From: Borislav Petkov <[email protected]>
When using perf stat on an AMD F15h system with the default hw events
attributes, some of the events don't get counted:
Performance counter stats for 'sleep 1':
0.749208 task-clock (msec) # 0.001 CPUs utilized
1 context-switches # 0.001 M/sec
0 cpu-migrations # 0.000 K/sec
54 page-faults # 0.072 M/sec
1,122,815 cycles # 1.499 GHz
286,740 stalled-cycles-frontend # 25.54% frontend cycles
idle
<not counted> stalled-cycles-backend
(0.00%)
^^^^^^^^^^^^
<not counted> instructions
(0.00%)
^^^^^^^^^^^^
<not counted> branches
(0.00%)
<not counted> branch-misses
(0.00%)
1.001550070 seconds time elapsed
The reason is that we have the HW watchdog consume one PMU counter
and when perf tries to schedule 6 events on 6 counters and some of
those counters are constrained to only a specific subset of PMCs by the
hardware, the event scheduling fails.
So let's disable the HW watchdog around a perf stat session running as
root and restore it after it to its previous state. This frees up the
one counter and the scheduling of the default events succeeds:
Performance counter stats for 'sleep 1':
0.806902 task-clock (msec) # 0.001 CPUs utilized
1 context-switches # 0.001 M/sec
0 cpu-migrations # 0.000 K/sec
55 page-faults # 0.068 M/sec
1,200,677 cycles # 1.488 GHz
308,044 stalled-cycles-frontend # 25.66% frontend cycles
idle
424,292 stalled-cycles-backend # 35.34% backend cycles idle
672,694 instructions # 0.56 insn per cycle
# 0.63 stalled cycles per
insn
132,965 branches # 164.785 M/sec
7,300 branch-misses # 5.49% of all branches
1.001689739 seconds time elapsed
There's a --dont-disable-hwdt option which preserves the old behavior.
Signed-off-by: Borislav Petkov <[email protected]>
---
tools/perf/builtin-stat.c | 41 +++++++++++++++++++++++++++++++++++++++++
1 file changed, 41 insertions(+)
diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index a02f2e965628..b2aa2ed3161c 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -146,6 +146,8 @@ static aggr_get_id_t aggr_get_id;
static bool append_file;
static const char *output_name;
static int output_fd;
+static bool keep_hwdt;
+static int prev_hwdt; /* previous HW watchdog state */
struct perf_stat {
bool record;
@@ -1539,6 +1541,39 @@ static void print_counters(struct timespec *ts, int
argc, const char **argv)
fflush(stat_config.output);
}
+static void perf_stat_toggle_hwdt(int on)
+{
+ static const char *p = "sys/kernel/nmi_watchdog";
+ int val;
+
+ if (keep_hwdt)
+ return;
+
+ if (geteuid())
+ return;
+
+ if (procfs__read_int(p, &val) < 0)
+ return;
+
+ /* Reenable only when it was enabled before. */
+ if (on) {
+ if (prev_hwdt)
+ goto write;
+ /* Disable HWDT only when it is enabled. */
+ } else {
+ prev_hwdt = val;
+
+ if (val)
+ goto write;
+ }
+
+ return;
+
+write:
+ if (procfs__write_int(p, on) < 0)
+ return;
+}
+
static volatile int signr = -1;
static void skip_signal(int signo)
@@ -1575,6 +1610,8 @@ static void sig_atexit(void)
sigprocmask(SIG_SETMASK, &oset, NULL);
+ perf_stat_toggle_hwdt(1);
+
if (signr == -1)
return;
@@ -1659,6 +1696,8 @@ static const struct option stat_options[] = {
"Only print computed metrics. No raw values",
enable_metric_only),
OPT_BOOLEAN(0, "topdown", &topdown_run,
"measure topdown level 1 statistics"),
+ OPT_BOOLEAN(0, "dont-disable-hwdt", &keep_hwdt,
+ "Do not disable HW NMI watchdog during the current
session"),
OPT_END()
};
@@ -2523,6 +2562,8 @@ int cmd_stat(int argc, const char **argv, const char
*prefix __maybe_unused)
if (perf_stat_init_aggr_mode())
goto out;
+ perf_stat_toggle_hwdt(0);
+
/*
* We dont want to block the signals - that would cause
* child tasks to inherit that and Ctrl-C would not work.
--
2.11.0
--
Regards/Gruss,
Boris.
Good mailing practices for 400: avoid top-posting and trim the reply.
