// 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.
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* The code was developed originally to detect the presence of SMIs on Intel
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* and AMD systems, although there is no dependency upon x86 herein.
|
*
|
* The classical example usage of this tracer is in detecting the presence of
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* SMIs or System Management Interrupts on Intel and AMD systems. An SMI is a
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* somewhat special form of hardware interrupt spawned from earlier CPU debug
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* modes in which the (BIOS/EFI/etc.) firmware arranges for the South Bridge
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* LPC (or other device) to generate a special interrupt under certain
|
* circumstances, for example, upon expiration of a special SMI timer device,
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* due to certain external thermal readings, on certain I/O address accesses,
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* 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.
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*
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* 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
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* somewhat brute force mechanism to detect these interrupts. In this case,
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* we do it by hogging all of the CPU(s) for configurable timer intervals,
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* sampling the built-in CPU timer, looking for discontiguous readings.
|
*
|
* WARNING: This implementation necessarily introduces latencies. Therefore,
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* 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>
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*
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* Includes useful feedback from Clark Williams <clark@redhat.com>
|
*
|
*/
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#include <linux/kthread.h>
|
#include <linux/tracefs.h>
|
#include <linux/uaccess.h>
|
#include <linux/cpumask.h>
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#include <linux/delay.h>
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#include <linux/sched/clock.h>
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#include "trace.h"
|
|
static struct trace_array *hwlat_trace;
|
|
#define U64STR_SIZE 22 /* 20 digits max */
|
|
#define BANNER "hwlat_detector: "
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#define DEFAULT_SAMPLE_WINDOW 1000000 /* 1s */
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#define DEFAULT_SAMPLE_WIDTH 500000 /* 0.5s */
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#define DEFAULT_LAT_THRESHOLD 10 /* 10us */
|
|
/* sampling thread*/
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static struct task_struct *hwlat_kthread;
|
|
static struct dentry *hwlat_sample_width; /* sample width us */
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static struct dentry *hwlat_sample_window; /* sample window us */
|
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/* Save the previous tracing_thresh value */
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static unsigned long save_tracing_thresh;
|
|
/* NMI timestamp counters */
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static u64 nmi_ts_start;
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static u64 nmi_total_ts;
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static int nmi_count;
|
static int nmi_cpu;
|
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/* Tells NMIs to call back to the hwlat tracer to record timestamps */
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bool trace_hwlat_callback_enabled;
|
|
/* If the user changed threshold, remember it */
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static u64 last_tracing_thresh = DEFAULT_LAT_THRESHOLD * NSEC_PER_USEC;
|
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/* Individual latency samples are stored here when detected. */
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struct hwlat_sample {
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u64 seqnum; /* unique sequence */
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u64 duration; /* delta */
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u64 outer_duration; /* delta (outer loop) */
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u64 nmi_total_ts; /* Total time spent in NMIs */
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struct timespec64 timestamp; /* wall time */
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int nmi_count; /* # NMIs during this sample */
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int count; /* # of iteratons over threash */
|
};
|
|
/* keep the global state somewhere. */
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static struct hwlat_data {
|
|
struct mutex lock; /* protect changes */
|
|
u64 count; /* total since reset */
|
|
u64 sample_window; /* total sampling window (on+off) */
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u64 sample_width; /* active sampling portion of window */
|
|
} hwlat_data = {
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.sample_window = DEFAULT_SAMPLE_WINDOW,
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.sample_width = DEFAULT_SAMPLE_WIDTH,
|
};
|
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static void trace_hwlat_sample(struct hwlat_sample *sample)
|
{
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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;
|
|
event = trace_buffer_lock_reserve(buffer, TRACE_HWLAT, sizeof(*entry),
|
tracing_gen_ctx());
|
if (!event)
|
return;
|
entry = ring_buffer_event_data(event);
|
entry->seqnum = sample->seqnum;
|
entry->duration = sample->duration;
|
entry->outer_duration = sample->outer_duration;
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entry->timestamp = sample->timestamp;
|
entry->nmi_total_ts = sample->nmi_total_ts;
|
entry->nmi_count = sample->nmi_count;
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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)
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#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.
|
*/
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static int get_sample(void)
|
{
|
struct trace_array *tr = hwlat_trace;
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struct hwlat_sample s;
|
time_type start, t1, t2, last_t2;
|
s64 diff, outer_diff, total, last_total = 0;
|
u64 sample = 0;
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u64 thresh = tracing_thresh;
|
u64 outer_sample = 0;
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int ret = -1;
|
unsigned int count = 0;
|
|
do_div(thresh, NSEC_PER_USEC); /* modifies interval value */
|
|
nmi_cpu = smp_processor_id();
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nmi_total_ts = 0;
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nmi_count = 0;
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/* Make sure NMIs see this first */
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barrier();
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trace_hwlat_callback_enabled = true;
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init_time(last_t2, 0);
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start = time_get(); /* start timestamp */
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outer_diff = 0;
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|
do {
|
|
t1 = time_get(); /* we'll look for a discontinuity */
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t2 = time_get();
|
|
if (time_u64(last_t2)) {
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/* Check the delta from outer loop (t2 to next t1) */
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outer_diff = time_to_us(time_sub(t1, last_t2));
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/* 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) {
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if (!count)
|
ktime_get_real_ts64(&s.timestamp);
|
count++;
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}
|
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/* This shouldn't happen */
|
if (diff < 0) {
|
pr_err(BANNER "time running backwards\n");
|
goto out;
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}
|
|
if (diff > sample)
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sample = diff; /* only want highest value */
|
|
} while (total <= hwlat_data.sample_width);
|
|
barrier(); /* finish the above in the view for NMIs */
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trace_hwlat_callback_enabled = false;
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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;
|
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/* We read in microseconds */
|
if (nmi_total_ts)
|
do_div(nmi_total_ts, NSEC_PER_USEC);
|
|
hwlat_data.count++;
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s.seqnum = hwlat_data.count;
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s.duration = sample;
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s.outer_duration = outer_sample;
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s.nmi_total_ts = nmi_total_ts;
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s.nmi_count = nmi_count;
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s.count = count;
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trace_hwlat_sample(&s);
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|
latency = max(sample, outer_sample);
|
|
/* Keep a running maximum ever recorded hardware latency */
|
if (latency > tr->max_latency) {
|
tr->max_latency = latency;
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latency_fsnotify(tr);
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}
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}
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|
out:
|
return ret;
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}
|
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static struct cpumask save_cpumask;
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static bool disable_migrate;
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static void move_to_next_cpu(void)
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{
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struct cpumask *current_mask = &save_cpumask;
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struct trace_array *tr = hwlat_trace;
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int next_cpu;
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|
if (disable_migrate)
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return;
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/*
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* 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, tr->tracing_cpumask);
|
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 (hwlat_kthread)
|
return 0;
|
|
/* Just pick the first CPU on first iteration */
|
get_online_cpus();
|
cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask);
|
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)
|
{
|
int ret;
|
struct dentry *top_dir;
|
|
ret = tracing_init_dentry();
|
if (ret)
|
return -ENOMEM;
|
|
top_dir = tracefs_create_dir("hwlat_detector", NULL);
|
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);
|
