perf tools: Add utility function to detect SMT status

Add an smt_on() function to return if SMT is enabled or disabled.  Used
in the next patch.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170811232634.30465-7-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/Build

@@ -22,6 +22,7 @@ libperf-y += rbtree.o
 libperf-y += libstring.o
 libperf-y += bitmap.o
 libperf-y += hweight.o
+libperf-y += smt.o
 libperf-y += quote.o
 libperf-y += strbuf.o
 libperf-y += string.o

tools/perf/util/smt.c

@@ -0,0 +1,44 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <linux/bitops.h>
+#include "api/fs/fs.h"
+#include "smt.h"
+
+int smt_on(void)
+{
+	static bool cached;
+	static int cached_result;
+	int cpu;
+	int ncpu;
+
+	if (cached)
+		return cached_result;
+
+	ncpu = sysconf(_SC_NPROCESSORS_CONF);
+	for (cpu = 0; cpu < ncpu; cpu++) {
+		unsigned long long siblings;
+		char *str;
+		size_t strlen;
+		char fn[256];
+
+		snprintf(fn, sizeof fn,
+			"devices/system/cpu/cpu%d/topology/thread_siblings",
+			cpu);
+		if (sysfs__read_str(fn, &str, &strlen) < 0)
+			continue;
+		/* Entry is hex, but does not have 0x, so need custom parser */
+		siblings = strtoull(str, NULL, 16);
+		free(str);
+		if (hweight64(siblings) > 1) {
+			cached_result = 1;
+			cached = true;
+			break;
+		}
+	}
+	if (!cached) {
+		cached_result = 0;
+		cached = true;
+	}
+	return cached_result;
+}

tools/perf/util/smt.h

@@ -0,0 +1,6 @@
+#ifndef SMT_H
+#define SMT_H 1
+
+int smt_on(void);
+
+#endif