Merge tag 'trace-v6.13-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace...

[drm/drm-misc.git] / kernel / trace / trace_hwlat.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 <williams@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 */

static struct dentry *hwlat_sample_width;       /* sample width us */
static struct dentry *hwlat_sample_window;      /* sample window us */
static struct dentry *hwlat_thread_mode;        /* hwlat thread mode */

enum {
        MODE_NONE = 0,
        MODE_ROUND_ROBIN,
        MODE_PER_CPU,
        MODE_MAX
};
static char *thread_mode_str[] = { "none", "round-robin", "per-cpu" };

/* Save the previous tracing_thresh value */
static unsigned long save_tracing_thresh;

/* runtime kthread data */
struct hwlat_kthread_data {
        struct task_struct      *kthread;
        /* NMI timestamp counters */
        u64                     nmi_ts_start;
        u64                     nmi_total_ts;
        int                     nmi_count;
        int                     nmi_cpu;
};

static struct hwlat_kthread_data hwlat_single_cpu_data;
static DEFINE_PER_CPU(struct hwlat_kthread_data, hwlat_per_cpu_data);

/* 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 iterations over thresh */
};

/* 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 */

        int     thread_mode;            /* thread mode */

} hwlat_data = {
        .sample_window          = DEFAULT_SAMPLE_WINDOW,
        .sample_width           = DEFAULT_SAMPLE_WIDTH,
        .thread_mode            = MODE_ROUND_ROBIN
};

static struct hwlat_kthread_data *get_cpu_data(void)
{
        if (hwlat_data.thread_mode == MODE_PER_CPU)
                return this_cpu_ptr(&hwlat_per_cpu_data);
        else
                return &hwlat_single_cpu_data;
}

static bool hwlat_busy;

static void trace_hwlat_sample(struct hwlat_sample *sample)
{
        struct trace_array *tr = hwlat_trace;
        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;
        entry->timestamp                = sample->timestamp;
        entry->nmi_total_ts             = sample->nmi_total_ts;
        entry->nmi_count                = sample->nmi_count;
        entry->count                    = sample->count;

        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)
{
        struct hwlat_kthread_data *kdata = get_cpu_data();

        if (!kdata->kthread)
                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)
                        kdata->nmi_ts_start = time_get();
                else
                        kdata->nmi_total_ts += time_get() - kdata->nmi_ts_start;
        }

        if (enter)
                kdata->nmi_count++;
}

/*
 * hwlat_err - report a hwlat error.
 */
#define hwlat_err(msg) ({                                                       \
        struct trace_array *tr = hwlat_trace;                                   \
                                                                                \
        trace_array_printk_buf(tr->array_buffer.buffer, _THIS_IP_, msg);        \
})

/**
 * 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 hwlat_kthread_data *kdata = get_cpu_data();
        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 */

        kdata->nmi_total_ts = 0;
        kdata->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) {
                                hwlat_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) {
                        hwlat_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) {
                        hwlat_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 (kdata->nmi_total_ts)
                        do_div(kdata->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 = kdata->nmi_total_ts;
                s.nmi_count = kdata->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 void move_to_next_cpu(void)
{
        struct cpumask *current_mask = &save_cpumask;
        struct trace_array *tr = hwlat_trace;
        int next_cpu;

        /*
         * 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 change_mode;

        cpus_read_lock();
        cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask);
        next_cpu = cpumask_next(raw_smp_processor_id(), current_mask);
        cpus_read_unlock();

        if (next_cpu >= nr_cpu_ids)
                next_cpu = cpumask_first(current_mask);

        if (next_cpu >= nr_cpu_ids) /* Shouldn't happen! */
                goto change_mode;

        cpumask_clear(current_mask);
        cpumask_set_cpu(next_cpu, current_mask);

        set_cpus_allowed_ptr(current, current_mask);
        return;

 change_mode:
        hwlat_data.thread_mode = MODE_NONE;
        pr_info(BANNER "cpumask changed while in round-robin mode, switching to mode none\n");
}

/*
 * 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()) {

                if (hwlat_data.thread_mode == MODE_ROUND_ROBIN)
                        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;
}

/*
 * stop_stop_kthread - Inform the hardware latency sampling/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_single_kthread(void)
{
        struct hwlat_kthread_data *kdata = get_cpu_data();
        struct task_struct *kthread;

        cpus_read_lock();
        kthread = kdata->kthread;

        if (!kthread)
                goto out_put_cpus;

        kthread_stop(kthread);
        kdata->kthread = NULL;

out_put_cpus:
        cpus_read_unlock();
}


/*
 * start_single_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_single_kthread(struct trace_array *tr)
{
        struct hwlat_kthread_data *kdata = get_cpu_data();
        struct cpumask *current_mask = &save_cpumask;
        struct task_struct *kthread;
        int next_cpu;

        cpus_read_lock();
        if (kdata->kthread)
                goto out_put_cpus;

        kthread = kthread_create(kthread_fn, NULL, "hwlatd");
        if (IS_ERR(kthread)) {
                pr_err(BANNER "could not start sampling thread\n");
                cpus_read_unlock();
                return -ENOMEM;
        }

        /* Just pick the first CPU on first iteration */
        cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask);

        if (hwlat_data.thread_mode == MODE_ROUND_ROBIN) {
                next_cpu = cpumask_first(current_mask);
                cpumask_clear(current_mask);
                cpumask_set_cpu(next_cpu, current_mask);

        }

        set_cpus_allowed_ptr(kthread, current_mask);

        kdata->kthread = kthread;
        wake_up_process(kthread);

out_put_cpus:
        cpus_read_unlock();
        return 0;
}

/*
 * stop_cpu_kthread - Stop a hwlat cpu kthread
 */
static void stop_cpu_kthread(unsigned int cpu)
{
        struct task_struct *kthread;

        kthread = per_cpu(hwlat_per_cpu_data, cpu).kthread;
        if (kthread)
                kthread_stop(kthread);
        per_cpu(hwlat_per_cpu_data, cpu).kthread = NULL;
}

/*
 * stop_per_cpu_kthreads - Inform the hardware latency sampling/detector kthread to stop
 *
 * This kicks the running hardware latency sampling/detector kernel threads and
 * tells it to stop sampling now. Use this on unload and at system shutdown.
 */
static void stop_per_cpu_kthreads(void)
{
        unsigned int cpu;

        cpus_read_lock();
        for_each_online_cpu(cpu)
                stop_cpu_kthread(cpu);
        cpus_read_unlock();
}

/*
 * start_cpu_kthread - Start a hwlat cpu kthread
 */
static int start_cpu_kthread(unsigned int cpu)
{
        struct task_struct *kthread;

        /* Do not start a new hwlatd thread if it is already running */
        if (per_cpu(hwlat_per_cpu_data, cpu).kthread)
                return 0;

        kthread = kthread_run_on_cpu(kthread_fn, NULL, cpu, "hwlatd/%u");
        if (IS_ERR(kthread)) {
                pr_err(BANNER "could not start sampling thread\n");
                return -ENOMEM;
        }

        per_cpu(hwlat_per_cpu_data, cpu).kthread = kthread;

        return 0;
}

#ifdef CONFIG_HOTPLUG_CPU
static void hwlat_hotplug_workfn(struct work_struct *dummy)
{
        struct trace_array *tr = hwlat_trace;
        unsigned int cpu = smp_processor_id();

        mutex_lock(&trace_types_lock);
        mutex_lock(&hwlat_data.lock);
        cpus_read_lock();

        if (!hwlat_busy || hwlat_data.thread_mode != MODE_PER_CPU)
                goto out_unlock;

        if (!cpu_online(cpu))
                goto out_unlock;
        if (!cpumask_test_cpu(cpu, tr->tracing_cpumask))
                goto out_unlock;

        start_cpu_kthread(cpu);

out_unlock:
        cpus_read_unlock();
        mutex_unlock(&hwlat_data.lock);
        mutex_unlock(&trace_types_lock);
}

static DECLARE_WORK(hwlat_hotplug_work, hwlat_hotplug_workfn);

/*
 * hwlat_cpu_init - CPU hotplug online callback function
 */
static int hwlat_cpu_init(unsigned int cpu)
{
        schedule_work_on(cpu, &hwlat_hotplug_work);
        return 0;
}

/*
 * hwlat_cpu_die - CPU hotplug offline callback function
 */
static int hwlat_cpu_die(unsigned int cpu)
{
        stop_cpu_kthread(cpu);
        return 0;
}

static void hwlat_init_hotplug_support(void)
{
        int ret;

        ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "trace/hwlat:online",
                                hwlat_cpu_init, hwlat_cpu_die);
        if (ret < 0)
                pr_warn(BANNER "Error to init cpu hotplug support\n");

        return;
}
#else /* CONFIG_HOTPLUG_CPU */
static void hwlat_init_hotplug_support(void)
{
        return;
}
#endif /* CONFIG_HOTPLUG_CPU */

/*
 * start_per_cpu_kthreads - Kick off the hardware latency sampling/detector kthreads
 *
 * This starts the kernel threads that will sit on potentially all cpus and
 * sample the CPU timestamp counter (TSC or similar) and look for potential
 * hardware latencies.
 */
static int start_per_cpu_kthreads(struct trace_array *tr)
{
        struct cpumask *current_mask = &save_cpumask;
        unsigned int cpu;
        int retval;

        cpus_read_lock();
        /*
         * Run only on CPUs in which hwlat is allowed to run.
         */
        cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask);

        for_each_cpu(cpu, current_mask) {
                retval = start_cpu_kthread(cpu);
                if (retval)
                        goto out_error;
        }
        cpus_read_unlock();

        return 0;

out_error:
        cpus_read_unlock();
        stop_per_cpu_kthreads();
        return retval;
}

static void *s_mode_start(struct seq_file *s, loff_t *pos)
{
        int mode = *pos;

        mutex_lock(&hwlat_data.lock);

        if (mode >= MODE_MAX)
                return NULL;

        return pos;
}

static void *s_mode_next(struct seq_file *s, void *v, loff_t *pos)
{
        int mode = ++(*pos);

        if (mode >= MODE_MAX)
                return NULL;

        return pos;
}

static int s_mode_show(struct seq_file *s, void *v)
{
        loff_t *pos = v;
        int mode = *pos;

        if (mode == hwlat_data.thread_mode)
                seq_printf(s, "[%s]", thread_mode_str[mode]);
        else
                seq_printf(s, "%s", thread_mode_str[mode]);

        if (mode < MODE_MAX - 1) /* if mode is any but last */
                seq_puts(s, " ");

        return 0;
}

static void s_mode_stop(struct seq_file *s, void *v)
{
        seq_puts(s, "\n");
        mutex_unlock(&hwlat_data.lock);
}

static const struct seq_operations thread_mode_seq_ops = {
        .start          = s_mode_start,
        .next           = s_mode_next,
        .show           = s_mode_show,
        .stop           = s_mode_stop
};

static int hwlat_mode_open(struct inode *inode, struct file *file)
{
        return seq_open(file, &thread_mode_seq_ops);
};

static void hwlat_tracer_start(struct trace_array *tr);
static void hwlat_tracer_stop(struct trace_array *tr);

/**
 * hwlat_mode_write - Write function for "mode" 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 "mode" interface
 * to the hardware latency detector. hwlatd has different operation modes.
 * The "none" sets the allowed cpumask for a single hwlatd thread at the
 * startup and lets the scheduler handle the migration. The default mode is
 * the "round-robin" one, in which a single hwlatd thread runs, migrating
 * among the allowed CPUs in a round-robin fashion. The "per-cpu" mode
 * creates one hwlatd thread per allowed CPU.
 */
static ssize_t hwlat_mode_write(struct file *filp, const char __user *ubuf,
                                 size_t cnt, loff_t *ppos)
{
        struct trace_array *tr = hwlat_trace;
        const char *mode;
        char buf[64];
        int ret, i;

        if (cnt >= sizeof(buf))
                return -EINVAL;

        if (copy_from_user(buf, ubuf, cnt))
                return -EFAULT;

        buf[cnt] = 0;

        mode = strstrip(buf);

        ret = -EINVAL;

        /*
         * trace_types_lock is taken to avoid concurrency on start/stop
         * and hwlat_busy.
         */
        mutex_lock(&trace_types_lock);
        if (hwlat_busy)
                hwlat_tracer_stop(tr);

        mutex_lock(&hwlat_data.lock);

        for (i = 0; i < MODE_MAX; i++) {
                if (strcmp(mode, thread_mode_str[i]) == 0) {
                        hwlat_data.thread_mode = i;
                        ret = cnt;
                }
        }

        mutex_unlock(&hwlat_data.lock);

        if (hwlat_busy)
                hwlat_tracer_start(tr);
        mutex_unlock(&trace_types_lock);

        *ppos += cnt;



        return ret;
}

/*
 * The width parameter is read/write using the generic trace_min_max_param
 * method. The *val is protected by the hwlat_data lock and is upper
 * bounded by the window parameter.
 */
static struct trace_min_max_param hwlat_width = {
        .lock           = &hwlat_data.lock,
        .val            = &hwlat_data.sample_width,
        .max            = &hwlat_data.sample_window,
        .min            = NULL,
};

/*
 * The window parameter is read/write using the generic trace_min_max_param
 * method. The *val is protected by the hwlat_data lock and is lower
 * bounded by the width parameter.
 */
static struct trace_min_max_param hwlat_window = {
        .lock           = &hwlat_data.lock,
        .val            = &hwlat_data.sample_window,
        .max            = NULL,
        .min            = &hwlat_data.sample_width,
};

static const struct file_operations thread_mode_fops = {
        .open           = hwlat_mode_open,
        .read           = seq_read,
        .llseek         = seq_lseek,
        .release        = seq_release,
        .write          = hwlat_mode_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", TRACE_MODE_WRITE,
                                                  top_dir,
                                                  &hwlat_window,
                                                  &trace_min_max_fops);
        if (!hwlat_sample_window)
                goto err;

        hwlat_sample_width = tracefs_create_file("width", TRACE_MODE_WRITE,
                                                 top_dir,
                                                 &hwlat_width,
                                                 &trace_min_max_fops);
        if (!hwlat_sample_width)
                goto err;

        hwlat_thread_mode = trace_create_file("mode", TRACE_MODE_WRITE,
                                              top_dir,
                                              NULL,
                                              &thread_mode_fops);
        if (!hwlat_thread_mode)
                goto err;

        return 0;

 err:
        tracefs_remove(top_dir);
        return -ENOMEM;
}

static void hwlat_tracer_start(struct trace_array *tr)
{
        int err;

        if (hwlat_data.thread_mode == MODE_PER_CPU)
                err = start_per_cpu_kthreads(tr);
        else
                err = start_single_kthread(tr);
        if (err)
                pr_err(BANNER "Cannot start hwlat kthread\n");
}

static void hwlat_tracer_stop(struct trace_array *tr)
{
        if (hwlat_data.thread_mode == MODE_PER_CPU)
                stop_per_cpu_kthreads();
        else
                stop_single_kthread();
}

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)
{
        hwlat_tracer_stop(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;

        mutex_init(&hwlat_data.lock);

        ret = register_tracer(&hwlat_tracer);
        if (ret)
                return ret;

        hwlat_init_hotplug_support();

        init_tracefs();

        return 0;
}
late_initcall(init_hwlat_tracer);