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trace_hwlat.c
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633 lines (518 loc) · 16.7 KB
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/*
* 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. <[email protected]>
* Copyright (C) 2013-2016 Steven Rostedt, Red Hat, Inc. <[email protected]>
*
* Includes useful feedback from Clark Williams <[email protected]>
*
* 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/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 */
};
/* 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;
entry->nmi_total_ts = sample->nmi_total_ts;
entry->nmi_count = sample->nmi_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;
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 */
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 */
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);
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) {
struct hwlat_sample s;
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;
ktime_get_real_ts64(&s.timestamp);
s.nmi_total_ts = nmi_total_ts;
s.nmi_count = nmi_count;
trace_hwlat_sample(&s);
/* Keep a running maximum ever recorded hardware latency */
if (sample > tr->max_latency)
tr->max_latency = sample;
}
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, than stop migrating for the duration
* of the current test.
*/
if (!cpumask_equal(current_mask, ¤t->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);
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;
/* 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_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;
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);