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 // 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_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;
     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)
 {
     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);
 
     sched_setaffinity(0, 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);
 
     }
 
     sched_setaffinity(kthread->pid, 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;
 
     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 (!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_online_cpu(cpu)
         per_cpu(hwlat_per_cpu_data, cpu).kthread = NULL;
 
     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)
         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);

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