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Subject[PATCH] perf stat: Add hint for SMI cost measurement
Date
From: Kan Liang <kan.liang@linux.intel.com>

Cstate may cause drift between aperf and cycles, which impact the
accuracy of SMI cost measurement.

Add smi_env_check() to check if cstate is completely disabled during the
measurement. If not, give a hint to user.

Update the document as well.

Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
---
tools/perf/Documentation/perf-stat.txt | 4 ++++
tools/perf/builtin-stat.c | 23 +++++++++++++++++++++++
2 files changed, 27 insertions(+)

diff --git a/tools/perf/Documentation/perf-stat.txt b/tools/perf/Documentation/perf-stat.txt
index 39c05f8..e0d8763 100644
--- a/tools/perf/Documentation/perf-stat.txt
+++ b/tools/perf/Documentation/perf-stat.txt
@@ -309,6 +309,10 @@ The output is SMI cycles%, equals to (aperf - unhalted core cycles) / aperf

Users who wants to get the actual value can apply --no-metric-only.

+Cstate may cause drift between aperf and cycles.
+Please completely disable cstate during the measurement.
+E.g. set idle=poll in grub
+
EXAMPLES
--------

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index c3625ec..ac273b4 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -87,6 +87,7 @@

#define DEFAULT_SEPARATOR " "
#define FREEZE_ON_SMI_PATH "devices/cpu/freeze_on_smi"
+#define CPUIDLE_CUR_DRV "devices/system/cpu/cpuidle/current_driver"

static void print_counters(struct timespec *ts, int argc, const char **argv);

@@ -1024,6 +1025,25 @@ __weak void arch_topdown_group_warn(void)
{
}

+static void smi_env_check(void)
+{
+ char *name;
+ size_t len;
+
+ if (sysfs__read_str(CPUIDLE_CUR_DRV, &name, &len)) {
+ pr_warning("Failed to check cstate status.\n");
+ return;
+ }
+
+ if (strncmp(name, "none", 4)) {
+ pr_warning("Cstate may cause drift between aperf and cycles. "
+ "Please completely disable cstate, "
+ "E.g. set idle=poll in grub\n");
+ }
+
+ free(name);
+}
+
/*
* Add default attributes, if there were no attributes specified or
* if -d/--detailed, -d -d or -d -d -d is used:
@@ -1196,6 +1216,9 @@ static int add_default_attributes(void)

if (pmu_have_event("msr", "aperf") &&
pmu_have_event("msr", "smi")) {
+
+ smi_env_check();
+
if (!force_metric_only)
stat_config.metric_only = true;
err = parse_events(evsel_list, smi_cost_attrs, &errinfo);
--
2.7.4
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