tracing: Add example and documentation for new __vstring() macro

Update the sample trace events to include an example that uses the new
__vstring() helpers for TRACE_EVENTS.

Link: https://lkml.kernel.org/r/20220715175555.16375a3b@gandalf.local.home
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/samples/trace_events/trace-events-sample.c b/samples/trace_events/trace-events-sample.c
index 4d34dc0..608c4ae 100644 (file)
--- a/samples/trace_events/trace-events-sample.c
+++ b/samples/trace_events/trace-events-sample.c
@@ -19,9 +19,10 @@ static const char *random_strings[] = {
        "One ring to rule them all"
 };
 
-static void simple_thread_func(int cnt)
+static void do_simple_thread_func(int cnt, const char *fmt, ...)
 {
        unsigned long bitmask[1] = {0xdeadbeefUL};
+       va_list va;
        int array[6];
        int len = cnt % 5;
        int i;
@@ -33,9 +34,13 @@ static void simple_thread_func(int cnt)
                array[i] = i + 1;
        array[i] = 0;
 
+       va_start(va, fmt);
+
        /* Silly tracepoints */
        trace_foo_bar("hello", cnt, array, random_strings[len],
-                     current->cpus_ptr);
+                     current->cpus_ptr, fmt, &va);
+
+       va_end(va);
 
        trace_foo_with_template_simple("HELLO", cnt);
 
@@ -48,6 +53,11 @@ static void simple_thread_func(int cnt)
        trace_foo_rel_loc("Hello __rel_loc", cnt, bitmask);
 }
 
+static void simple_thread_func(int cnt)
+{
+       do_simple_thread_func(cnt, "iter=%d", cnt);
+}
+
 static int simple_thread(void *arg)
 {
        int cnt = 0;

diff --git a/samples/trace_events/trace-events-sample.h b/samples/trace_events/trace-events-sample.h
index cbbbb83..1a92226 100644 (file)
--- a/samples/trace_events/trace-events-sample.h
+++ b/samples/trace_events/trace-events-sample.h
@@ -141,6 +141,27 @@
  *         In most cases, the __assign_str() macro will take the same
  *         parameters as the __string() macro had to declare the string.
  *
+ *   __vstring: This is similar to __string() but instead of taking a
+ *         dynamic length, it takes a variable list va_list 'va' variable.
+ *         Some event callers already have a message from parameters saved
+ *         in a va_list. Passing in the format and the va_list variable
+ *         will save just enough on the ring buffer for that string.
+ *         Note, the va variable used is a pointer to a va_list, not
+ *         to the va_list directly.
+ *
+ *           (va_list *va)
+ *
+ *         __vstring(foo, fmt, va)  is similar to:  vsnprintf(foo, fmt, va)
+ *
+ *         To assign the string, use the helper macro __assign_vstr().
+ *
+ *         __assign_vstr(foo, fmt, va);
+ *
+ *         In most cases, the __assign_vstr() macro will take the same
+ *         parameters as the __vstring() macro had to declare the string.
+ *         Use __get_str() to retrieve the __vstring() just like it would for
+ *         __string().
+ *
  *   __string_len: This is a helper to a __dynamic_array, but it understands
  *        that the array has characters in it, and with the combined
  *         use of __assign_str_len(), it will allocate 'len' + 1 bytes
@@ -256,9 +277,10 @@ TRACE_DEFINE_ENUM(TRACE_SAMPLE_ZOO);
 TRACE_EVENT(foo_bar,
 
        TP_PROTO(const char *foo, int bar, const int *lst,
-                const char *string, const struct cpumask *mask),
+                const char *string, const struct cpumask *mask,
+                const char *fmt, va_list *va),
 
-       TP_ARGS(foo, bar, lst, string, mask),
+       TP_ARGS(foo, bar, lst, string, mask, fmt, va),
 
        TP_STRUCT__entry(
                __array(        char,   foo,    10              )
@@ -266,6 +288,7 @@ TRACE_EVENT(foo_bar,
                __dynamic_array(int,    list,   __length_of(lst))
                __string(       str,    string                  )
                __bitmask(      cpus,   num_possible_cpus()     )
+               __vstring(      vstr,   fmt,    va              )
        ),
 
        TP_fast_assign(
@@ -274,10 +297,11 @@ TRACE_EVENT(foo_bar,
                memcpy(__get_dynamic_array(list), lst,
                       __length_of(lst) * sizeof(int));
                __assign_str(str, string);
+               __assign_vstr(vstr, fmt, va);
                __assign_bitmask(cpus, cpumask_bits(mask), num_possible_cpus());
        ),
 
-       TP_printk("foo %s %d %s %s %s %s (%s)", __entry->foo, __entry->bar,
+       TP_printk("foo %s %d %s %s %s %s (%s) %s", __entry->foo, __entry->bar,
 
 /*
  * Notice here the use of some helper functions. This includes:
@@ -321,7 +345,7 @@ TRACE_EVENT(foo_bar,
                  __print_array(__get_dynamic_array(list),
                                __get_dynamic_array_len(list) / sizeof(int),
                                sizeof(int)),
-                 __get_str(str), __get_bitmask(cpus))
+                 __get_str(str), __get_bitmask(cpus), __get_str(vstr))
 );
 
 /*