linux/kernel/trace/trace_hwlat.c
Steven Rostedt (VMware) b396bfdebf tracing: Have hwlat ts be first instance and record count of instances
The hwlat tracer runs a loop of width time during a given window. It then
reports the max latency over a given threshold and records a timestamp. But
this timestamp is the time after the width has finished, and not the time it
actually triggered.

Record the actual time when the latency was greater than the threshold as
well as the number of times it was greater in a given width per window.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
2020-03-03 17:33:43 -05:00

650 lines
17 KiB
C

// SPDX-License-Identifier: GPL-2.0
/*
* trace_hwlat.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>
*
*/
#include <linux/kthread.h>
#include <linux/tracefs.h>
#include <linux/uaccess.h>
#include <linux/cpumask.h>
#include <linux/delay.h>
#include <linux/sched/clock.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;
/* 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;
/* Individual latency samples are stored here when detected. */
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 timespec64 timestamp; /* wall time */
int nmi_count; /* # NMIs during this sample */
int count; /* # of iteratons over threash */
};
/* 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 trace_buffer *buffer = tr->array_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;
entry->nmi_total_ts = sample->nmi_total_ts;
entry->nmi_count = sample->nmi_count;
entry->count = sample->count;
if (!call_filter_check_discard(call, entry, buffer, event))
trace_buffer_unlock_commit_nostack(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
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
*
* 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;
struct hwlat_sample s;
time_type start, t1, t2, last_t2;
s64 diff, outer_diff, total, last_total = 0;
u64 sample = 0;
u64 thresh = tracing_thresh;
u64 outer_sample = 0;
int ret = -1;
unsigned int count = 0;
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 */
outer_diff = 0;
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) */
outer_diff = time_to_us(time_sub(t1, last_t2));
/* This shouldn't happen */
if (outer_diff < 0) {
pr_err(BANNER "time running backwards\n");
goto out;
}
if (outer_diff > outer_sample)
outer_sample = outer_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 */
if (diff > thresh || outer_diff > thresh) {
if (!count)
ktime_get_real_ts64(&s.timestamp);
count++;
}
/* 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);
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 */
if (sample > thresh || outer_sample > thresh) {
u64 latency;
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.nmi_total_ts = nmi_total_ts;
s.nmi_count = nmi_count;
s.count = count;
trace_hwlat_sample(&s);
latency = max(sample, outer_sample);
/* Keep a running maximum ever recorded hardware latency */
if (latency > tr->max_latency) {
tr->max_latency = latency;
latency_fsnotify(tr);
}
}
out:
return ret;
}
static struct cpumask save_cpumask;
static bool disable_migrate;
static void move_to_next_cpu(void)
{
struct cpumask *current_mask = &save_cpumask;
int next_cpu;
if (disable_migrate)
return;
/*
* If for some reason the user modifies the CPU affinity
* of this thread, then stop migrating for the duration
* of the current test.
*/
if (!cpumask_equal(current_mask, current->cpus_ptr))
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);
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
*
* 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.
*
* Executes one loop interaction on each CPU in tracing_cpumask sysfs file.
*/
static int kthread_fn(void *data)
{
u64 interval;
while (!kthread_should_stop()) {
move_to_next_cpu();
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 cpumask *current_mask = &save_cpumask;
struct task_struct *kthread;
int next_cpu;
if (WARN_ON(hwlat_kthread))
return 0;
/* Just pick the first CPU on first iteration */
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);
kthread = kthread_create(kthread_fn, NULL, "hwlatd");
if (IS_ERR(kthread)) {
pr_err(BANNER "could not start sampling thread\n");
return -ENOMEM;
}
cpumask_clear(current_mask);
cpumask_set_cpu(next_cpu, current_mask);
sched_setaffinity(kthread->pid, current_mask);
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(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;
disable_migrate = false;
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);