tracing: Have hist_debug show what function a field uses

When CONFIG_HIST_TRIGGERS_DEBUG is enabled, each trace event has a
"hist_debug" file that explains the histogram internal data. This is very
useful for debugging histograms.

One bit of data that was missing from this file was what function a
histogram field uses to process its data. The hist_field structure now has
a fn_num that is used by a switch statement in hist_fn_call() to call a
function directly (to avoid spectre mitigations).

Instead of displaying that number, create a string array that maps to the
histogram function enums so that the function for a field may be
displayed:

 ~# cat /sys/kernel/tracing/events/sched/sched_switch/hist_debug
[..]
hist_data: 0000000043d62762

  n_vals: 2
  n_keys: 1
  n_fields: 3

  val fields:

    hist_data->fields[0]:
      flags:
        VAL: HIST_FIELD_FL_HITCOUNT
      type: u64
      size: 8
      is_signed: 0
      function: hist_field_counter()

    hist_data->fields[1]:
      flags:
        HIST_FIELD_FL_VAR
      var.name: __arg_3921_2
      var.idx (into tracing_map_elt.vars[]): 0
      type: unsigned long[]
      size: 128
      is_signed: 0
      function: hist_field_nop()

  key fields:

    hist_data->fields[2]:
      flags:
        HIST_FIELD_FL_KEY
      ftrace_event_field name: prev_pid
      type: pid_t
      size: 8
      is_signed: 1
      function: hist_field_s32()

The "function:" field above is added.

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20260122203822.58df4d80@gandalf.local.home
Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Tested-by: Tom Zanussi <zanussi@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: rostedt

kernel/trace/trace_events_hist.c

@@ -105,38 +105,44 @@ enum field_op_id {
 	FIELD_OP_MULT,
 };
 
+#define FIELD_FUNCS					\
+	C(NOP,			"nop"),			\
+	C(VAR_REF,		"var_ref"),		\
+	C(COUNTER,		"counter"),		\
+	C(CONST,		"const"),		\
+	C(LOG2,			"log2"),		\
+	C(BUCKET,		"bucket"),		\
+	C(TIMESTAMP,		"timestamp"),		\
+	C(CPU,			"cpu"),			\
+	C(COMM,			"comm"),		\
+	C(STRING,		"string"),		\
+	C(DYNSTRING,		"dynstring"),		\
+	C(RELDYNSTRING,		"reldynstring"),	\
+	C(PSTRING,		"pstring"),		\
+	C(S64,			"s64"),			\
+	C(U64,			"u64"),			\
+	C(S32,			"s32"),			\
+	C(U32,			"u32"),			\
+	C(S16,			"s16"),			\
+	C(U16,			"u16"),			\
+	C(S8,			"s8"),			\
+	C(U8,			"u8"),			\
+	C(UMINUS,		"uminus"),		\
+	C(MINUS,		"minus"),		\
+	C(PLUS,			"plus"),		\
+	C(DIV,			"div"),			\
+	C(MULT,			"mult"),		\
+	C(DIV_POWER2,		"div_power2"),		\
+	C(DIV_NOT_POWER2,	"div_not_power2"),	\
+	C(DIV_MULT_SHIFT,	"div_mult_shift"),	\
+	C(EXECNAME,		"execname"),		\
+	C(STACK,		"stack"),
+
+#undef C
+#define C(a, b)		HIST_FIELD_FN_##a
+
 enum hist_field_fn {
-	HIST_FIELD_FN_NOP,
-	HIST_FIELD_FN_VAR_REF,
-	HIST_FIELD_FN_COUNTER,
-	HIST_FIELD_FN_CONST,
-	HIST_FIELD_FN_LOG2,
-	HIST_FIELD_FN_BUCKET,
-	HIST_FIELD_FN_TIMESTAMP,
-	HIST_FIELD_FN_CPU,
-	HIST_FIELD_FN_COMM,
-	HIST_FIELD_FN_STRING,
-	HIST_FIELD_FN_DYNSTRING,
-	HIST_FIELD_FN_RELDYNSTRING,
-	HIST_FIELD_FN_PSTRING,
-	HIST_FIELD_FN_S64,
-	HIST_FIELD_FN_U64,
-	HIST_FIELD_FN_S32,
-	HIST_FIELD_FN_U32,
-	HIST_FIELD_FN_S16,
-	HIST_FIELD_FN_U16,
-	HIST_FIELD_FN_S8,
-	HIST_FIELD_FN_U8,
-	HIST_FIELD_FN_UMINUS,
-	HIST_FIELD_FN_MINUS,
-	HIST_FIELD_FN_PLUS,
-	HIST_FIELD_FN_DIV,
-	HIST_FIELD_FN_MULT,
-	HIST_FIELD_FN_DIV_POWER2,
-	HIST_FIELD_FN_DIV_NOT_POWER2,
-	HIST_FIELD_FN_DIV_MULT_SHIFT,
-	HIST_FIELD_FN_EXECNAME,
-	HIST_FIELD_FN_STACK,
+	FIELD_FUNCS
 };
 
 /*
@@ -5854,6 +5860,12 @@ const struct file_operations event_hist_fops = {
 };
 
 #ifdef CONFIG_HIST_TRIGGERS_DEBUG
+
+#undef C
+#define C(a, b)		b
+
+static const char * const field_funcs[] = { FIELD_FUNCS };
+
 static void hist_field_debug_show_flags(struct seq_file *m,
 					unsigned long flags)
 {
@@ -5918,6 +5930,7 @@ static int hist_field_debug_show(struct seq_file *m,
 	seq_printf(m, "      type: %s\n", field->type);
 	seq_printf(m, "      size: %u\n", field->size);
 	seq_printf(m, "      is_signed: %u\n", field->is_signed);
+	seq_printf(m, "      function: hist_field_%s()\n", field_funcs[field->fn_num]);
 
 	return 0;
 }