace record and size +is the size of the data item, in bytes. + +For example, here's the information displayed for the 'sched_wakeup' +event: + +# cat /debug/tracing/events/sched/sched_wakeup/format + +name: sched_wakeup +ID: 60 +format: + field:unsigned short common_type; offset:0; size:2; + field:unsigned char common_flags; offset:2; size:1; + field:unsigned char common_preempt_count; offset:3; size:1; + field:int common_pid; offset:4; size:4; + field:int common_tgid; offset:8; size:4; + + field:char comm[TASK_COMM_LEN]; offset:12; size:16; + field:pid_t pid; offset:28; size:4; + field:int prio; offset:32; size:4; + field:int success; offset:36; size:4; + field:int cpu; offset:40; size:4; + +print fmt: "task %s:%d [%d] success=%d [%03d]", REC->comm, REC->pid, + REC->prio, REC->success, REC->cpu + +This event contains 10 fields, the first 5 common and the remaining 5 +event-specific. All the fields for this event are numeric, except for +'comm' which is a string, a distinction important for event filtering. + +5. Event filtering +================== + +Trace events can be filtered in the kernel by associating boolean +'filter expressions' with them. As soon as an event is logged into +the trace buffer, its fields are checked against the filter expression +associated with that event type. An event with field values that +'match' the filter will appear in the trace output, and an event whose +values don't match will be discarded. An event with no filter +associated with it matches everything, and is the default when no +filter has been set for an event. + +5.1 Expression syntax +--------------------- + +A filter expression consists of one or more 'predicates' that can be +combined using the logical operators '&&' and '||'. A predicate is +simply a clause that compares the value of a field contained within a +logged event with a constant value and returns either 0 or 1 depending +on whether the field value matched (1) or didn't match (0): + + field-name relational-operator value + +Parentheses can be used to provide arbitrary logical groupings and +double-quotes can be used to prevent the shell from interpreting +operators as shell metacharacters. + +The field-names available for use in filters can be found in the +'format' files for trace events (see section 4). + +The relational-operators depend on the type of the field being tested: + +The operators available for numeric fields are: + +==, !=, <, <=, >, >= + +And for string fields they are: + +==, != + +Currently, only exact string matches are supported. + +Currently, the maximum number of predicates in a filter is 16. + +5.2 Setting filters +------------------- + +A filter for an individual event is set by writing a filter expression +to the 'filter' file for the given event. + +For example: + +# cd /debug/tracing/events/sched/sched_wakeup +# echo "common_preempt_count > 4" > filter + +A slightly more involved example: + +# cd /debug/tracing/events/sched/sched_signal_send +# echo "((sig >= 10 && sig < 15) || sig == 17) && comm != bash" > filter + +If there is an error in the expression, you'll get an 'Invalid +argument' error when setting it, and the erroneous string along with +an error message can be seen by looking at the filter e.g.: + +# cd /debug/tracing/events/sched/sched_signal_send +# echo "((sig >= 10 && sig < 15) || dsig == 17) && comm != bash" > filter +-bash: echo: write error: Invalid argument +# cat filter +((sig >= 10 && sig < 15) || dsig == 17) && comm != bash +^ +parse_error: Field not found + +Currently the caret ('^') for an error always appears at the beginning of +the filter string; the error message should still be useful though +even without more accurate position info. + +5.3 Clearing filters +-------------------- + +To clear the filter for an event, write a '0' to the event's filter +file. + +To clear the filters for all events in a subsystem, write a '0' to the +subsystem's filter file. + +5.3 Subsystem filters +--------------------- + +For convenience, filters for every event in a subsystem can be set or +cleared as a group by writing a filter expression into the filter file +at the root of the subsytem. Note however, that if a filter for any +event within the subsystem lacks a field specified in the subsystem +filter, or if the filter can't be applied for any other reason, the +filter for that event will retain its previous setting. This can +result in an unintended mixture of filters which could lead to +confusing (to the user who might think different filters are in +effect) trace output. Only filters that reference just the common +fields can be guaranteed to propagate successfully to all events. + +Here are a few subsystem filter examples that also illustrate the +above points: + +Clear the filters on all events in the sched subsytem: + +# cd /sys/kernel/debug/tracing/events/sched +# echo 0 > filter +# cat sched_switch/filter +none +# cat sched_wakeup/filter +none + +Set a filter using only common fields for all events in the sched +subsytem (all events end up with the same filter): + +# cd /sys/kernel/debug/tracing/events/sched +# echo common_pid == 0 > filter +# cat sched_switch/filter +common_pid == 0 +# cat sched_wakeup/filter +common_pid == 0 + +Attempt to set a filter using a non-common field for all events in the +sched subsytem (all events but those that have a prev_pid field retain +their old filters): + +# cd /sys/kernel/debug/tracing/events/sched +# echo prev_pid == 0 > filter +# cat sched_switch/filter +prev_pid == 0 +# cat sched_wakeup/filter +common_pid == 0 diff --git a/Documentation/trace/ftrace-design.txt b/Documentation/trace/ftrace-design.txt new file mode 100644 index 00000000000..7003e10f10f --- /dev/null +++ b/Documentation/trace/ftrace-design.txt @@ -0,0 +1,233 @@ + function tracer guts + ==================== + +Introduction +------------ + +Here we will cover the architecture pieces that the common function tracing +code relies on for proper functioning. Things are broken down into increasing +complexity so that you can start simple and at least get basic functionality. + +Note that this focuses on architecture implementation details only. If you +want more explanation of a feature in terms of common code, review the common +ftrace.txt file. + + +Prerequisites +------------- + +Ftrace relies on these features being implemented: + STACKTRACE_SUPPORT - implement save_stack_trace() + TRACE_IRQFLAGS_SUPPORT - implement include/asm/irqflags.h + + +HAVE_FUNCTION_TRACER +-------------------- + +You will need to implement the mcount and the ftrace_stub functions. + +The exact mcount symbol name will depend on your toolchain. Some call it +"mcount", "_mcount", or even "__mcount". You can probably figure it out by +running something like: + $ echo 'main(){}' | gcc -x c -S -o - - -pg | grep mcount + call mcount +We'll make the assumption below that the symbol is "mcount" just to keep things +nice and simple in the examples. + +Keep in mind that the ABI that is in effect inside of the mcount function is +*highly* architecture/toolchain specific. We cannot help you in this regard, +sorry. Dig up some old documentation and/or find someone more familiar than +you to bang ideas off of. Typically, register usage (argument/scratch/etc...) +is a major issue at this point, especially in relation to the location of the +mcount call (before/after function prologue). You might also want to look at +how glibc has implemented the mcount function for your architecture. It might +be (semi-)relevant. + +The mcount function should check the function pointer ftrace_trace_function +to see if it is set to ftrace_stub. If it is, there is nothing for you to do, +so return immediately. If it isn't, then call that function in the same way +the mcount function normally calls __mcount_internal -- the first argument is +the "frompc" while the second argument is the "selfpc" (adjusted to remove the +size of the mcount call that is embedded in the function). + +For example, if the function foo() calls bar(), when the bar() function calls +mcount(), the arguments mcount() will pass to the tracer are: + "frompc" - the address bar() will use to return to foo() + "selfpc" - the address bar() (with _mcount() size adjustment) + +Also keep in mind that this mcount function will be called *a lot*, so +optimizing for the default case of no tracer will help the smooth running of +your system when tracing is disabled. So the start of the mcount function is +typically the bare min with checking things before returning. That also means +the code flow should usually kept linear (i.e. no branching in the nop case). +This is of course an optimization and not a hard requirement. + +Here is some pseudo code that should help (these functions should actually be +implemented in assembly): + +void ftrace_stub(void) +{ + return; +} + +void mcount(void) +{ + /* save any bare state needed in order to do initial checking */ + + extern void (*ftrace_trace_function)(unsigned long, unsigned long); + if (ftrace_trace_function != ftrace_stub) + goto do_trace; + + /* restore any bare state */ + + return; + +do_trace: + + /* save all state needed by the ABI (see paragraph above) */ + + unsigned long frompc = ...; + unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE; + ftrace_trace_function(frompc, selfpc); + + /* restore all state needed by the ABI */ +} + +Don't forget to export mcount for modules ! +extern void mcount(void); +EXPORT_SYMBOL(mcount); + + +HAVE_FUNCTION_TRACE_MCOUNT_TEST +------------------------------- + +This is an optional optimization for the normal case when tracing is turned off +in the system. If you do not enable this Kconfig option, the common ftrace +code will take care of doing the checking for you. + +To support this feature, you only need to check the function_trace_stop +variable in the mcount function. If it is non-zero, there is no tracing to be +done at all, so you can return. + +This additional pseudo code would simply be: +void mcount(void) +{ + /* save any bare state needed in order to do initial checking */ + ++ if (function_trace_stop) ++ return; + + extern void (*ftrace_trace_function)(unsigned long, unsigned long); + if (ftrace_trace_function != ftrace_stub) +... + + +HAVE_FUNCTION_GRAPH_TRACER +-------------------------- + +Deep breath ... time to do some real work. Here you will need to update the +mcount function to check ftrace graph function pointers, as well as implement +some functions to save (hijack) and restore the return address. + +The mcount function should check the function pointers ftrace_graph_return +(compare to ftrace_stub) and ftrace_graph_entry (compare to +ftrace_graph_entry_stub). If either of those are not set to the relevant stub +function, call the arch-specific function ftrace_graph_caller which in turn +calls the arch-specific function prepare_ftrace_return. Neither of these +function names are strictly required, but you should use them anyways to stay +consistent across the architecture ports -- easier to compare & contrast +things. + +The arguments to prepare_ftrace_return are slightly different than what are +passed to ftrace_trace_function. The second argument "selfpc" is the same, +but the first argument should be a pointer to the "frompc". Typically this is +located on the stack. This allows the function to hijack the return address +temporarily to have it point to the arch-specific function return_to_handler. +That function will simply call the common ftrace_return_to_handler function and +that will return the original return address with which, you can return to the +original call site. + +Here is the updated mcount pseudo code: +void mcount(void) +{ +... + if (ftrace_trace_function != ftrace_stub) + goto do_trace; + ++#ifdef CONFIG_FUNCTION_GRAPH_TRACER ++ extern void (*ftrace_graph_return)(...); ++ extern void (*ftrace_graph_entry)(...); ++ if (ftrace_graph_return != ftrace_stub || ++ ftrace_graph_entry != ftrace_graph_entry_stub) ++ ftrace_graph_caller(); ++#endif + + /* restore any bare state */ +... + +Here is the pseudo code for the new ftrace_graph_caller assembly function: +#ifdef CONFIG_FUNCTION_GRAPH_TRACER +void ftrace_graph_caller(void) +{ + /* save all state needed by the ABI */ + + unsigned long *frompc = &...; + unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE; + prepare_ftrace_return(frompc, selfpc); + + /* restore all state needed by the ABI */ +} +#endif + +For information on how to implement prepare_ftrace_return(), simply look at +the x86 version. The only architecture-specific piece in it is the setup of +the fault recovery table (the asm(...) code). The rest should be the same +across architectures. + +Here is the pseudo code for the new return_to_handler assembly function. Note +that the ABI that applies here is different from what applies to the mcount +code. Since you are returning from a function (after the epilogue), you might +be able to skimp on things saved/restored (usually just registers used to pass +return values). + +#ifdef CONFIG_FUNCTION_GRAPH_TRACER +void return_to_handler(void) +{ + /* save all state needed by the ABI (see paragraph above) */ + + void (*original_return_point)(void) = ftrace_return_to_handler(); + + /* restore all state needed by the ABI */ + + /* this is usually either a return or a jump */ + original_return_point(); +} +#endif + + +HAVE_FTRACE_NMI_ENTER +--------------------- + +If you can't trace NMI functions, then skip this option. + +<details to be filled> + + +HAVE_FTRACE_SYSCALLS +--------------------- + +<details to be filled> + + +HAVE_FTRACE_MCOUNT_RECORD +------------------------- + +See scripts/recordmcount.pl for more info. + +<details to be filled> + + +HAVE_DYNAMIC_FTRACE +--------------------- + +<details to be filled> diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt index 355d0f1f8c5..1b6292bbdd6 100644 --- a/Documentation/trace/ftrace.txt +++ b/Documentation/trace/ftrace.txt @@ -26,6 +26,12 @@ disabled, and more (ftrace allows for tracer plugins, which means that the list of tracers can always grow). +Implementation Details +---------------------- + +See ftrace-design.txt for details for arch porters and such. + + The File System --------------- |