[PATCH v2 0/4] tracing: Add Hardware Latency detector tracer

[PATCH v2 0/4] tracing: Add Hardware Latency detector tracer

[Steven Rostedt]

This adds the PREEMPT_RT hwlat detector as a Linux tracer in mainline.
In the PREEMPT_RT patch set, it is a separate entity that is controlled
by the debugfs file system. I found that it is better suited as a
latency tracer in the tracing directory, as it follows pretty much the
same paradigm as the other latency tracers that already exist. All
I had to add was a hwlat_detector directory that contained a window
and width for the period and duration respectively of the test. But
the samples would just write to the tracing ring buffer and the max
latency would be stored in tracing_max_latency, and the threshold can
be set by the existing tracing_threshold. The last patch also adds a
new feature that would have the kthread migrate after each period to
another CPU specified by tracing_cpumask.


Here's the documention that is added to hwlat_detector.txt in the
Documentation directory:

Introduction:
-------------

The tracer hwlat_detector is a special purpose tracer that is used to
detect large system latencies induced by the behavior of certain underlying
hardware or firmware, independent of Linux itself. The code was developed
originally to detect SMIs (System Management Interrupts) on x86 systems,
however there is nothing x86 specific about this patchset. It was
originally written for use by the "RT" patch since the Real Time
kernel is highly latency sensitive.

SMIs are not serviced by the Linux kernel, which means that it does not
even know that they are occuring. SMIs are instead set up by BIOS code
and are serviced by BIOS code, usually for "critical" events such as
management of thermal sensors and fans. Sometimes though, SMIs are used for
other tasks and those tasks can spend an inordinate amount of time in the
handler (sometimes measured in milliseconds). Obviously this is a problem if
you are trying to keep event service latencies down in the microsecond range.

The hardware latency detector works by hogging one of the cpus for configurable
amounts of time (with interrupts disabled), polling the CPU Time Stamp Counter
for some period, then looking for gaps in the TSC data. Any gap indicates a
time when the polling was interrupted and since the interrupts are disabled,
the only thing that could do that would be an SMI or other hardware hiccup
(or an NMI, but those can be tracked).

Note that the hwlat detector should *NEVER* be used in a production environment.
It is intended to be run manually to determine if the hardware platform has a
problem with long system firmware service routines.

Usage:
------

Write the ASCII text "hwlat" into the current_tracer file of the tracing system
(mounted at /sys/kernel/tracing or /sys/kernel/tracing). It is possible to
redefine the threshold in microseconds (us) above which latency spikes will
be taken into account.

Example:

	# echo hwlat > /sys/kernel/tracing/current_tracer
	# echo 100 > /sys/kernel/tracing/tracing_thresh

The /sys/kernel/tracing/hwlat_detector interface contains the following files:

width			- time period to sample with CPUs held (usecs)
			  must be less than the total window size (enforced)
window			- total period of sampling, width being inside (usecs)

By default the width is set to 500,000 and window to 1,000,000, meaning that
for every 1,000,000 usecs (1s) the hwlat detector will spin for 500,000 usecs
(0.5s). If tracing_thresh contains zero when hwlat tracer is enabled, it will
change to a default of 10 usecs. If any latencies that exceed the threshold is
observed then the data will be written to the tracing ring buffer.

The minimum sleep time between periods is 1 millisecond. Even if width
is less than 1 millisecond apart from window, to allow the system to not
be totally starved.

If tracing_thresh was zero when hwlat detector was started, it will be set
back to zero if another tracer is loaded. Note, the last value in
tracing_thresh that hwlat detector had will be saved and this value will
be restored in tracing_thresh if it is still zero when hwlat detector is
started again.

The following tracing directory files are used by the hwlat_detector:

in /sys/kernel/tracing:

 tracing_threshold	- minimum latency value to be considered (usecs)
 tracing_max_latency	- maximum hardware latency actually observed (usecs)
 hwlat_detector/width	- specified amount of time to spin within window (usecs)
 hwlat_detector/window	- amount of time between (width) runs (usecs)

Changes since v1:

 . Added checks for CONFIG_GENERIC_SCHED_CLOCK, and if that is set,
   only nmi_counts will be recorded when an NMI is triggered, but not
   the time in NMI, because the generic sched_clock is not NMI safe.
   (suggested by Sebastian Andrzej Siewior)

 . Changed the format a little to be a little nicer looking.

   Was: [99] inner:13 outer:16 ts:1470352534.886878855
   Now: #99 inner/outer(us):   13/16   ts:1470352534.886878855

 . Included the NMI patch into the main patch set.

 . Holding off SMI counting until I have an arch_smi_count() that isn't
   so "really yucky".

Jon Masters (1):
      tracing: Add documentation for hwlat_detector tracer

Steven Rostedt (Red Hat) (3):
      tracing: Added hardware latency tracer
      tracing: Have hwlat trace migrate across tracing_cpumask CPUs
      tracing: Add NMI tracing in hwlat detector

----
 Documentation/trace/hwlat_detector.txt |  79 ++++
 arch/sh/kernel/ftrace.c                |   4 +-
 include/linux/ftrace_irq.h             |  31 +-
 kernel/trace/Kconfig                   |  35 ++
 kernel/trace/Makefile                  |   1 +
 kernel/trace/trace.c                   |   2 +-
 kernel/trace/trace.h                   |   3 +
 kernel/trace/trace_entries.h           |  27 ++
 kernel/trace/trace_hwlat.c             | 633 +++++++++++++++++++++++++++++++++
 kernel/trace/trace_output.c            |  66 ++++
 10 files changed, 874 insertions(+), 7 deletions(-)
 create mode 100644 Documentation/trace/hwlat_detector.txt
 create mode 100644 kernel/trace/trace_hwlat.c

[PATCH v2 1/4] tracing: Added hardware latency tracer

[Steven Rostedt]

[-- Attachment #1: 0001-tracing-Added-hardware-latency-tracer.patch --]
[-- Type: text/plain, Size: 23671 bytes --]

From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

The hardware latency tracer has been in the PREEMPT_RT patch for some time.
It is used to detect possible SMIs or any other hardware interruptions that
the kernel is unaware of. Note, NMIs may also be detected, but that may be
good to note as well.

The logic is pretty simple. It simply creates a thread that spins on a
single CPU for a specified amount of time (width) within a periodic window
(window). These numbers may be adjusted by their cooresponding names in

   /sys/kernel/tracing/hwlat_detector/

The defaults are window = 1000000 us (1 second)
                 width  =  500000 us (1/2 second)

The loop consists of:

	t1 = trace_clock_local();
	t2 = trace_clock_local();

Where trace_clock_local() is a variant of sched_clock().

The difference of t2 - t1 is recorded as the "inner" timestamp and also the
timestamp  t1 - prev_t2 is recorded as the "outer" timestamp. If either of
these differences are greater than the time denoted in
/sys/kernel/tracing/tracing_thresh then it records the event.

When this tracer is started, and tracing_thresh is zero, it changes to the
default threshold of 10 us.

The hwlat tracer in the PREEMPT_RT patch was originally written by
Jon Masters. I have modified it quite a bit and turned it into a
tracer.

Based-on-code-by: Jon Masters <jcm@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/trace/Kconfig         |  35 +++
 kernel/trace/Makefile        |   1 +
 kernel/trace/trace.c         |   2 +-
 kernel/trace/trace.h         |   3 +
 kernel/trace/trace_entries.h |  23 ++
 kernel/trace/trace_hwlat.c   | 527 +++++++++++++++++++++++++++++++++++++++++++
 kernel/trace/trace_output.c  |  52 +++++
 7 files changed, 642 insertions(+), 1 deletion(-)
 create mode 100644 kernel/trace/trace_hwlat.c

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index f4b86e8ca1e7..72c07c2ffd79 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -221,6 +221,41 @@ config SCHED_TRACER
 	  This tracer tracks the latency of the highest priority task
 	  to be scheduled in, starting from the point it has woken up.
 
+config HWLAT_TRACER
+	bool "Tracer to detect hardware latencies (like SMIs)"
+	select GENERIC_TRACER
+	help
+	 This tracer, when enabled will create one or more kernel threads,
+	 depening on what the cpumask file is set to, which each thread
+	 spinning in a loop looking for interruptions caused by
+	 something other than the kernel. For example, if a
+	 System Management Interrupt (SMI) takes a noticeable amount of
+	 time, this tracer will detect it. This is useful for testing
+	 if a system is reliable for Real Time tasks.
+
+	 Some files are created in the tracing directory when this
+	 is enabled:
+
+	   hwlat_detector/width   - time in usecs for how long to spin for
+	   hwlat_detector/window  - time in usecs between the start of each
+				     iteration
+
+	 A kernel thread is created that will spin with interrupts disabled
+	 for "width" microseconds in every "widow" cycle. It will not spin
+	 for "window - width" microseconds, where the system can
+	 continue to operate.
+
+	 The output will appear in the trace and trace_pipe files.
+
+	 When the tracer is not running, it has no affect on the system,
+	 but when it is running, it can cause the system to be
+	 periodically non responsive. Do not run this tracer on a
+	 production system.
+
+	 To enable this tracer, echo in "hwlat" into the current_tracer
+	 file. Every time a latency is greater than tracing_thresh, it will
+	 be recorded into the ring buffer.
+
 config ENABLE_DEFAULT_TRACERS
 	bool "Trace process context switches and events"
 	depends on !GENERIC_TRACER
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index 979e7bfbde7a..e57980845549 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -37,6 +37,7 @@ obj-$(CONFIG_FUNCTION_TRACER) += trace_functions.o
 obj-$(CONFIG_IRQSOFF_TRACER) += trace_irqsoff.o
 obj-$(CONFIG_PREEMPT_TRACER) += trace_irqsoff.o
 obj-$(CONFIG_SCHED_TRACER) += trace_sched_wakeup.o
+obj-$(CONFIG_HWLAT_TRACER) += trace_hwlat.o
 obj-$(CONFIG_NOP_TRACER) += trace_nop.o
 obj-$(CONFIG_STACK_TRACER) += trace_stack.o
 obj-$(CONFIG_MMIOTRACE) += trace_mmiotrace.o
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index dade4c9559cc..474cc814e16d 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1047,7 +1047,7 @@ void disable_trace_on_warning(void)
  *
  * Shows real state of the ring buffer if it is enabled or not.
  */
-static int tracer_tracing_is_on(struct trace_array *tr)
+int tracer_tracing_is_on(struct trace_array *tr)
 {
 	if (tr->trace_buffer.buffer)
 		return ring_buffer_record_is_on(tr->trace_buffer.buffer);
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index f783df416726..1d866b0c1567 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -38,6 +38,7 @@ enum trace_type {
 	TRACE_USER_STACK,
 	TRACE_BLK,
 	TRACE_BPUTS,
+	TRACE_HWLAT,
 
 	__TRACE_LAST_TYPE,
 };
@@ -326,6 +327,7 @@ extern void __ftrace_bad_type(void);
 		IF_ASSIGN(var, ent, struct print_entry, TRACE_PRINT);	\
 		IF_ASSIGN(var, ent, struct bprint_entry, TRACE_BPRINT);	\
 		IF_ASSIGN(var, ent, struct bputs_entry, TRACE_BPUTS);	\
+		IF_ASSIGN(var, ent, struct hwlat_entry, TRACE_HWLAT);	\
 		IF_ASSIGN(var, ent, struct trace_mmiotrace_rw,		\
 			  TRACE_MMIO_RW);				\
 		IF_ASSIGN(var, ent, struct trace_mmiotrace_map,		\
@@ -571,6 +573,7 @@ void tracing_reset_current(int cpu);
 void tracing_reset_all_online_cpus(void);
 int tracing_open_generic(struct inode *inode, struct file *filp);
 bool tracing_is_disabled(void);
+int tracer_tracing_is_on(struct trace_array *tr);
 struct dentry *trace_create_file(const char *name,
 				 umode_t mode,
 				 struct dentry *parent,
diff --git a/kernel/trace/trace_entries.h b/kernel/trace/trace_entries.h
index 5c30efcda5e6..70d47dd99359 100644
--- a/kernel/trace/trace_entries.h
+++ b/kernel/trace/trace_entries.h
@@ -322,3 +322,26 @@ FTRACE_ENTRY(branch, trace_branch,
 	FILTER_OTHER
 );
 
+
+FTRACE_ENTRY(hwlat, hwlat_entry,
+
+	TRACE_HWLAT,
+
+	F_STRUCT(
+		__field(	u64,			duration	)
+		__field(	u64,			outer_duration	)
+		__field_struct( struct timespec,	timestamp	)
+		__field_desc(	long,	timestamp,	tv_sec		)
+		__field_desc(	long,	timestamp,	tv_nsec		)
+		__field(	unsigned int,		seqnum		)
+	),
+
+	F_printk("cnt:%u\tts:%010lu.%010lu\tinner:%llu\touter:%llu\n",
+		 __entry->seqnum,
+		 __entry->tv_sec,
+		 __entry->tv_nsec,
+		 __entry->duration,
+		 __entry->outer_duration),
+
+	FILTER_OTHER
+);
diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
new file mode 100644
index 000000000000..08dfabe4e862
--- /dev/null
+++ b/kernel/trace/trace_hwlat.c
@@ -0,0 +1,527 @@
+/*
+ * trace_hwlatdetect.c - A simple Hardware Latency detector.
+ *
+ * Use this tracer to detect large system latencies induced by the behavior of
+ * certain underlying system hardware or firmware, independent of Linux itself.
+ * The code was developed originally to detect the presence of SMIs on Intel
+ * and AMD systems, although there is no dependency upon x86 herein.
+ *
+ * The classical example usage of this tracer is in detecting the presence of
+ * SMIs or System Management Interrupts on Intel and AMD systems. An SMI is a
+ * somewhat special form of hardware interrupt spawned from earlier CPU debug
+ * modes in which the (BIOS/EFI/etc.) firmware arranges for the South Bridge
+ * LPC (or other device) to generate a special interrupt under certain
+ * circumstances, for example, upon expiration of a special SMI timer device,
+ * due to certain external thermal readings, on certain I/O address accesses,
+ * and other situations. An SMI hits a special CPU pin, triggers a special
+ * SMI mode (complete with special memory map), and the OS is unaware.
+ *
+ * Although certain hardware-inducing latencies are necessary (for example,
+ * a modern system often requires an SMI handler for correct thermal control
+ * and remote management) they can wreak havoc upon any OS-level performance
+ * guarantees toward low-latency, especially when the OS is not even made
+ * aware of the presence of these interrupts. For this reason, we need a
+ * somewhat brute force mechanism to detect these interrupts. In this case,
+ * we do it by hogging all of the CPU(s) for configurable timer intervals,
+ * sampling the built-in CPU timer, looking for discontiguous readings.
+ *
+ * WARNING: This implementation necessarily introduces latencies. Therefore,
+ *          you should NEVER use this tracer while running in a production
+ *          environment requiring any kind of low-latency performance
+ *          guarantee(s).
+ *
+ * Copyright (C) 2008-2009 Jon Masters, Red Hat, Inc. <jcm@redhat.com>
+ * Copyright (C) 2013-2016 Steven Rostedt, Red Hat, Inc. <srostedt@redhat.com>
+ *
+ * Includes useful feedback from Clark Williams <clark@redhat.com>
+ *
+ * This file is licensed under the terms of the GNU General Public
+ * License version 2. This program is licensed "as is" without any
+ * warranty of any kind, whether express or implied.
+ */
+#include <linux/kthread.h>
+#include <linux/tracefs.h>
+#include <linux/uaccess.h>
+#include <linux/delay.h>
+#include "trace.h"
+
+static struct trace_array	*hwlat_trace;
+
+#define U64STR_SIZE		22			/* 20 digits max */
+
+#define BANNER			"hwlat_detector: "
+#define DEFAULT_SAMPLE_WINDOW	1000000			/* 1s */
+#define DEFAULT_SAMPLE_WIDTH	500000			/* 0.5s */
+#define DEFAULT_LAT_THRESHOLD	10			/* 10us */
+
+/* sampling thread*/
+static struct task_struct *hwlat_kthread;
+
+static struct dentry *hwlat_sample_width;	/* sample width us */
+static struct dentry *hwlat_sample_window;	/* sample window us */
+
+/* Save the previous tracing_thresh value */
+static unsigned long save_tracing_thresh;
+
+/* If the user changed threshold, remember it */
+static u64 last_tracing_thresh = DEFAULT_LAT_THRESHOLD * NSEC_PER_USEC;
+
+/* Individual latency samples are stored here when detected. */
+struct hwlat_sample {
+	u64		seqnum;		/* unique sequence */
+	u64		duration;	/* delta */
+	u64		outer_duration;	/* delta (outer loop) */
+	struct timespec	timestamp;	/* wall time */
+};
+
+/* keep the global state somewhere. */
+static struct hwlat_data {
+
+	struct mutex lock;		/* protect changes */
+
+	u64	count;			/* total since reset */
+
+	u64	sample_window;		/* total sampling window (on+off) */
+	u64	sample_width;		/* active sampling portion of window */
+
+} hwlat_data = {
+	.sample_window		= DEFAULT_SAMPLE_WINDOW,
+	.sample_width		= DEFAULT_SAMPLE_WIDTH,
+};
+
+static void trace_hwlat_sample(struct hwlat_sample *sample)
+{
+	struct trace_array *tr = hwlat_trace;
+	struct trace_event_call *call = &event_hwlat;
+	struct ring_buffer *buffer = tr->trace_buffer.buffer;
+	struct ring_buffer_event *event;
+	struct hwlat_entry *entry;
+	unsigned long flags;
+	int pc;
+
+	pc = preempt_count();
+	local_save_flags(flags);
+
+	event = trace_buffer_lock_reserve(buffer, TRACE_HWLAT, sizeof(*entry),
+					  flags, pc);
+	if (!event)
+		return;
+	entry	= ring_buffer_event_data(event);
+	entry->seqnum			= sample->seqnum;
+	entry->duration			= sample->duration;
+	entry->outer_duration		= sample->outer_duration;
+	entry->timestamp		= sample->timestamp;
+
+	if (!call_filter_check_discard(call, entry, buffer, event))
+		__buffer_unlock_commit(buffer, event);
+}
+
+/* Macros to encapsulate the time capturing infrastructure */
+#define time_type	u64
+#define time_get()	trace_clock_local()
+#define time_to_us(x)	div_u64(x, 1000)
+#define time_sub(a, b)	((a) - (b))
+#define init_time(a, b)	(a = b)
+#define time_u64(a)	a
+
+/**
+ * get_sample - sample the CPU TSC and look for likely hardware latencies
+ *
+ * Used to repeatedly capture the CPU TSC (or similar), looking for potential
+ * hardware-induced latency. Called with interrupts disabled and with
+ * hwlat_data.lock held.
+ */
+static int get_sample(void)
+{
+	struct trace_array *tr = hwlat_trace;
+	time_type start, t1, t2, last_t2;
+	s64 diff, total, last_total = 0;
+	u64 sample = 0;
+	u64 thresh = tracing_thresh;
+	u64 outer_sample = 0;
+	int ret = -1;
+
+	do_div(thresh, NSEC_PER_USEC); /* modifies interval value */
+
+	init_time(last_t2, 0);
+	start = time_get(); /* start timestamp */
+
+	do {
+
+		t1 = time_get();	/* we'll look for a discontinuity */
+		t2 = time_get();
+
+		if (time_u64(last_t2)) {
+			/* Check the delta from outer loop (t2 to next t1) */
+			diff = time_to_us(time_sub(t1, last_t2));
+			/* This shouldn't happen */
+			if (diff < 0) {
+				pr_err(BANNER "time running backwards\n");
+				goto out;
+			}
+			if (diff > outer_sample)
+				outer_sample = diff;
+		}
+		last_t2 = t2;
+
+		total = time_to_us(time_sub(t2, start)); /* sample width */
+
+		/* Check for possible overflows */
+		if (total < last_total) {
+			pr_err("Time total overflowed\n");
+			break;
+		}
+		last_total = total;
+
+		/* This checks the inner loop (t1 to t2) */
+		diff = time_to_us(time_sub(t2, t1));     /* current diff */
+
+		/* This shouldn't happen */
+		if (diff < 0) {
+			pr_err(BANNER "time running backwards\n");
+			goto out;
+		}
+
+		if (diff > sample)
+			sample = diff; /* only want highest value */
+
+	} while (total <= hwlat_data.sample_width);
+
+	ret = 0;
+
+	/* If we exceed the threshold value, we have found a hardware latency */
+	if (sample > thresh || outer_sample > thresh) {
+		struct hwlat_sample s;
+
+		ret = 1;
+
+		hwlat_data.count++;
+		s.seqnum = hwlat_data.count;
+		s.duration = sample;
+		s.outer_duration = outer_sample;
+		s.timestamp = CURRENT_TIME;
+		trace_hwlat_sample(&s);
+
+		/* Keep a running maximum ever recorded hardware latency */
+		if (sample > tr->max_latency)
+			tr->max_latency = sample;
+	}
+
+out:
+	return ret;
+}
+
+/*
+ * kthread_fn - The CPU time sampling/hardware latency detection kernel thread
+ *
+ * Used to periodically sample the CPU TSC via a call to get_sample. We
+ * disable interrupts, which does (intentionally) introduce latency since we
+ * need to ensure nothing else might be running (and thus preempting).
+ * Obviously this should never be used in production environments.
+ *
+ * Currently this runs on which ever CPU it was scheduled on, but most
+ * real-world hardware latency situations occur across several CPUs,
+ * but we might later generalize this if we find there are any actualy
+ * systems with alternate SMI delivery or other hardware latencies.
+ */
+static int kthread_fn(void *data)
+{
+	u64 interval;
+
+	while (!kthread_should_stop()) {
+
+		local_irq_disable();
+		get_sample();
+		local_irq_enable();
+
+		mutex_lock(&hwlat_data.lock);
+		interval = hwlat_data.sample_window - hwlat_data.sample_width;
+		mutex_unlock(&hwlat_data.lock);
+
+		do_div(interval, USEC_PER_MSEC); /* modifies interval value */
+
+		/* Always sleep for at least 1ms */
+		if (interval < 1)
+			interval = 1;
+
+		if (msleep_interruptible(interval))
+			break;
+	}
+
+	return 0;
+}
+
+/**
+ * start_kthread - Kick off the hardware latency sampling/detector kthread
+ *
+ * This starts the kernel thread that will sit and sample the CPU timestamp
+ * counter (TSC or similar) and look for potential hardware latencies.
+ */
+static int start_kthread(struct trace_array *tr)
+{
+	struct task_struct *kthread;
+
+	kthread = kthread_create(kthread_fn, NULL, "hwlatd");
+	if (IS_ERR(kthread)) {
+		pr_err(BANNER "could not start sampling thread\n");
+		return -ENOMEM;
+	}
+	hwlat_kthread = kthread;
+	wake_up_process(kthread);
+
+	return 0;
+}
+
+/**
+ * stop_kthread - Inform the hardware latency samping/detector kthread to stop
+ *
+ * This kicks the running hardware latency sampling/detector kernel thread and
+ * tells it to stop sampling now. Use this on unload and at system shutdown.
+ */
+static void stop_kthread(void)
+{
+	if (!hwlat_kthread)
+		return;
+	kthread_stop(hwlat_kthread);
+	hwlat_kthread = NULL;
+}
+
+/*
+ * hwlat_read - Wrapper read function for reading both window and width
+ * @filp: The active open file structure
+ * @ubuf: The userspace provided buffer to read value into
+ * @cnt: The maximum number of bytes to read
+ * @ppos: The current "file" position
+ *
+ * This function provides a generic read implementation for the global state
+ * "hwlat_data" structure filesystem entries.
+ */
+static ssize_t hwlat_read(struct file *filp, char __user *ubuf,
+			  size_t cnt, loff_t *ppos)
+{
+	char buf[U64STR_SIZE];
+	u64 *entry = filp->private_data;
+	u64 val;
+	int len;
+
+	if (!entry)
+		return -EFAULT;
+
+	if (cnt > sizeof(buf))
+		cnt = sizeof(buf);
+
+	val = *entry;
+
+	len = snprintf(buf, sizeof(buf), "%llu\n", val);
+
+	return simple_read_from_buffer(ubuf, cnt, ppos, buf, len);
+}
+
+/**
+ * hwlat_width_write - Write function for "width" entry
+ * @filp: The active open file structure
+ * @ubuf: The user buffer that contains the value to write
+ * @cnt: The maximum number of bytes to write to "file"
+ * @ppos: The current position in @file
+ *
+ * This function provides a write implementation for the "width" interface
+ * to the hardware latency detector. It can be used to configure
+ * for how many us of the total window us we will actively sample for any
+ * hardware-induced latency periods. Obviously, it is not possible to
+ * sample constantly and have the system respond to a sample reader, or,
+ * worse, without having the system appear to have gone out to lunch. It
+ * is enforced that width is less that the total window size.
+ */
+static ssize_t
+hwlat_width_write(struct file *filp, const char __user *ubuf,
+		  size_t cnt, loff_t *ppos)
+{
+	u64 val;
+	int err;
+
+	err = kstrtoull_from_user(ubuf, cnt, 10, &val);
+	if (err)
+		return err;
+
+	mutex_lock(&hwlat_data.lock);
+	if (val < hwlat_data.sample_window)
+		hwlat_data.sample_width = val;
+	else
+		err = -EINVAL;
+	mutex_unlock(&hwlat_data.lock);
+
+	if (err)
+		return err;
+
+	return cnt;
+}
+
+/**
+ * hwlat_window_write - Write function for "window" entry
+ * @filp: The active open file structure
+ * @ubuf: The user buffer that contains the value to write
+ * @cnt: The maximum number of bytes to write to "file"
+ * @ppos: The current position in @file
+ *
+ * This function provides a write implementation for the "window" interface
+ * to the hardware latency detetector. The window is the total time
+ * in us that will be considered one sample period. Conceptually, windows
+ * occur back-to-back and contain a sample width period during which
+ * actual sampling occurs. Can be used to write a new total window size. It
+ * is enfoced that any value written must be greater than the sample width
+ * size, or an error results.
+ */
+static ssize_t
+hwlat_window_write(struct file *filp, const char __user *ubuf,
+		   size_t cnt, loff_t *ppos)
+{
+	u64 val;
+	int err;
+
+	err = kstrtoull_from_user(ubuf, cnt, 10, &val);
+	if (err)
+		return err;
+
+	mutex_lock(&hwlat_data.lock);
+	if (hwlat_data.sample_width < val)
+		hwlat_data.sample_window = val;
+	else
+		err = -EINVAL;
+	mutex_unlock(&hwlat_data.lock);
+
+	if (err)
+		return err;
+
+	return cnt;
+}
+
+static const struct file_operations width_fops = {
+	.open		= tracing_open_generic,
+	.read		= hwlat_read,
+	.write		= hwlat_width_write,
+};
+
+static const struct file_operations window_fops = {
+	.open		= tracing_open_generic,
+	.read		= hwlat_read,
+	.write		= hwlat_window_write,
+};
+
+/**
+ * init_tracefs - A function to initialize the tracefs interface files
+ *
+ * This function creates entries in tracefs for "hwlat_detector".
+ * It creates the hwlat_detector directory in the tracing directory,
+ * and within that directory is the count, width and window files to
+ * change and view those values.
+ */
+static int init_tracefs(void)
+{
+	struct dentry *d_tracer;
+	struct dentry *top_dir;
+
+	d_tracer = tracing_init_dentry();
+	if (IS_ERR(d_tracer))
+		return -ENOMEM;
+
+	top_dir = tracefs_create_dir("hwlat_detector", d_tracer);
+	if (!top_dir)
+		return -ENOMEM;
+
+	hwlat_sample_window = tracefs_create_file("window", 0640,
+						  top_dir,
+						  &hwlat_data.sample_window,
+						  &window_fops);
+	if (!hwlat_sample_window)
+		goto err;
+
+	hwlat_sample_width = tracefs_create_file("width", 0644,
+						 top_dir,
+						 &hwlat_data.sample_width,
+						 &width_fops);
+	if (!hwlat_sample_width)
+		goto err;
+
+	return 0;
+
+ err:
+	tracefs_remove_recursive(top_dir);
+	return -ENOMEM;
+}
+
+static void hwlat_tracer_start(struct trace_array *tr)
+{
+	int err;
+
+	err = start_kthread(tr);
+	if (err)
+		pr_err(BANNER "Cannot start hwlat kthread\n");
+}
+
+static void hwlat_tracer_stop(struct trace_array *tr)
+{
+	stop_kthread();
+}
+
+static bool hwlat_busy;
+
+static int hwlat_tracer_init(struct trace_array *tr)
+{
+	/* Only allow one instance to enable this */
+	if (hwlat_busy)
+		return -EBUSY;
+
+	hwlat_trace = tr;
+
+	hwlat_data.count = 0;
+	tr->max_latency = 0;
+	save_tracing_thresh = tracing_thresh;
+
+	/* tracing_thresh is in nsecs, we speak in usecs */
+	if (!tracing_thresh)
+		tracing_thresh = last_tracing_thresh;
+
+	if (tracer_tracing_is_on(tr))
+		hwlat_tracer_start(tr);
+
+	hwlat_busy = true;
+
+	return 0;
+}
+
+static void hwlat_tracer_reset(struct trace_array *tr)
+{
+	stop_kthread();
+
+	/* the tracing threshold is static between runs */
+	last_tracing_thresh = tracing_thresh;
+
+	tracing_thresh = save_tracing_thresh;
+	hwlat_busy = false;
+}
+
+static struct tracer hwlat_tracer __read_mostly =
+{
+	.name		= "hwlat",
+	.init		= hwlat_tracer_init,
+	.reset		= hwlat_tracer_reset,
+	.start		= hwlat_tracer_start,
+	.stop		= hwlat_tracer_stop,
+	.allow_instances = true,
+};
+
+__init static int init_hwlat_tracer(void)
+{
+	int ret;
+
+	mutex_init(&hwlat_data.lock);
+
+	ret = register_tracer(&hwlat_tracer);
+	if (ret)
+		return ret;
+
+	init_tracefs();
+
+	return 0;
+}
+late_initcall(init_hwlat_tracer);
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index 0bb9cf2d53e6..d67a562df259 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -1098,6 +1098,57 @@ static struct trace_event trace_user_stack_event = {
 	.funcs		= &trace_user_stack_funcs,
 };
 
+/* TRACE_HWLAT */
+static enum print_line_t
+trace_hwlat_print(struct trace_iterator *iter, int flags,
+		  struct trace_event *event)
+{
+	struct trace_entry *entry = iter->ent;
+	struct trace_seq *s = &iter->seq;
+	struct hwlat_entry *field;
+
+	trace_assign_type(field, entry);
+
+	trace_seq_printf(s, "#%-5u inner/outer(us): %4llu/%-5llu ts:%ld.%09ld\n",
+			 field->seqnum,
+			 field->duration,
+			 field->outer_duration,
+			 field->timestamp.tv_sec,
+			 field->timestamp.tv_nsec);
+
+	return trace_handle_return(s);
+}
+
+
+static enum print_line_t
+trace_hwlat_raw(struct trace_iterator *iter, int flags,
+		struct trace_event *event)
+{
+	struct hwlat_entry *field;
+	struct trace_seq *s = &iter->seq;
+
+	trace_assign_type(field, iter->ent);
+
+	trace_seq_printf(s, "%llu %lld %ld %09ld %u\n",
+			 field->duration,
+			 field->outer_duration,
+			 field->timestamp.tv_sec,
+			 field->timestamp.tv_nsec,
+			 field->seqnum);
+
+	return trace_handle_return(s);
+}
+
+static struct trace_event_functions trace_hwlat_funcs = {
+	.trace		= trace_hwlat_print,
+	.raw		= trace_hwlat_raw,
+};
+
+static struct trace_event trace_hwlat_event = {
+	.type		= TRACE_HWLAT,
+	.funcs		= &trace_hwlat_funcs,
+};
+
 /* TRACE_BPUTS */
 static enum print_line_t
 trace_bputs_print(struct trace_iterator *iter, int flags,
@@ -1233,6 +1284,7 @@ static struct trace_event *events[] __initdata = {
 	&trace_bputs_event,
 	&trace_bprint_event,
 	&trace_print_event,
+	&trace_hwlat_event,
 	NULL
 };
 
-- 
2.8.1

[PATCH v2 2/4] tracing: Add documentation for hwlat_detector tracer

[Steven Rostedt]

[-- Attachment #1: 0002-tracing-Add-documentation-for-hwlat_detector-tracer.patch --]
[-- Type: text/plain, Size: 4251 bytes --]

From: Jon Masters <jcm@redhat.com>

Added the documentation on how to use th hwlat_detector.

Signed-off-by: Jon Masters <jcm@redhat.com>
[ Various updates and modified to show hwlat as a tracer ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 Documentation/trace/hwlat_detector.txt | 73 ++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 Documentation/trace/hwlat_detector.txt

diff --git a/Documentation/trace/hwlat_detector.txt b/Documentation/trace/hwlat_detector.txt
new file mode 100644
index 000000000000..c02e8ef800cf
--- /dev/null
+++ b/Documentation/trace/hwlat_detector.txt
@@ -0,0 +1,73 @@
+Introduction:
+-------------
+
+The tracer hwlat_detector is a special purpose tracer that is used to
+detect large system latencies induced by the behavior of certain underlying
+hardware or firmware, independent of Linux itself. The code was developed
+originally to detect SMIs (System Management Interrupts) on x86 systems,
+however there is nothing x86 specific about this patchset. It was
+originally written for use by the "RT" patch since the Real Time
+kernel is highly latency sensitive.
+
+SMIs are not serviced by the Linux kernel, which means that it does not
+even know that they are occuring. SMIs are instead set up by BIOS code
+and are serviced by BIOS code, usually for "critical" events such as
+management of thermal sensors and fans. Sometimes though, SMIs are used for
+other tasks and those tasks can spend an inordinate amount of time in the
+handler (sometimes measured in milliseconds). Obviously this is a problem if
+you are trying to keep event service latencies down in the microsecond range.
+
+The hardware latency detector works by hogging one of the cpus for configurable
+amounts of time (with interrupts disabled), polling the CPU Time Stamp Counter
+for some period, then looking for gaps in the TSC data. Any gap indicates a
+time when the polling was interrupted and since the interrupts are disabled,
+the only thing that could do that would be an SMI or other hardware hiccup
+(or an NMI, but those can be tracked).
+
+Note that the hwlat detector should *NEVER* be used in a production environment.
+It is intended to be run manually to determine if the hardware platform has a
+problem with long system firmware service routines.
+
+Usage:
+------
+
+Write the ASCII text "hwlat" into the current_tracer file of the tracing system
+(mounted at /sys/kernel/tracing or /sys/kernel/tracing). It is possible to
+redefine the threshold in microseconds (us) above which latency spikes will
+be taken into account.
+
+Example:
+
+	# echo hwlat > /sys/kernel/tracing/current_tracer
+	# echo 100 > /sys/kernel/tracing/tracing_thresh
+
+The /sys/kernel/tracing/hwlat_detector interface contains the following files:
+
+width			- time period to sample with CPUs held (usecs)
+			  must be less than the total window size (enforced)
+window			- total period of sampling, width being inside (usecs)
+
+By default the width is set to 500,000 and window to 1,000,000, meaning that
+for every 1,000,000 usecs (1s) the hwlat detector will spin for 500,000 usecs
+(0.5s). If tracing_thresh contains zero when hwlat tracer is enabled, it will
+change to a default of 10 usecs. If any latencies that exceed the threshold is
+observed then the data will be written to the tracing ring buffer.
+
+The minimum sleep time between periods is 1 millisecond. Even if width
+is less than 1 millisecond apart from window, to allow the system to not
+be totally starved.
+
+If tracing_thresh was zero when hwlat detector was started, it will be set
+back to zero if another tracer is loaded. Note, the last value in
+tracing_thresh that hwlat detector had will be saved and this value will
+be restored in tracing_thresh if it is still zero when hwlat detector is
+started again.
+
+The following tracing directory files are used by the hwlat_detector:
+
+in /sys/kernel/tracing:
+
+ tracing_threshold	- minimum latency value to be considered (usecs)
+ tracing_max_latency	- maximum hardware latency actually observed (usecs)
+ hwlat_detector/width	- specified amount of time to spin within window (usecs)
+ hwlat_detector/window	- amount of time between (width) runs (usecs)
-- 
2.8.1

[PATCH v2 3/4] tracing: Have hwlat trace migrate across tracing_cpumask CPUs

[Steven Rostedt]

[-- Attachment #1: 0003-tracing-Have-hwlat-trace-migrate-across-tracing_cpum.patch --]
[-- Type: text/plain, Size: 3611 bytes --]

From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

Instead of having the hwlat detector thread stay on one CPU, have it migrate
across all the CPUs specified by tracing_cpumask. If the user modifies the
thread's CPU affinity, the migration will stop until the next instance that
the tracer is instantiated. The migration happens at the end of each window
(period).

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 Documentation/trace/hwlat_detector.txt |  6 ++++
 kernel/trace/trace_hwlat.c             | 55 ++++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+)

diff --git a/Documentation/trace/hwlat_detector.txt b/Documentation/trace/hwlat_detector.txt
index c02e8ef800cf..3207717a0d1a 100644
--- a/Documentation/trace/hwlat_detector.txt
+++ b/Documentation/trace/hwlat_detector.txt
@@ -69,5 +69,11 @@ in /sys/kernel/tracing:
 
  tracing_threshold	- minimum latency value to be considered (usecs)
  tracing_max_latency	- maximum hardware latency actually observed (usecs)
+ tracing_cpumask	- the CPUs to move the hwlat thread across
  hwlat_detector/width	- specified amount of time to spin within window (usecs)
  hwlat_detector/window	- amount of time between (width) runs (usecs)
+
+The hwlat detector's kernel thread will migrate across each CPU specified in
+tracing_cpumask between each window. To limit the migration, either modify
+tracing_cpumask, or modify the hwlat kernel thread (named [hwlatd]) CPU
+affinity directly, and the migration will stop.
diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index 08dfabe4e862..65aab3914a56 100644
--- a/kernel/trace/trace_hwlat.c
+++ b/kernel/trace/trace_hwlat.c
@@ -42,6 +42,7 @@
 #include <linux/kthread.h>
 #include <linux/tracefs.h>
 #include <linux/uaccess.h>
+#include <linux/cpumask.h>
 #include <linux/delay.h>
 #include "trace.h"
 
@@ -211,6 +212,57 @@ out:
 	return ret;
 }
 
+static struct cpumask save_cpumask;
+static bool disable_migrate;
+
+static void move_to_next_cpu(void)
+{
+	static struct cpumask *current_mask;
+	int next_cpu;
+
+	if (disable_migrate)
+		return;
+
+	/* Just pick the first CPU on first iteration */
+	if (!current_mask) {
+		current_mask = &save_cpumask;
+		get_online_cpus();
+		cpumask_and(current_mask, cpu_online_mask, tracing_buffer_mask);
+		put_online_cpus();
+		next_cpu = cpumask_first(current_mask);
+		goto set_affinity;
+	}
+
+	/*
+	 * If for some reason the user modifies the CPU affinity
+	 * of this thread, than stop migrating for the duration
+	 * of the current test.
+	 */
+	if (!cpumask_equal(current_mask, &current->cpus_allowed))
+		goto disable;
+
+	get_online_cpus();
+	cpumask_and(current_mask, cpu_online_mask, tracing_buffer_mask);
+	next_cpu = cpumask_next(smp_processor_id(), current_mask);
+	put_online_cpus();
+
+	if (next_cpu >= nr_cpu_ids)
+		next_cpu = cpumask_first(current_mask);
+
+ set_affinity:
+	if (next_cpu >= nr_cpu_ids) /* Shouldn't happen! */
+		goto disable;
+
+	cpumask_clear(current_mask);
+	cpumask_set_cpu(next_cpu, current_mask);
+
+	sched_setaffinity(0, current_mask);
+	return;
+
+ disable:
+	disable_migrate = true;
+}
+
 /*
  * kthread_fn - The CPU time sampling/hardware latency detection kernel thread
  *
@@ -230,6 +282,8 @@ static int kthread_fn(void *data)
 
 	while (!kthread_should_stop()) {
 
+		move_to_next_cpu();
+
 		local_irq_disable();
 		get_sample();
 		local_irq_enable();
@@ -473,6 +527,7 @@ static int hwlat_tracer_init(struct trace_array *tr)
 
 	hwlat_trace = tr;
 
+	disable_migrate = false;
 	hwlat_data.count = 0;
 	tr->max_latency = 0;
 	save_tracing_thresh = tracing_thresh;
-- 
2.8.1

[PATCH v2 4/4] tracing: Add NMI tracing in hwlat detector

[Steven Rostedt]

[-- Attachment #1: 0004-tracing-Add-NMI-tracing-in-hwlat-detector.patch --]
[-- Type: text/plain, Size: 8452 bytes --]

From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

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:

 #3     inner/outer(us):    9/9     ts:1470836488.206734548
 #4     inner/outer(us):    0/8     ts:1470836497.140808588
 #5     inner/outer(us):    0/6     ts:1470836499.140825168 nmi-total:5 nmi-count:1
 #6     inner/outer(us):    9/9     ts:1470836501.140841748

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@goodmis.org>
---
 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 65aab3914a56..b97286c48735 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 d67a562df259..3fc20422c166 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, "#%-5u inner/outer(us): %4llu/%-5llu ts:%ld.%09ld\n",
+	trace_seq_printf(s, "#%-5u inner/outer(us): %4llu/%-5llu 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);
 }
 
-- 
2.8.1

Re: [PATCH v2 0/4] tracing: Add Hardware Latency detector tracer

[Daniel Bristot de Oliveira]

On 08/10/2016 10:53 AM, Steven Rostedt wrote:
> This adds the PREEMPT_RT hwlat detector as a Linux tracer in mainline.
> In the PREEMPT_RT patch set, it is a separate entity that is controlled
> by the debugfs file system. I found that it is better suited as a
> latency tracer in the tracing directory, as it follows pretty much the
> same paradigm as the other latency tracers that already exist. All
> I had to add was a hwlat_detector directory that contained a window
> and width for the period and duration respectively of the test. But
> the samples would just write to the tracing ring buffer and the max
> latency would be stored in tracing_max_latency, and the threshold can
> be set by the existing tracing_threshold. The last patch also adds a
> new feature that would have the kthread migrate after each period to
> another CPU specified by tracing_cpumask.

Hi!

I tested this patchset in a system which I can cause SMIs. The results
are consistent with the latency I see when I run cyclictest in this box
and cause SMIs on it. The tracer will be more accurate, as expected. So:

Tested-by: Daniel Bristot de Oliveira <bristot@redhat.com>

Regarding SMI count, when I added SMI count support to cyclictest I
based hardware support check on turbostat's implementation, at

tools/power/x86/turbostat/turbostat.c:probe_nhm_msrs()

It is for user-space, but I think it is a good starting point... Just
thinking aloud.

-- Daniel

Re: [PATCH v2 0/4] tracing: Add Hardware Latency detector tracer

[Peter Zijlstra]

On Wed, Aug 10, 2016 at 09:53:56AM -0400, Steven Rostedt wrote:
> Changes since v1:
> 
>  . Added checks for CONFIG_GENERIC_SCHED_CLOCK, and if that is set,
>    only nmi_counts will be recorded when an NMI is triggered, but not
>    the time in NMI, because the generic sched_clock is not NMI safe.
>    (suggested by Sebastian Andrzej Siewior)

So any of the platforms using GENERIC_SCHED_CLOCK _have_ NMIs ?

In any case, for those you could probably use ktime_get_mono_fast_ns().

Re: [PATCH v2 0/4] tracing: Add Hardware Latency detector tracer

[Steven Rostedt]

On Fri, 19 Aug 2016 16:09:28 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Aug 10, 2016 at 09:53:56AM -0400, Steven Rostedt wrote:
> > Changes since v1:
> > 
> >  . Added checks for CONFIG_GENERIC_SCHED_CLOCK, and if that is set,
> >    only nmi_counts will be recorded when an NMI is triggered, but not
> >    the time in NMI, because the generic sched_clock is not NMI safe.
> >    (suggested by Sebastian Andrzej Siewior)  
> 
> So any of the platforms using GENERIC_SCHED_CLOCK _have_ NMIs ?

>From what I gather, the answer is no, so I don't think this is an issue.

> 
> In any case, for those you could probably use ktime_get_mono_fast_ns().

Is that safe to call from NMI? Looking into the code I see:

	now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);

where timekeeping_get_ns() has:

	timekeeping_get_delta()

which does:

	read_seqcount_begin()

which has (eventually):

repeat:
	ret = READ_ONCE(s->sequence);
	if (unlikely(ret & 1)) {
		cpu_relax();
		goto repeat;
	}
	return ret;

Which I think could cause a deadlock from an NMI.

-- Steve

Re: [PATCH v2 0/4] tracing: Add Hardware Latency detector tracer

[Peter Zijlstra]

On Fri, Aug 19, 2016 at 10:26:20AM -0400, Steven Rostedt wrote:
> > In any case, for those you could probably use ktime_get_mono_fast_ns().
> 
> Is that safe to call from NMI?

It should be, we were very careful to make it so. Also read the comment
with __ktime_get_fast_ns().

> Looking into the code I see:
> 
> 	now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);
> 
> where timekeeping_get_ns() has:
> 
> 	timekeeping_get_delta()
> 
> which does:
> 
> 	read_seqcount_begin()
> 
> which has (eventually):

That's a bug in CONFIG_DEBUG_TIMEKEEPING, if you look at the
!timekeeping_get_delta() it does no such thing.

John, looks like 4ca22c2648f9 ("timekeeping: Add warnings when overflows
or underflows are observed") buggered things.

Re: [PATCH v2 0/4] tracing: Add Hardware Latency detector tracer

[John Stultz]

On Fri, Aug 19, 2016 at 8:05 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Fri, Aug 19, 2016 at 10:26:20AM -0400, Steven Rostedt wrote:
>> > In any case, for those you could probably use ktime_get_mono_fast_ns().
>>
>> Is that safe to call from NMI?
>
> It should be, we were very careful to make it so. Also read the comment
> with __ktime_get_fast_ns().
>
>> Looking into the code I see:
>>
>>       now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);
>>
>> where timekeeping_get_ns() has:
>>
>>       timekeeping_get_delta()
>>
>> which does:
>>
>>       read_seqcount_begin()
>>
>> which has (eventually):
>
> That's a bug in CONFIG_DEBUG_TIMEKEEPING, if you look at the
> !timekeeping_get_delta() it does no such thing.
>
> John, looks like 4ca22c2648f9 ("timekeeping: Add warnings when overflows
> or underflows are observed") buggered things.

Oof. Apologies. I missed that the fast methods called that helper as well.


Any objection to open-coding it for the fast method, like (sorry for
the whitespace damage here):

@@ -401,7 +401,10 @@ static __always_inline u64
__ktime_get_fast_ns(struct tk_fast *tkf)
        do {
                seq = raw_read_seqcount_latch(&tkf->seq);
                tkr = tkf->base + (seq & 0x01);
-               now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);
+               now = ktime_to_ns(tkr->base);
+
+               now += clocksource_delta(tkr->read(tkr->clock),
+                                        tkr->cycle_last, tkr->mask);
        } while (read_seqcount_retry(&tkf->seq, seq));

        return now;

If not I'll get such a patch sorted, tested and tagged for -stable.

thanks
-john

Re: [PATCH v2 1/4] tracing: Added hardware latency tracer

[Nilay Vaish]

On 10 August 2016 at 08:53, Steven Rostedt <rostedt@goodmis.org> wrote:
> diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
> new file mode 100644
> index 000000000000..08dfabe4e862
> --- /dev/null
> +++ b/kernel/trace/trace_hwlat.c
> @@ -0,0 +1,527 @@
> +/*
> + * trace_hwlatdetect.c - A simple Hardware Latency detector.
> + *
> + * Use this tracer to detect large system latencies induced by the behavior of
> + * certain underlying system hardware or firmware, independent of Linux itself.
> + * The code was developed originally to detect the presence of SMIs on Intel
> + * and AMD systems, although there is no dependency upon x86 herein.
> + *
> + * The classical example usage of this tracer is in detecting the presence of
> + * SMIs or System Management Interrupts on Intel and AMD systems. An SMI is a
> + * somewhat special form of hardware interrupt spawned from earlier CPU debug
> + * modes in which the (BIOS/EFI/etc.) firmware arranges for the South Bridge
> + * LPC (or other device) to generate a special interrupt under certain
> + * circumstances, for example, upon expiration of a special SMI timer device,
> + * due to certain external thermal readings, on certain I/O address accesses,
> + * and other situations. An SMI hits a special CPU pin, triggers a special
> + * SMI mode (complete with special memory map), and the OS is unaware.
> + *
> + * Although certain hardware-inducing latencies are necessary (for example,
> + * a modern system often requires an SMI handler for correct thermal control
> + * and remote management) they can wreak havoc upon any OS-level performance
> + * guarantees toward low-latency, especially when the OS is not even made
> + * aware of the presence of these interrupts. For this reason, we need a
> + * somewhat brute force mechanism to detect these interrupts. In this case,
> + * we do it by hogging all of the CPU(s) for configurable timer intervals,
> + * sampling the built-in CPU timer, looking for discontiguous readings.
> + *
> + * WARNING: This implementation necessarily introduces latencies. Therefore,
> + *          you should NEVER use this tracer while running in a production
> + *          environment requiring any kind of low-latency performance
> + *          guarantee(s).
> + *
> + * Copyright (C) 2008-2009 Jon Masters, Red Hat, Inc. <jcm@redhat.com>
> + * Copyright (C) 2013-2016 Steven Rostedt, Red Hat, Inc. <srostedt@redhat.com>
> + *
> + * Includes useful feedback from Clark Williams <clark@redhat.com>
> + *
> + * This file is licensed under the terms of the GNU General Public
> + * License version 2. This program is licensed "as is" without any
> + * warranty of any kind, whether express or implied.
> + */
> +#include <linux/kthread.h>
> +#include <linux/tracefs.h>
> +#include <linux/uaccess.h>
> +#include <linux/delay.h>
> +#include "trace.h"
> +
> +static struct trace_array      *hwlat_trace;
> +
> +#define U64STR_SIZE            22                      /* 20 digits max */
> +
> +#define BANNER                 "hwlat_detector: "
> +#define DEFAULT_SAMPLE_WINDOW  1000000                 /* 1s */
> +#define DEFAULT_SAMPLE_WIDTH   500000                  /* 0.5s */
> +#define DEFAULT_LAT_THRESHOLD  10                      /* 10us */
> +
> +/* sampling thread*/
> +static struct task_struct *hwlat_kthread;
> +
> +static struct dentry *hwlat_sample_width;      /* sample width us */
> +static struct dentry *hwlat_sample_window;     /* sample window us */
> +
> +/* Save the previous tracing_thresh value */
> +static unsigned long save_tracing_thresh;
> +
> +/* If the user changed threshold, remember it */
> +static u64 last_tracing_thresh = DEFAULT_LAT_THRESHOLD * NSEC_PER_USEC;
> +
> +/* Individual latency samples are stored here when detected. */
> +struct hwlat_sample {
> +       u64             seqnum;         /* unique sequence */
> +       u64             duration;       /* delta */
> +       u64             outer_duration; /* delta (outer loop) */
> +       struct timespec timestamp;      /* wall time */
> +};
> +
> +/* keep the global state somewhere. */
> +static struct hwlat_data {
> +
> +       struct mutex lock;              /* protect changes */
> +
> +       u64     count;                  /* total since reset */
> +
> +       u64     sample_window;          /* total sampling window (on+off) */
> +       u64     sample_width;           /* active sampling portion of window */
> +
> +} hwlat_data = {
> +       .sample_window          = DEFAULT_SAMPLE_WINDOW,
> +       .sample_width           = DEFAULT_SAMPLE_WIDTH,
> +};
> +
> +static void trace_hwlat_sample(struct hwlat_sample *sample)
> +{
> +       struct trace_array *tr = hwlat_trace;
> +       struct trace_event_call *call = &event_hwlat;

Steven, where is this variable event_hwlat declared?  To me it seems
that some macro is declaring it (most likely DEFINE_EVENT) but I was
not able to figure out the chain that ends up in the declaration.

--
Nilay

Re: [PATCH v2 1/4] tracing: Added hardware latency tracer

[Steven Rostedt]

On Mon, 22 Aug 2016 12:27:58 -0500
Nilay Vaish <nilayvaish@gmail.com> wrote:


> > +static void trace_hwlat_sample(struct hwlat_sample *sample)
> > +{
> > +       struct trace_array *tr = hwlat_trace;
> > +       struct trace_event_call *call = &event_hwlat;  
> 
> Steven, where is this variable event_hwlat declared?  To me it seems
> that some macro is declaring it (most likely DEFINE_EVENT) but I was
> not able to figure out the chain that ends up in the declaration.

Yeah, it's created similarly to how trace_events are created.

It's created via kernel/trace/trace_export.c using
kernel/trace/trace_entries.h

-- Steve

Re: [PATCH v2 0/4] tracing: Add Hardware Latency detector tracer

[Peter Zijlstra]

On Fri, Aug 19, 2016 at 09:36:41AM -0700, John Stultz wrote:
> On Fri, Aug 19, 2016 at 8:05 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > John, looks like 4ca22c2648f9 ("timekeeping: Add warnings when overflows
> > or underflows are observed") buggered things.
> 
> Oof. Apologies. I missed that the fast methods called that helper as well.
> 
> 
> Any objection to open-coding it for the fast method, like (sorry for
> the whitespace damage here):
> 
> @@ -401,7 +401,10 @@ static __always_inline u64
> __ktime_get_fast_ns(struct tk_fast *tkf)
>         do {
>                 seq = raw_read_seqcount_latch(&tkf->seq);
>                 tkr = tkf->base + (seq & 0x01);
> -               now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);
> +               now = ktime_to_ns(tkr->base);
> +
> +               now += clocksource_delta(tkr->read(tkr->clock),
> +                                        tkr->cycle_last, tkr->mask);
>         } while (read_seqcount_retry(&tkf->seq, seq));
> 
>         return now;
> 
> If not I'll get such a patch sorted, tested and tagged for -stable.

Works for me, Thomas?

Re: [PATCH v2 0/4] tracing: Add Hardware Latency detector tracer

[Thomas Gleixner]

On Tue, 23 Aug 2016, Peter Zijlstra wrote:
> On Fri, Aug 19, 2016 at 09:36:41AM -0700, John Stultz wrote:
> > On Fri, Aug 19, 2016 at 8:05 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > > John, looks like 4ca22c2648f9 ("timekeeping: Add warnings when overflows
> > > or underflows are observed") buggered things.
> > 
> > Oof. Apologies. I missed that the fast methods called that helper as well.
> > 
> > 
> > Any objection to open-coding it for the fast method, like (sorry for
> > the whitespace damage here):
> > 
> > @@ -401,7 +401,10 @@ static __always_inline u64
> > __ktime_get_fast_ns(struct tk_fast *tkf)
> >         do {
> >                 seq = raw_read_seqcount_latch(&tkf->seq);
> >                 tkr = tkf->base + (seq & 0x01);
> > -               now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);
> > +               now = ktime_to_ns(tkr->base);
> > +
> > +               now += clocksource_delta(tkr->read(tkr->clock),
> > +                                        tkr->cycle_last, tkr->mask);
> >         } while (read_seqcount_retry(&tkf->seq, seq));
> > 
> >         return now;
> > 
> > If not I'll get such a patch sorted, tested and tagged for -stable.
> 
> Works for me, Thomas?

No objections.