Commit e7c15cd8 authored by Steven Rostedt (Red Hat)'s avatar Steven Rostedt (Red Hat) Committed by Steven Rostedt

tracing: Added hardware latency tracer

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


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
Based-on-code-by: default avatarJon Masters <>
Signed-off-by: default avatarSteven Rostedt <>
parent 8861dd30
......@@ -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.
bool "Tracer to detect hardware latencies (like SMIs)"
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
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.
bool "Trace process context switches and events"
depends on !GENERIC_TRACER
......@@ -41,6 +41,7 @@ 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
......@@ -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);
......@@ -38,6 +38,7 @@ enum trace_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, \
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,
......@@ -322,3 +322,26 @@ FTRACE_ENTRY(branch, trace_branch,
FTRACE_ENTRY(hwlat, hwlat_entry,
__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 )
* 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. <>
* Copyright (C) 2013-2016 Steven Rostedt, Red Hat, Inc. <>
* Includes useful feedback from Clark Williams <>
* 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();
event = trace_buffer_lock_reserve(buffer, TRACE_HWLAT, sizeof(*entry),
flags, pc);
if (!event)
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");
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;
s.seqnum = hwlat_data.count;
s.duration = sample;
s.outer_duration = outer_sample;
s.timestamp = CURRENT_TIME;
/* Keep a running maximum ever recorded hardware latency */
if (sample > tr->max_latency)
tr->max_latency = sample;
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()) {
interval = hwlat_data.sample_window - hwlat_data.sample_width;
do_div(interval, USEC_PER_MSEC); /* modifies interval value */
/* Always sleep for at least 1ms */
if (interval < 1)
interval = 1;
if (msleep_interruptible(interval))
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;
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)
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;
if (val < hwlat_data.sample_window)
hwlat_data.sample_width = val;
err = -EINVAL;
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;
if (hwlat_data.sample_width < val)
hwlat_data.sample_window = val;
err = -EINVAL;
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,
if (!hwlat_sample_window)
goto err;
hwlat_sample_width = tracefs_create_file("width", 0644,
if (!hwlat_sample_width)
goto err;
return 0;
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)
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_busy = true;
return 0;
static void hwlat_tracer_reset(struct trace_array *tr)
/* 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;
ret = register_tracer(&hwlat_tracer);
if (ret)
return ret;