linux-kernel - [PATCH v3] perf tools: fix detecting SMT at machines with more than 32 cpus

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Date:   Mon, 04 May 2020 16:16:58 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     linux-kernel@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: [PATCH v3] perf tools: fix detecting SMT at machines with more than
 32 cpus

Cpu bitmap is split into 32 bit words. For system with more than 32 cores
threads are always in different words thus first word never has two bits:
cpu0: "0000,00000100,00000001", cpu 79: "8000,00000080,00000000".

Instead of parsing bitmap read "core_cpus_list" or "thread_siblings_list"
and simply check presence of ',' or '-' in it.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Fixes: de5077c4e38f ("perf tools: Add utility function to detect SMT status")
---
 tools/perf/util/smt.c |   39 ++++++++++++++++-----------------------
 1 file changed, 16 insertions(+), 23 deletions(-)

diff --git a/tools/perf/util/smt.c b/tools/perf/util/smt.c
index 20bacd5972ad..ef713981725a 100644
--- a/tools/perf/util/smt.c
+++ b/tools/perf/util/smt.c
@@ -1,6 +1,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
+#include <string.h>
 #include <linux/bitops.h>
 #include "api/fs/fs.h"
 #include "smt.h"
@@ -9,6 +10,7 @@ int smt_on(void)
 {
 	static bool cached;
 	static int cached_result;
+	char *str = NULL;
 	int cpu;
 	int ncpu;
 
@@ -20,33 +22,24 @@ int smt_on(void)
 
 	ncpu = sysconf(_SC_NPROCESSORS_CONF);
 	for (cpu = 0; cpu < ncpu; cpu++) {
-		unsigned long long siblings;
-		char *str;
-		size_t strlen;
 		char fn[256];
+		size_t len;
 
-		snprintf(fn, sizeof fn,
-			"devices/system/cpu/cpu%d/topology/core_cpus", cpu);
-		if (sysfs__read_str(fn, &str, &strlen) < 0) {
-			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;
+		snprintf(fn, sizeof(fn), "devices/system/cpu/cpu%d/topology/%s",
+			 cpu, "core_cpus_list");
+		if (sysfs__read_str(fn, &str, &len) > 0)
+			break;
+
+		snprintf(fn, sizeof(fn), "devices/system/cpu/cpu%d/topology/%s",
+			 cpu, "thread_siblings_list");
+		if (sysfs__read_str(fn, &str, &len) > 0)
 			break;
-		}
 	}
-	if (!cached) {
-		cached_result = 0;
+
+	// ',' or '-' should be in string if list contains more than one cpu
+	cached_result = str && (strchr(str, ',') || strchr(str, '-'));
+	free(str);
 done:
-		cached = true;
-	}
+	cached = true;
 	return cached_result;
 }