perf evlist: Expose perf_event_mlock_kb_in_pages() helper

When the user doesn't set --mmap-pages, perf_evlist__mmap() will do it
by reading the maximum possible for a non-root user from the
/proc/sys/kernel/perf_event_mlock_kb file.

Expose that function so that 'perf trace' can, for root users, to bump
mmap-pages to a higher value for root, based on the contents of this
proc file.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-xay69plylwibpb3l4isrpl1k@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evlist.c

@@ -986,26 +986,34 @@ static int perf_evlist__mmap_per_thread(struct perf_evlist *evlist,
 	return -1;
 }
 
-static size_t perf_evlist__mmap_size(unsigned long pages)
+unsigned long perf_event_mlock_kb_in_pages(void)
 {
-	if (pages == UINT_MAX) {
-		int max;
+	unsigned long pages;
+	int max;
 
-		if (sysctl__read_int("kernel/perf_event_mlock_kb", &max) < 0) {
-			/*
-			 * Pick a once upon a time good value, i.e. things look
-			 * strange since we can't read a sysctl value, but lets not
-			 * die yet...
-			 */
-			max = 512;
-		} else {
-			max -= (page_size / 1024);
-		}
+	if (sysctl__read_int("kernel/perf_event_mlock_kb", &max) < 0) {
+		/*
+		 * Pick a once upon a time good value, i.e. things look
+		 * strange since we can't read a sysctl value, but lets not
+		 * die yet...
+		 */
+		max = 512;
+	} else {
+		max -= (page_size / 1024);
+	}
+
+	pages = (max * 1024) / page_size;
+	if (!is_power_of_2(pages))
+		pages = rounddown_pow_of_two(pages);
 
-		pages = (max * 1024) / page_size;
-		if (!is_power_of_2(pages))
-			pages = rounddown_pow_of_two(pages);
-	} else if (!is_power_of_2(pages))
+	return pages;
+}
+
+static size_t perf_evlist__mmap_size(unsigned long pages)
+{
+	if (pages == UINT_MAX)
+		pages = perf_event_mlock_kb_in_pages();
+	else if (!is_power_of_2(pages))
 		return 0;
 
 	return (pages + 1) * page_size;

tools/perf/util/evlist.h

@@ -158,6 +158,8 @@ int perf_evlist__parse_mmap_pages(const struct option *opt,
 				  const char *str,
 				  int unset);
 
+unsigned long perf_event_mlock_kb_in_pages(void);
+
 int perf_evlist__mmap_ex(struct perf_evlist *evlist, unsigned int pages,
 			 bool overwrite, unsigned int auxtrace_pages,
 			 bool auxtrace_overwrite);