linux/kernel/trace/trace_hwlat.c
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   1// SPDX-License-Identifier: GPL-2.0
   2/*
   3 * trace_hwlatdetect.c - A simple Hardware Latency detector.
   4 *
   5 * Use this tracer to detect large system latencies induced by the behavior of
   6 * certain underlying system hardware or firmware, independent of Linux itself.
   7 * The code was developed originally to detect the presence of SMIs on Intel
   8 * and AMD systems, although there is no dependency upon x86 herein.
   9 *
  10 * The classical example usage of this tracer is in detecting the presence of
  11 * SMIs or System Management Interrupts on Intel and AMD systems. An SMI is a
  12 * somewhat special form of hardware interrupt spawned from earlier CPU debug
  13 * modes in which the (BIOS/EFI/etc.) firmware arranges for the South Bridge
  14 * LPC (or other device) to generate a special interrupt under certain
  15 * circumstances, for example, upon expiration of a special SMI timer device,
  16 * due to certain external thermal readings, on certain I/O address accesses,
  17 * and other situations. An SMI hits a special CPU pin, triggers a special
  18 * SMI mode (complete with special memory map), and the OS is unaware.
  19 *
  20 * Although certain hardware-inducing latencies are necessary (for example,
  21 * a modern system often requires an SMI handler for correct thermal control
  22 * and remote management) they can wreak havoc upon any OS-level performance
  23 * guarantees toward low-latency, especially when the OS is not even made
  24 * aware of the presence of these interrupts. For this reason, we need a
  25 * somewhat brute force mechanism to detect these interrupts. In this case,
  26 * we do it by hogging all of the CPU(s) for configurable timer intervals,
  27 * sampling the built-in CPU timer, looking for discontiguous readings.
  28 *
  29 * WARNING: This implementation necessarily introduces latencies. Therefore,
  30 *          you should NEVER use this tracer while running in a production
  31 *          environment requiring any kind of low-latency performance
  32 *          guarantee(s).
  33 *
  34 * Copyright (C) 2008-2009 Jon Masters, Red Hat, Inc. <jcm@redhat.com>
  35 * Copyright (C) 2013-2016 Steven Rostedt, Red Hat, Inc. <srostedt@redhat.com>
  36 *
  37 * Includes useful feedback from Clark Williams <clark@redhat.com>
  38 *
  39 */
  40#include <linux/kthread.h>
  41#include <linux/tracefs.h>
  42#include <linux/uaccess.h>
  43#include <linux/cpumask.h>
  44#include <linux/delay.h>
  45#include <linux/sched/clock.h>
  46#include "trace.h"
  47
  48static struct trace_array       *hwlat_trace;
  49
  50#define U64STR_SIZE             22                      /* 20 digits max */
  51
  52#define BANNER                  "hwlat_detector: "
  53#define DEFAULT_SAMPLE_WINDOW   1000000                 /* 1s */
  54#define DEFAULT_SAMPLE_WIDTH    500000                  /* 0.5s */
  55#define DEFAULT_LAT_THRESHOLD   10                      /* 10us */
  56
  57/* sampling thread*/
  58static struct task_struct *hwlat_kthread;
  59
  60static struct dentry *hwlat_sample_width;       /* sample width us */
  61static struct dentry *hwlat_sample_window;      /* sample window us */
  62
  63/* Save the previous tracing_thresh value */
  64static unsigned long save_tracing_thresh;
  65
  66/* NMI timestamp counters */
  67static u64 nmi_ts_start;
  68static u64 nmi_total_ts;
  69static int nmi_count;
  70static int nmi_cpu;
  71
  72/* Tells NMIs to call back to the hwlat tracer to record timestamps */
  73bool trace_hwlat_callback_enabled;
  74
  75/* If the user changed threshold, remember it */
  76static u64 last_tracing_thresh = DEFAULT_LAT_THRESHOLD * NSEC_PER_USEC;
  77
  78/* Individual latency samples are stored here when detected. */
  79struct hwlat_sample {
  80        u64                     seqnum;         /* unique sequence */
  81        u64                     duration;       /* delta */
  82        u64                     outer_duration; /* delta (outer loop) */
  83        u64                     nmi_total_ts;   /* Total time spent in NMIs */
  84        struct timespec64       timestamp;      /* wall time */
  85        int                     nmi_count;      /* # NMIs during this sample */
  86};
  87
  88/* keep the global state somewhere. */
  89static struct hwlat_data {
  90
  91        struct mutex lock;              /* protect changes */
  92
  93        u64     count;                  /* total since reset */
  94
  95        u64     sample_window;          /* total sampling window (on+off) */
  96        u64     sample_width;           /* active sampling portion of window */
  97
  98} hwlat_data = {
  99        .sample_window          = DEFAULT_SAMPLE_WINDOW,
 100        .sample_width           = DEFAULT_SAMPLE_WIDTH,
 101};
 102
 103static void trace_hwlat_sample(struct hwlat_sample *sample)
 104{
 105        struct trace_array *tr = hwlat_trace;
 106        struct trace_event_call *call = &event_hwlat;
 107        struct ring_buffer *buffer = tr->trace_buffer.buffer;
 108        struct ring_buffer_event *event;
 109        struct hwlat_entry *entry;
 110        unsigned long flags;
 111        int pc;
 112
 113        pc = preempt_count();
 114        local_save_flags(flags);
 115
 116        event = trace_buffer_lock_reserve(buffer, TRACE_HWLAT, sizeof(*entry),
 117                                          flags, pc);
 118        if (!event)
 119                return;
 120        entry   = ring_buffer_event_data(event);
 121        entry->seqnum                   = sample->seqnum;
 122        entry->duration                 = sample->duration;
 123        entry->outer_duration           = sample->outer_duration;
 124        entry->timestamp                = sample->timestamp;
 125        entry->nmi_total_ts             = sample->nmi_total_ts;
 126        entry->nmi_count                = sample->nmi_count;
 127
 128        if (!call_filter_check_discard(call, entry, buffer, event))
 129                trace_buffer_unlock_commit_nostack(buffer, event);
 130}
 131
 132/* Macros to encapsulate the time capturing infrastructure */
 133#define time_type       u64
 134#define time_get()      trace_clock_local()
 135#define time_to_us(x)   div_u64(x, 1000)
 136#define time_sub(a, b)  ((a) - (b))
 137#define init_time(a, b) (a = b)
 138#define time_u64(a)     a
 139
 140void trace_hwlat_callback(bool enter)
 141{
 142        if (smp_processor_id() != nmi_cpu)
 143                return;
 144
 145        /*
 146         * Currently trace_clock_local() calls sched_clock() and the
 147         * generic version is not NMI safe.
 148         */
 149        if (!IS_ENABLED(CONFIG_GENERIC_SCHED_CLOCK)) {
 150                if (enter)
 151                        nmi_ts_start = time_get();
 152                else
 153                        nmi_total_ts += time_get() - nmi_ts_start;
 154        }
 155
 156        if (enter)
 157                nmi_count++;
 158}
 159
 160/**
 161 * get_sample - sample the CPU TSC and look for likely hardware latencies
 162 *
 163 * Used to repeatedly capture the CPU TSC (or similar), looking for potential
 164 * hardware-induced latency. Called with interrupts disabled and with
 165 * hwlat_data.lock held.
 166 */
 167static int get_sample(void)
 168{
 169        struct trace_array *tr = hwlat_trace;
 170        time_type start, t1, t2, last_t2;
 171        s64 diff, total, last_total = 0;
 172        u64 sample = 0;
 173        u64 thresh = tracing_thresh;
 174        u64 outer_sample = 0;
 175        int ret = -1;
 176
 177        do_div(thresh, NSEC_PER_USEC); /* modifies interval value */
 178
 179        nmi_cpu = smp_processor_id();
 180        nmi_total_ts = 0;
 181        nmi_count = 0;
 182        /* Make sure NMIs see this first */
 183        barrier();
 184
 185        trace_hwlat_callback_enabled = true;
 186
 187        init_time(last_t2, 0);
 188        start = time_get(); /* start timestamp */
 189
 190        do {
 191
 192                t1 = time_get();        /* we'll look for a discontinuity */
 193                t2 = time_get();
 194
 195                if (time_u64(last_t2)) {
 196                        /* Check the delta from outer loop (t2 to next t1) */
 197                        diff = time_to_us(time_sub(t1, last_t2));
 198                        /* This shouldn't happen */
 199                        if (diff < 0) {
 200                                pr_err(BANNER "time running backwards\n");
 201                                goto out;
 202                        }
 203                        if (diff > outer_sample)
 204                                outer_sample = diff;
 205                }
 206                last_t2 = t2;
 207
 208                total = time_to_us(time_sub(t2, start)); /* sample width */
 209
 210                /* Check for possible overflows */
 211                if (total < last_total) {
 212                        pr_err("Time total overflowed\n");
 213                        break;
 214                }
 215                last_total = total;
 216
 217                /* This checks the inner loop (t1 to t2) */
 218                diff = time_to_us(time_sub(t2, t1));     /* current diff */
 219
 220                /* This shouldn't happen */
 221                if (diff < 0) {
 222                        pr_err(BANNER "time running backwards\n");
 223                        goto out;
 224                }
 225
 226                if (diff > sample)
 227                        sample = diff; /* only want highest value */
 228
 229        } while (total <= hwlat_data.sample_width);
 230
 231        barrier(); /* finish the above in the view for NMIs */
 232        trace_hwlat_callback_enabled = false;
 233        barrier(); /* Make sure nmi_total_ts is no longer updated */
 234
 235        ret = 0;
 236
 237        /* If we exceed the threshold value, we have found a hardware latency */
 238        if (sample > thresh || outer_sample > thresh) {
 239                struct hwlat_sample s;
 240
 241                ret = 1;
 242
 243                /* We read in microseconds */
 244                if (nmi_total_ts)
 245                        do_div(nmi_total_ts, NSEC_PER_USEC);
 246
 247                hwlat_data.count++;
 248                s.seqnum = hwlat_data.count;
 249                s.duration = sample;
 250                s.outer_duration = outer_sample;
 251                ktime_get_real_ts64(&s.timestamp);
 252                s.nmi_total_ts = nmi_total_ts;
 253                s.nmi_count = nmi_count;
 254                trace_hwlat_sample(&s);
 255
 256                /* Keep a running maximum ever recorded hardware latency */
 257                if (sample > tr->max_latency)
 258                        tr->max_latency = sample;
 259                if (outer_sample > tr->max_latency)
 260                        tr->max_latency = outer_sample;
 261        }
 262
 263out:
 264        return ret;
 265}
 266
 267static struct cpumask save_cpumask;
 268static bool disable_migrate;
 269
 270static void move_to_next_cpu(void)
 271{
 272        struct cpumask *current_mask = &save_cpumask;
 273        int next_cpu;
 274
 275        if (disable_migrate)
 276                return;
 277        /*
 278         * If for some reason the user modifies the CPU affinity
 279         * of this thread, than stop migrating for the duration
 280         * of the current test.
 281         */
 282        if (!cpumask_equal(current_mask, current->cpus_ptr))
 283                goto disable;
 284
 285        get_online_cpus();
 286        cpumask_and(current_mask, cpu_online_mask, tracing_buffer_mask);
 287        next_cpu = cpumask_next(smp_processor_id(), current_mask);
 288        put_online_cpus();
 289
 290        if (next_cpu >= nr_cpu_ids)
 291                next_cpu = cpumask_first(current_mask);
 292
 293        if (next_cpu >= nr_cpu_ids) /* Shouldn't happen! */
 294                goto disable;
 295
 296        cpumask_clear(current_mask);
 297        cpumask_set_cpu(next_cpu, current_mask);
 298
 299        sched_setaffinity(0, current_mask);
 300        return;
 301
 302 disable:
 303        disable_migrate = true;
 304}
 305
 306/*
 307 * kthread_fn - The CPU time sampling/hardware latency detection kernel thread
 308 *
 309 * Used to periodically sample the CPU TSC via a call to get_sample. We
 310 * disable interrupts, which does (intentionally) introduce latency since we
 311 * need to ensure nothing else might be running (and thus preempting).
 312 * Obviously this should never be used in production environments.
 313 *
 314 * Executes one loop interaction on each CPU in tracing_cpumask sysfs file.
 315 */
 316static int kthread_fn(void *data)
 317{
 318        u64 interval;
 319
 320        while (!kthread_should_stop()) {
 321
 322                move_to_next_cpu();
 323
 324                local_irq_disable();
 325                get_sample();
 326                local_irq_enable();
 327
 328                mutex_lock(&hwlat_data.lock);
 329                interval = hwlat_data.sample_window - hwlat_data.sample_width;
 330                mutex_unlock(&hwlat_data.lock);
 331
 332                do_div(interval, USEC_PER_MSEC); /* modifies interval value */
 333
 334                /* Always sleep for at least 1ms */
 335                if (interval < 1)
 336                        interval = 1;
 337
 338                if (msleep_interruptible(interval))
 339                        break;
 340        }
 341
 342        return 0;
 343}
 344
 345/**
 346 * start_kthread - Kick off the hardware latency sampling/detector kthread
 347 *
 348 * This starts the kernel thread that will sit and sample the CPU timestamp
 349 * counter (TSC or similar) and look for potential hardware latencies.
 350 */
 351static int start_kthread(struct trace_array *tr)
 352{
 353        struct cpumask *current_mask = &save_cpumask;
 354        struct task_struct *kthread;
 355        int next_cpu;
 356
 357        if (WARN_ON(hwlat_kthread))
 358                return 0;
 359
 360        /* Just pick the first CPU on first iteration */
 361        current_mask = &save_cpumask;
 362        get_online_cpus();
 363        cpumask_and(current_mask, cpu_online_mask, tracing_buffer_mask);
 364        put_online_cpus();
 365        next_cpu = cpumask_first(current_mask);
 366
 367        kthread = kthread_create(kthread_fn, NULL, "hwlatd");
 368        if (IS_ERR(kthread)) {
 369                pr_err(BANNER "could not start sampling thread\n");
 370                return -ENOMEM;
 371        }
 372
 373        cpumask_clear(current_mask);
 374        cpumask_set_cpu(next_cpu, current_mask);
 375        sched_setaffinity(kthread->pid, current_mask);
 376
 377        hwlat_kthread = kthread;
 378        wake_up_process(kthread);
 379
 380        return 0;
 381}
 382
 383/**
 384 * stop_kthread - Inform the hardware latency samping/detector kthread to stop
 385 *
 386 * This kicks the running hardware latency sampling/detector kernel thread and
 387 * tells it to stop sampling now. Use this on unload and at system shutdown.
 388 */
 389static void stop_kthread(void)
 390{
 391        if (!hwlat_kthread)
 392                return;
 393        kthread_stop(hwlat_kthread);
 394        hwlat_kthread = NULL;
 395}
 396
 397/*
 398 * hwlat_read - Wrapper read function for reading both window and width
 399 * @filp: The active open file structure
 400 * @ubuf: The userspace provided buffer to read value into
 401 * @cnt: The maximum number of bytes to read
 402 * @ppos: The current "file" position
 403 *
 404 * This function provides a generic read implementation for the global state
 405 * "hwlat_data" structure filesystem entries.
 406 */
 407static ssize_t hwlat_read(struct file *filp, char __user *ubuf,
 408                          size_t cnt, loff_t *ppos)
 409{
 410        char buf[U64STR_SIZE];
 411        u64 *entry = filp->private_data;
 412        u64 val;
 413        int len;
 414
 415        if (!entry)
 416                return -EFAULT;
 417
 418        if (cnt > sizeof(buf))
 419                cnt = sizeof(buf);
 420
 421        val = *entry;
 422
 423        len = snprintf(buf, sizeof(buf), "%llu\n", val);
 424
 425        return simple_read_from_buffer(ubuf, cnt, ppos, buf, len);
 426}
 427
 428/**
 429 * hwlat_width_write - Write function for "width" entry
 430 * @filp: The active open file structure
 431 * @ubuf: The user buffer that contains the value to write
 432 * @cnt: The maximum number of bytes to write to "file"
 433 * @ppos: The current position in @file
 434 *
 435 * This function provides a write implementation for the "width" interface
 436 * to the hardware latency detector. It can be used to configure
 437 * for how many us of the total window us we will actively sample for any
 438 * hardware-induced latency periods. Obviously, it is not possible to
 439 * sample constantly and have the system respond to a sample reader, or,
 440 * worse, without having the system appear to have gone out to lunch. It
 441 * is enforced that width is less that the total window size.
 442 */
 443static ssize_t
 444hwlat_width_write(struct file *filp, const char __user *ubuf,
 445                  size_t cnt, loff_t *ppos)
 446{
 447        u64 val;
 448        int err;
 449
 450        err = kstrtoull_from_user(ubuf, cnt, 10, &val);
 451        if (err)
 452                return err;
 453
 454        mutex_lock(&hwlat_data.lock);
 455        if (val < hwlat_data.sample_window)
 456                hwlat_data.sample_width = val;
 457        else
 458                err = -EINVAL;
 459        mutex_unlock(&hwlat_data.lock);
 460
 461        if (err)
 462                return err;
 463
 464        return cnt;
 465}
 466
 467/**
 468 * hwlat_window_write - Write function for "window" entry
 469 * @filp: The active open file structure
 470 * @ubuf: The user buffer that contains the value to write
 471 * @cnt: The maximum number of bytes to write to "file"
 472 * @ppos: The current position in @file
 473 *
 474 * This function provides a write implementation for the "window" interface
 475 * to the hardware latency detetector. The window is the total time
 476 * in us that will be considered one sample period. Conceptually, windows
 477 * occur back-to-back and contain a sample width period during which
 478 * actual sampling occurs. Can be used to write a new total window size. It
 479 * is enfoced that any value written must be greater than the sample width
 480 * size, or an error results.
 481 */
 482static ssize_t
 483hwlat_window_write(struct file *filp, const char __user *ubuf,
 484                   size_t cnt, loff_t *ppos)
 485{
 486        u64 val;
 487        int err;
 488
 489        err = kstrtoull_from_user(ubuf, cnt, 10, &val);
 490        if (err)
 491                return err;
 492
 493        mutex_lock(&hwlat_data.lock);
 494        if (hwlat_data.sample_width < val)
 495                hwlat_data.sample_window = val;
 496        else
 497                err = -EINVAL;
 498        mutex_unlock(&hwlat_data.lock);
 499
 500        if (err)
 501                return err;
 502
 503        return cnt;
 504}
 505
 506static const struct file_operations width_fops = {
 507        .open           = tracing_open_generic,
 508        .read           = hwlat_read,
 509        .write          = hwlat_width_write,
 510};
 511
 512static const struct file_operations window_fops = {
 513        .open           = tracing_open_generic,
 514        .read           = hwlat_read,
 515        .write          = hwlat_window_write,
 516};
 517
 518/**
 519 * init_tracefs - A function to initialize the tracefs interface files
 520 *
 521 * This function creates entries in tracefs for "hwlat_detector".
 522 * It creates the hwlat_detector directory in the tracing directory,
 523 * and within that directory is the count, width and window files to
 524 * change and view those values.
 525 */
 526static int init_tracefs(void)
 527{
 528        struct dentry *d_tracer;
 529        struct dentry *top_dir;
 530
 531        d_tracer = tracing_init_dentry();
 532        if (IS_ERR(d_tracer))
 533                return -ENOMEM;
 534
 535        top_dir = tracefs_create_dir("hwlat_detector", d_tracer);
 536        if (!top_dir)
 537                return -ENOMEM;
 538
 539        hwlat_sample_window = tracefs_create_file("window", 0640,
 540                                                  top_dir,
 541                                                  &hwlat_data.sample_window,
 542                                                  &window_fops);
 543        if (!hwlat_sample_window)
 544                goto err;
 545
 546        hwlat_sample_width = tracefs_create_file("width", 0644,
 547                                                 top_dir,
 548                                                 &hwlat_data.sample_width,
 549                                                 &width_fops);
 550        if (!hwlat_sample_width)
 551                goto err;
 552
 553        return 0;
 554
 555 err:
 556        tracefs_remove_recursive(top_dir);
 557        return -ENOMEM;
 558}
 559
 560static void hwlat_tracer_start(struct trace_array *tr)
 561{
 562        int err;
 563
 564        err = start_kthread(tr);
 565        if (err)
 566                pr_err(BANNER "Cannot start hwlat kthread\n");
 567}
 568
 569static void hwlat_tracer_stop(struct trace_array *tr)
 570{
 571        stop_kthread();
 572}
 573
 574static bool hwlat_busy;
 575
 576static int hwlat_tracer_init(struct trace_array *tr)
 577{
 578        /* Only allow one instance to enable this */
 579        if (hwlat_busy)
 580                return -EBUSY;
 581
 582        hwlat_trace = tr;
 583
 584        disable_migrate = false;
 585        hwlat_data.count = 0;
 586        tr->max_latency = 0;
 587        save_tracing_thresh = tracing_thresh;
 588
 589        /* tracing_thresh is in nsecs, we speak in usecs */
 590        if (!tracing_thresh)
 591                tracing_thresh = last_tracing_thresh;
 592
 593        if (tracer_tracing_is_on(tr))
 594                hwlat_tracer_start(tr);
 595
 596        hwlat_busy = true;
 597
 598        return 0;
 599}
 600
 601static void hwlat_tracer_reset(struct trace_array *tr)
 602{
 603        stop_kthread();
 604
 605        /* the tracing threshold is static between runs */
 606        last_tracing_thresh = tracing_thresh;
 607
 608        tracing_thresh = save_tracing_thresh;
 609        hwlat_busy = false;
 610}
 611
 612static struct tracer hwlat_tracer __read_mostly =
 613{
 614        .name           = "hwlat",
 615        .init           = hwlat_tracer_init,
 616        .reset          = hwlat_tracer_reset,
 617        .start          = hwlat_tracer_start,
 618        .stop           = hwlat_tracer_stop,
 619        .allow_instances = true,
 620};
 621
 622__init static int init_hwlat_tracer(void)
 623{
 624        int ret;
 625
 626        mutex_init(&hwlat_data.lock);
 627
 628        ret = register_tracer(&hwlat_tracer);
 629        if (ret)
 630                return ret;
 631
 632        init_tracefs();
 633
 634        return 0;
 635}
 636late_initcall(init_hwlat_tracer);
 637