perf evlist: Do not use hard coded value for a mmap_pages default

So far what is in there by default is what we were using: 512KB + the
control page, but the admin may change that, and if it does to a smaller
value, all calls to tooling for non root users start failing, requiring
that the user manually set --mmap_pages/-m.

Use instead what is in /proc/sys/kernel/perf_event_mlock_kb.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-2f6mtm8xu3wo5lhkql6jdblh@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evlist.c

@@ -893,10 +893,22 @@ static int perf_evlist__mmap_per_thread(struct perf_evlist *evlist,
 
 static size_t perf_evlist__mmap_size(unsigned long pages)
 {
-	/* 512 kiB: default amount of unprivileged mlocked memory */
-	if (pages == UINT_MAX)
-		pages = (512 * 1024) / page_size;
-	else if (!is_power_of_2(pages))
+	if (pages == UINT_MAX) {
+		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);
+		}
+
+		pages = (max * 1024) / page_size;
+	} else if (!is_power_of_2(pages))
 		return 0;
 
 	return (pages + 1) * page_size;