perf ftrace: Add option '-m/--buffer-size' to set per-cpu buffer size

This adds an option '-m/--buffer-size' to allow us set the size of per-cpu
tracing buffer.

Committer testing:

Before running with this option:

  # find /sys/kernel/tracing/ -name buffer_size_kb | xargs cat
  1408
  1408
  1408
  1408
  1408
  1408
  1408
  1408
  1408
  #

Then, run:

  # perf ftrace -m 2048K | head -10
   2)               |  mutex_unlock() {
   2)   ==========> |
   2)               |    smp_irq_work_interrupt() {
   2)               |      irq_enter() {
   2)   0.121 us    |        rcu_irq_enter();
   2)   0.128 us    |        irqtime_account_irq();
   2)   0.719 us    |      }
   2)               |      __wake_up() {
   2)               |        __wake_up_common_lock() {
   2)   0.105 us    |          _raw_spin_lock_irqsave();
  #

Now look at those tracefs knobs:

  # find /sys/kernel/tracing/ -name buffer_size_kb | xargs cat
  2048
  2048
  2048
  2048
  2048
  2048
  2048
  2048
  2048
  #

This should be similar to the -m option in the other perf tools, such as
'perf record', 'perf trace', etc.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200808023141.14227-5-changbin.du@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-ftrace.txt

@@ -53,6 +53,11 @@ OPTIONS
 	Ranges of CPUs are specified with -: 0-2.
 	Default is to trace on all online CPUs.
 
+-m::
+--buffer-size::
+	Set the size of per-cpu tracing buffer, <size> is expected to
+	be a number with appended unit character - B/K/M/G.
+
 -T::
 --trace-funcs=::
 	Select function tracer and set function filter on the given

tools/perf/builtin-ftrace.c

@@ -26,6 +26,7 @@
 #include "thread_map.h"
 #include "util/cap.h"
 #include "util/config.h"
+#include "util/units.h"
 
 #define DEFAULT_TRACER  "function_graph"
 
@@ -39,6 +40,7 @@ struct perf_ftrace {
 	struct list_head	graph_funcs;
 	struct list_head	nograph_funcs;
 	int			graph_depth;
+	unsigned long		percpu_buffer_size;
 };
 
 struct filter_entry {
@@ -324,6 +326,21 @@ static int set_tracing_depth(struct perf_ftrace *ftrace)
 	return 0;
 }
 
+static int set_tracing_percpu_buffer_size(struct perf_ftrace *ftrace)
+{
+	int ret;
+
+	if (ftrace->percpu_buffer_size == 0)
+		return 0;
+
+	ret = write_tracing_file_int("buffer_size_kb",
+				     ftrace->percpu_buffer_size / 1024);
+	if (ret < 0)
+		return ret;
+
+	return 0;
+}
+
 static int __cmd_ftrace(struct perf_ftrace *ftrace, int argc, const char **argv)
 {
 	char *trace_file;
@@ -388,6 +405,11 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace, int argc, const char **argv)
 		goto out_reset;
 	}
 
+	if (set_tracing_percpu_buffer_size(ftrace) < 0) {
+		pr_err("failed to set tracing per-cpu buffer size\n");
+		goto out_reset;
+	}
+
 	if (write_tracing_file("current_tracer", ftrace->tracer) < 0) {
 		pr_err("failed to set current_tracer to %s\n", ftrace->tracer);
 		goto out_reset;
@@ -506,6 +528,37 @@ static void delete_filter_func(struct list_head *head)
 	}
 }
 
+static int parse_buffer_size(const struct option *opt,
+			     const char *str, int unset)
+{
+	unsigned long *s = (unsigned long *)opt->value;
+	static struct parse_tag tags_size[] = {
+		{ .tag  = 'B', .mult = 1       },
+		{ .tag  = 'K', .mult = 1 << 10 },
+		{ .tag  = 'M', .mult = 1 << 20 },
+		{ .tag  = 'G', .mult = 1 << 30 },
+		{ .tag  = 0 },
+	};
+	unsigned long val;
+
+	if (unset) {
+		*s = 0;
+		return 0;
+	}
+
+	val = parse_tag_value(str, tags_size);
+	if (val != (unsigned long) -1) {
+		if (val < 1024) {
+			pr_err("buffer size too small, must larger than 1KB.");
+			return -1;
+		}
+		*s = val;
+		return 0;
+	}
+
+	return -1;
+}
+
 static void select_tracer(struct perf_ftrace *ftrace)
 {
 	bool graph = !list_empty(&ftrace->graph_funcs) ||
@@ -560,6 +613,8 @@ int cmd_ftrace(int argc, const char **argv)
 		     "Set nograph filter on given functions", parse_filter_func),
 	OPT_INTEGER('D', "graph-depth", &ftrace.graph_depth,
 		    "Max depth for function graph tracer"),
+	OPT_CALLBACK('m', "buffer-size", &ftrace.percpu_buffer_size, "size",
+		     "size of per cpu buffer", parse_buffer_size),
 	OPT_END()
 	};