linux-kernel - [PATCH] perf tool: fix detection of active SMT

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Date:   Wed, 29 Apr 2020 13:00:39 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     linux-kernel@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Kan Liang <kan.liang@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Dmitry Monakhov <dmtrmonakhov@...dex-team.ru>
Subject: [PATCH] perf tool: fix detection of active SMT

SMT now could be disabled via "/sys/devices/system/cpu/smt/control".
Status shown in "/sys/devices/system/cpu/smt/active" simply as "0" / "1".

If this knob isn't here fallback to checking topology but fix couple bugs:

Check access("devices/system/cpu/cpu%d/topology/core_cpus", F_OK) fails,
unless current directory is "/sys". Simply try read this file first.

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")
Fixes: 0ccdb8407a46 ("perf tools: Apply new CPU topology sysfs attributes")
---
 tools/perf/util/smt.c |   41 +++++++++++++++++++++--------------------
 1 file changed, 21 insertions(+), 20 deletions(-)

diff --git a/tools/perf/util/smt.c b/tools/perf/util/smt.c
index 3b791ef2cd50..c398528d1006 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"
@@ -9,39 +10,39 @@ int smt_on(void)
 {
 	static bool cached;
 	static int cached_result;
+	int active;
 	int cpu;
 	int ncpu;
+	char *str = NULL;
+	size_t strlen;
 
 	if (cached)
 		return cached_result;
 
+	if (sysfs__read_int("devices/system/cpu/smt/active", &active) > 0)
+		goto done;
+
 	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/core_cpus", cpu);
-		if (access(fn, F_OK) == -1) {
-			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, &strlen) > 0)
+			break;
+
+		snprintf(fn, sizeof(fn), "devices/system/cpu/cpu%d/topology/%s",
+			 cpu, "thread_siblings_list");
+		if (sysfs__read_str(fn, &str, &strlen) > 0)
 			break;
-		}
 	}
+
+	active = str && (strchr(str, ',') != NULL || strchr(str, '-') != NULL);
+	free(str);
+
+done:
 	if (!cached) {
-		cached_result = 0;
+		cached_result = active;
 		cached = true;
 	}
 	return cached_result;