[PATCH v4 2/3] perf tsc: Add cpuinfo fall back for arch_get_tsc_freq

[Ian Rogers]

The CPUID method of arch_get_tsc_freq fails for older Intel processors,
such as Skylake. Compute using /proc/cpuinfo.

Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
 tools/perf/arch/x86/util/tsc.c | 52 +++++++++++++++++++++++++++++++---
 1 file changed, 48 insertions(+), 4 deletions(-)

diff --git a/tools/perf/arch/x86/util/tsc.c b/tools/perf/arch/x86/util/tsc.c
index b69144f22489..eb2b5195bd02 100644
--- a/tools/perf/arch/x86/util/tsc.c
+++ b/tools/perf/arch/x86/util/tsc.c
@@ -1,7 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/types.h>
+#include <math.h>
 #include <string.h>
 
+#include "../../../util/debug.h"
 #include "../../../util/tsc.h"
 #include "cpuid.h"
 
@@ -14,6 +16,44 @@ u64 rdtsc(void)
 	return low | ((u64)high) << 32;
 }
 
+/*
+ * Derive the TSC frequency in Hz from the /proc/cpuinfo, for example:
+ * ...
+ * model name      : Intel(R) Xeon(R) Gold 6154 CPU @ 3.00GHz
+ * ...
+ * will return 3000000000.
+ */
+static double cpuinfo_tsc_freq(void)
+{
+	double result = 0;
+	FILE *cpuinfo;
+	char *line = NULL;
+	size_t len = 0;
+
+	cpuinfo = fopen("/proc/cpuinfo", "r");
+	if (!cpuinfo) {
+		pr_err("Failed to read /proc/cpuinfo for TSC frequency");
+		return NAN;
+	}
+	while (getline(&line, &len, cpuinfo) > 0) {
+		if (!strncmp(line, "model name", 10)) {
+			char *pos = strstr(line + 11, " @ ");
+
+			if (pos && sscanf(pos, " @ %lfGHz", &result) == 1) {
+				result *= 1000000000;
+				goto out;
+			}
+		}
+	}
+out:
+	if (fpclassify(result) == FP_ZERO)
+		pr_err("Failed to find TSC frequency in /proc/cpuinfo");
+
+	free(line);
+	fclose(cpuinfo);
+	return result;
+}
+
 double arch_get_tsc_freq(void)
 {
 	unsigned int a, b, c, d, lvl;
@@ -33,13 +73,17 @@ double arch_get_tsc_freq(void)
 	 * Don't support Time Stamp Counter and
 	 * Nominal Core Crystal Clock Information Leaf.
 	 */
-	if (lvl < 0x15)
-		return 0;
+	if (lvl < 0x15) {
+		tsc = cpuinfo_tsc_freq();
+		return tsc;
+	}
 
 	cpuid(0x15, 0, &a, &b, &c, &d);
 	/* TSC frequency is not enumerated */
-	if (!a || !b || !c)
-		return 0;
+	if (!a || !b || !c) {
+		tsc = cpuinfo_tsc_freq();
+		return tsc;
+	}
 
 	tsc = (double)c * (double)b / (double)a;
 	return tsc;
-- 
2.37.0.170.g444d1eabd0-goog