Re: [RFC][PATCH 4/3] tracing: Add NMI tracing in hwlat detector

From: Steven Rostedt
Date: Tue Aug 09 2016 - 13:23:03 EST

On Fri, 5 Aug 2016 17:40:43 +0200
Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote:

> > For x86, it has its own NMI safe sched_clock. I could make this "NMI"
> > code depend on:
> >
> > #ifndef CONFIG_GENERIC_SCHED_CLOCK
>
> that would be nice. That would be disable approx $(git grep
> sched_clock_register | wc -l) users but better than a lock up I guess.

How about this patch. It still records nmi_count, just not the time
spent in the NMI.

-- Steve


As NMIs can also cause latency when interrupts are disabled, the hwlat
detectory has no way to know if the latency it detects is from an NMI or an
SMI or some other hardware glitch.

As ftrace_nmi_enter/exit() funtions are no longer used (except for sh, which
isn't supported anymore), I converted those to "arch_ftrace_nmi_enter/exit"
and use ftrace_nmi_enter/exit() to check if hwlat detector is tracing or
not, and if so, it calls into the hwlat utility.

Since the hwlat detector only has a single kthread that is spinning with
interrupts disabled, it marks what CPU it is on, and if the NMI callback
happens on that CPU, it records the time spent in that NMI. This is added to
the output that is generated by the hwlat detector as:

[1] inner:0 outer:8 ts:1470329211.586481492
[2] inner:0 outer:4 ts:1470329231.619726657 nmi-total:4 nmi-count:1
[3] inner:0 outer:5 ts:1470329234.550761052 nmi-total:4 nmi-count:1
[4] inner:8 outer:0 ts:1470329263.655911668

All time is still tracked in microseconds.

The NMI information is only shown when an NMI occurred during the sample.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
arch/sh/kernel/ftrace.c | 4 ++--
include/linux/ftrace_irq.h | 31 +++++++++++++++++++++++----
kernel/trace/trace_entries.h | 8 +++++--
kernel/trace/trace_hwlat.c | 51 ++++++++++++++++++++++++++++++++++++++++++++
kernel/trace/trace_output.c | 16 +++++++++++++-
5 files changed, 101 insertions(+), 9 deletions(-)

diff --git a/arch/sh/kernel/ftrace.c b/arch/sh/kernel/ftrace.c
index 38993e09ef03..deb9eba3a43d 100644
--- a/arch/sh/kernel/ftrace.c
+++ b/arch/sh/kernel/ftrace.c
@@ -139,7 +139,7 @@ static void ftrace_mod_code(void)
clear_mod_flag();
}

-void ftrace_nmi_enter(void)
+void arch_ftrace_nmi_enter(void)
{
if (atomic_inc_return(&nmi_running) & MOD_CODE_WRITE_FLAG) {
smp_rmb();
@@ -150,7 +150,7 @@ void ftrace_nmi_enter(void)
smp_mb();
}

-void ftrace_nmi_exit(void)
+void arch_ftrace_nmi_exit(void)
{
/* Finish all executions before clearing nmi_running */
smp_mb();
diff --git a/include/linux/ftrace_irq.h b/include/linux/ftrace_irq.h
index dca7bf8cffe2..4ec2c9b205f2 100644
--- a/include/linux/ftrace_irq.h
+++ b/include/linux/ftrace_irq.h
@@ -3,11 +3,34 @@


#ifdef CONFIG_FTRACE_NMI_ENTER
-extern void ftrace_nmi_enter(void);
-extern void ftrace_nmi_exit(void);
+extern void arch_ftrace_nmi_enter(void);
+extern void arch_ftrace_nmi_exit(void);
#else
-static inline void ftrace_nmi_enter(void) { }
-static inline void ftrace_nmi_exit(void) { }
+static inline void arch_ftrace_nmi_enter(void) { }
+static inline void arch_ftrace_nmi_exit(void) { }
#endif

+#ifdef CONFIG_HWLAT_TRACER
+extern bool trace_hwlat_callback_enabled;
+extern void trace_hwlat_callback(bool enter);
+#endif
+
+static inline void ftrace_nmi_enter(void)
+{
+#ifdef CONFIG_HWLAT_TRACER
+ if (trace_hwlat_callback_enabled)
+ trace_hwlat_callback(true);
+#endif
+ arch_ftrace_nmi_enter();
+}
+
+static inline void ftrace_nmi_exit(void)
+{
+ arch_ftrace_nmi_exit();
+#ifdef CONFIG_HWLAT_TRACER
+ if (trace_hwlat_callback_enabled)
+ trace_hwlat_callback(false);
+#endif
+}
+
#endif /* _LINUX_FTRACE_IRQ_H */
diff --git a/kernel/trace/trace_entries.h b/kernel/trace/trace_entries.h
index 70d47dd99359..d1cc37e78f99 100644
--- a/kernel/trace/trace_entries.h
+++ b/kernel/trace/trace_entries.h
@@ -330,18 +330,22 @@ FTRACE_ENTRY(hwlat, hwlat_entry,
F_STRUCT(
__field( u64, duration )
__field( u64, outer_duration )
+ __field( u64, nmi_total_ts )
__field_struct( struct timespec, timestamp )
__field_desc( long, timestamp, tv_sec )
__field_desc( long, timestamp, tv_nsec )
+ __field( unsigned int, nmi_count )
__field( unsigned int, seqnum )
),

- F_printk("cnt:%u\tts:%010lu.%010lu\tinner:%llu\touter:%llu\n",
+ F_printk("cnt:%u\tts:%010lu.%010lu\tinner:%llu\touter:%llunmi-ts:%llu\tnmi-count:%u\n",
__entry->seqnum,
__entry->tv_sec,
__entry->tv_nsec,
__entry->duration,
- __entry->outer_duration),
+ __entry->outer_duration,
+ __entry->nmi_total_ts,
+ __entry->nmi_count),

FILTER_OTHER
);
diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index f0b5b2944ee7..2a668e55dcc6 100644
--- a/kernel/trace/trace_hwlat.c
+++ b/kernel/trace/trace_hwlat.c
@@ -64,6 +64,15 @@ static struct dentry *hwlat_sample_window; /* sample window us */
/* Save the previous tracing_thresh value */
static unsigned long save_tracing_thresh;

+/* NMI timestamp counters */
+static u64 nmi_ts_start;
+static u64 nmi_total_ts;
+static int nmi_count;
+static int nmi_cpu;
+
+/* Tells NMIs to call back to the hwlat tracer to record timestamps */
+bool trace_hwlat_callback_enabled;
+
/* If the user changed threshold, remember it */
static u64 last_tracing_thresh = DEFAULT_LAT_THRESHOLD * NSEC_PER_USEC;

@@ -72,7 +81,9 @@ struct hwlat_sample {
u64 seqnum; /* unique sequence */
u64 duration; /* delta */
u64 outer_duration; /* delta (outer loop) */
+ u64 nmi_total_ts; /* Total time spent in NMIs */
struct timespec timestamp; /* wall time */
+ int nmi_count; /* # NMIs during this sample */
};

/* keep the global state somewhere. */
@@ -112,6 +123,8 @@ static void trace_hwlat_sample(struct hwlat_sample *sample)
entry->duration = sample->duration;
entry->outer_duration = sample->outer_duration;
entry->timestamp = sample->timestamp;
+ entry->nmi_total_ts = sample->nmi_total_ts;
+ entry->nmi_count = sample->nmi_count;

if (!call_filter_check_discard(call, entry, buffer, event))
__buffer_unlock_commit(buffer, event);
@@ -125,6 +138,26 @@ static void trace_hwlat_sample(struct hwlat_sample *sample)
#define init_time(a, b) (a = b)
#define time_u64(a) a

+void trace_hwlat_callback(bool enter)
+{
+ if (smp_processor_id() != nmi_cpu)
+ return;
+
+ /*
+ * Currently trace_clock_local() calls sched_clock() and the
+ * generic version is not NMI safe.
+ */
+ if (!IS_ENABLED(CONFIG_GENERIC_SCHED_CLOCK)) {
+ if (enter)
+ nmi_ts_start = time_get();
+ else
+ nmi_total_ts = time_get() - nmi_ts_start;
+ }
+
+ if (enter)
+ nmi_count++;
+}
+
/**
* get_sample - sample the CPU TSC and look for likely hardware latencies
*
@@ -144,6 +177,14 @@ static int get_sample(void)

do_div(thresh, NSEC_PER_USEC); /* modifies interval value */

+ nmi_cpu = smp_processor_id();
+ nmi_total_ts = 0;
+ nmi_count = 0;
+ /* Make sure NMIs see this first */
+ barrier();
+
+ trace_hwlat_callback_enabled = true;
+
init_time(last_t2, 0);
start = time_get(); /* start timestamp */

@@ -188,6 +229,10 @@ static int get_sample(void)

} while (total <= hwlat_data.sample_width);

+ barrier(); /* finish the above in the view for NMIs */
+ trace_hwlat_callback_enabled = false;
+ barrier(); /* Make sure nmi_total_ts is no longer updated */
+
ret = 0;

/* If we exceed the threshold value, we have found a hardware latency */
@@ -196,11 +241,17 @@ static int get_sample(void)

ret = 1;

+ /* We read in microseconds */
+ if (nmi_total_ts)
+ do_div(nmi_total_ts, NSEC_PER_USEC);
+
hwlat_data.count++;
s.seqnum = hwlat_data.count;
s.duration = sample;
s.outer_duration = outer_sample;
s.timestamp = CURRENT_TIME;
+ s.nmi_total_ts = nmi_total_ts;
+ s.nmi_count = nmi_count;
trace_hwlat_sample(&s);

/* Keep a running maximum ever recorded hardware latency */
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index cc5db62a3929..5478a97e8db3 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -1109,13 +1109,27 @@ trace_hwlat_print(struct trace_iterator *iter, int flags,

trace_assign_type(field, entry);

- trace_seq_printf(s, "[%u] inner:%llu outer:%lld ts:%ld.%09ld\n",
+ trace_seq_printf(s, "[%u] inner:%llu outer:%lld ts:%ld.%09ld",
field->seqnum,
field->duration,
field->outer_duration,
field->timestamp.tv_sec,
field->timestamp.tv_nsec);

+ if (field->nmi_count) {
+ /*
+ * The generic sched_clock() is not NMI safe, thus
+ * we only record the count and not the time.
+ */
+ if (!IS_ENABLED(CONFIG_GENERIC_SCHED_CLOCK))
+ trace_seq_printf(s, " nmi-total:%llu",
+ field->nmi_total_ts);
+ trace_seq_printf(s, " nmi-count:%u",
+ field->nmi_count);
+ }
+
+ trace_seq_putc(s, '\n');
+
return trace_handle_return(s);
}

--
1.9.3