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Linux/kernel/trace/trace_hwlat.c

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  1 // SPDX-License-Identifier: GPL-2.0
  2 /*
  3  * trace_hwlat.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 
 48 static 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*/
 58 static struct task_struct *hwlat_kthread;
 59 
 60 static struct dentry *hwlat_sample_width;       /* sample width us */
 61 static struct dentry *hwlat_sample_window;      /* sample window us */
 62 
 63 /* Save the previous tracing_thresh value */
 64 static unsigned long save_tracing_thresh;
 65 
 66 /* NMI timestamp counters */
 67 static u64 nmi_ts_start;
 68 static u64 nmi_total_ts;
 69 static int nmi_count;
 70 static int nmi_cpu;
 71 
 72 /* Tells NMIs to call back to the hwlat tracer to record timestamps */
 73 bool trace_hwlat_callback_enabled;
 74 
 75 /* If the user changed threshold, remember it */
 76 static u64 last_tracing_thresh = DEFAULT_LAT_THRESHOLD * NSEC_PER_USEC;
 77 
 78 /* Individual latency samples are stored here when detected. */
 79 struct 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. */
 89 static 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 
103 static 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 
140 void 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  */
167 static 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                 u64 latency;
241 
242                 ret = 1;
243 
244                 /* We read in microseconds */
245                 if (nmi_total_ts)
246                         do_div(nmi_total_ts, NSEC_PER_USEC);
247 
248                 hwlat_data.count++;
249                 s.seqnum = hwlat_data.count;
250                 s.duration = sample;
251                 s.outer_duration = outer_sample;
252                 ktime_get_real_ts64(&s.timestamp);
253                 s.nmi_total_ts = nmi_total_ts;
254                 s.nmi_count = nmi_count;
255                 trace_hwlat_sample(&s);
256 
257                 latency = max(sample, outer_sample);
258 
259                 /* Keep a running maximum ever recorded hardware latency */
260                 if (latency > tr->max_latency) {
261                         tr->max_latency = latency;
262                         latency_fsnotify(tr);
263                 }
264         }
265 
266 out:
267         return ret;
268 }
269 
270 static struct cpumask save_cpumask;
271 static bool disable_migrate;
272 
273 static void move_to_next_cpu(void)
274 {
275         struct cpumask *current_mask = &save_cpumask;
276         int next_cpu;
277 
278         if (disable_migrate)
279                 return;
280         /*
281          * If for some reason the user modifies the CPU affinity
282          * of this thread, then stop migrating for the duration
283          * of the current test.
284          */
285         if (!cpumask_equal(current_mask, current->cpus_ptr))
286                 goto disable;
287 
288         get_online_cpus();
289         cpumask_and(current_mask, cpu_online_mask, tracing_buffer_mask);
290         next_cpu = cpumask_next(smp_processor_id(), current_mask);
291         put_online_cpus();
292 
293         if (next_cpu >= nr_cpu_ids)
294                 next_cpu = cpumask_first(current_mask);
295 
296         if (next_cpu >= nr_cpu_ids) /* Shouldn't happen! */
297                 goto disable;
298 
299         cpumask_clear(current_mask);
300         cpumask_set_cpu(next_cpu, current_mask);
301 
302         sched_setaffinity(0, current_mask);
303         return;
304 
305  disable:
306         disable_migrate = true;
307 }
308 
309 /*
310  * kthread_fn - The CPU time sampling/hardware latency detection kernel thread
311  *
312  * Used to periodically sample the CPU TSC via a call to get_sample. We
313  * disable interrupts, which does (intentionally) introduce latency since we
314  * need to ensure nothing else might be running (and thus preempting).
315  * Obviously this should never be used in production environments.
316  *
317  * Executes one loop interaction on each CPU in tracing_cpumask sysfs file.
318  */
319 static int kthread_fn(void *data)
320 {
321         u64 interval;
322 
323         while (!kthread_should_stop()) {
324 
325                 move_to_next_cpu();
326 
327                 local_irq_disable();
328                 get_sample();
329                 local_irq_enable();
330 
331                 mutex_lock(&hwlat_data.lock);
332                 interval = hwlat_data.sample_window - hwlat_data.sample_width;
333                 mutex_unlock(&hwlat_data.lock);
334 
335                 do_div(interval, USEC_PER_MSEC); /* modifies interval value */
336 
337                 /* Always sleep for at least 1ms */
338                 if (interval < 1)
339                         interval = 1;
340 
341                 if (msleep_interruptible(interval))
342                         break;
343         }
344 
345         return 0;
346 }
347 
348 /**
349  * start_kthread - Kick off the hardware latency sampling/detector kthread
350  *
351  * This starts the kernel thread that will sit and sample the CPU timestamp
352  * counter (TSC or similar) and look for potential hardware latencies.
353  */
354 static int start_kthread(struct trace_array *tr)
355 {
356         struct cpumask *current_mask = &save_cpumask;
357         struct task_struct *kthread;
358         int next_cpu;
359 
360         if (WARN_ON(hwlat_kthread))
361                 return 0;
362 
363         /* Just pick the first CPU on first iteration */
364         current_mask = &save_cpumask;
365         get_online_cpus();
366         cpumask_and(current_mask, cpu_online_mask, tracing_buffer_mask);
367         put_online_cpus();
368         next_cpu = cpumask_first(current_mask);
369 
370         kthread = kthread_create(kthread_fn, NULL, "hwlatd");
371         if (IS_ERR(kthread)) {
372                 pr_err(BANNER "could not start sampling thread\n");
373                 return -ENOMEM;
374         }
375 
376         cpumask_clear(current_mask);
377         cpumask_set_cpu(next_cpu, current_mask);
378         sched_setaffinity(kthread->pid, current_mask);
379 
380         hwlat_kthread = kthread;
381         wake_up_process(kthread);
382 
383         return 0;
384 }
385 
386 /**
387  * stop_kthread - Inform the hardware latency samping/detector kthread to stop
388  *
389  * This kicks the running hardware latency sampling/detector kernel thread and
390  * tells it to stop sampling now. Use this on unload and at system shutdown.
391  */
392 static void stop_kthread(void)
393 {
394         if (!hwlat_kthread)
395                 return;
396         kthread_stop(hwlat_kthread);
397         hwlat_kthread = NULL;
398 }
399 
400 /*
401  * hwlat_read - Wrapper read function for reading both window and width
402  * @filp: The active open file structure
403  * @ubuf: The userspace provided buffer to read value into
404  * @cnt: The maximum number of bytes to read
405  * @ppos: The current "file" position
406  *
407  * This function provides a generic read implementation for the global state
408  * "hwlat_data" structure filesystem entries.
409  */
410 static ssize_t hwlat_read(struct file *filp, char __user *ubuf,
411                           size_t cnt, loff_t *ppos)
412 {
413         char buf[U64STR_SIZE];
414         u64 *entry = filp->private_data;
415         u64 val;
416         int len;
417 
418         if (!entry)
419                 return -EFAULT;
420 
421         if (cnt > sizeof(buf))
422                 cnt = sizeof(buf);
423 
424         val = *entry;
425 
426         len = snprintf(buf, sizeof(buf), "%llu\n", val);
427 
428         return simple_read_from_buffer(ubuf, cnt, ppos, buf, len);
429 }
430 
431 /**
432  * hwlat_width_write - Write function for "width" entry
433  * @filp: The active open file structure
434  * @ubuf: The user buffer that contains the value to write
435  * @cnt: The maximum number of bytes to write to "file"
436  * @ppos: The current position in @file
437  *
438  * This function provides a write implementation for the "width" interface
439  * to the hardware latency detector. It can be used to configure
440  * for how many us of the total window us we will actively sample for any
441  * hardware-induced latency periods. Obviously, it is not possible to
442  * sample constantly and have the system respond to a sample reader, or,
443  * worse, without having the system appear to have gone out to lunch. It
444  * is enforced that width is less that the total window size.
445  */
446 static ssize_t
447 hwlat_width_write(struct file *filp, const char __user *ubuf,
448                   size_t cnt, loff_t *ppos)
449 {
450         u64 val;
451         int err;
452 
453         err = kstrtoull_from_user(ubuf, cnt, 10, &val);
454         if (err)
455                 return err;
456 
457         mutex_lock(&hwlat_data.lock);
458         if (val < hwlat_data.sample_window)
459                 hwlat_data.sample_width = val;
460         else
461                 err = -EINVAL;
462         mutex_unlock(&hwlat_data.lock);
463 
464         if (err)
465                 return err;
466 
467         return cnt;
468 }
469 
470 /**
471  * hwlat_window_write - Write function for "window" entry
472  * @filp: The active open file structure
473  * @ubuf: The user buffer that contains the value to write
474  * @cnt: The maximum number of bytes to write to "file"
475  * @ppos: The current position in @file
476  *
477  * This function provides a write implementation for the "window" interface
478  * to the hardware latency detetector. The window is the total time
479  * in us that will be considered one sample period. Conceptually, windows
480  * occur back-to-back and contain a sample width period during which
481  * actual sampling occurs. Can be used to write a new total window size. It
482  * is enfoced that any value written must be greater than the sample width
483  * size, or an error results.
484  */
485 static ssize_t
486 hwlat_window_write(struct file *filp, const char __user *ubuf,
487                    size_t cnt, loff_t *ppos)
488 {
489         u64 val;
490         int err;
491 
492         err = kstrtoull_from_user(ubuf, cnt, 10, &val);
493         if (err)
494                 return err;
495 
496         mutex_lock(&hwlat_data.lock);
497         if (hwlat_data.sample_width < val)
498                 hwlat_data.sample_window = val;
499         else
500                 err = -EINVAL;
501         mutex_unlock(&hwlat_data.lock);
502 
503         if (err)
504                 return err;
505 
506         return cnt;
507 }
508 
509 static const struct file_operations width_fops = {
510         .open           = tracing_open_generic,
511         .read           = hwlat_read,
512         .write          = hwlat_width_write,
513 };
514 
515 static const struct file_operations window_fops = {
516         .open           = tracing_open_generic,
517         .read           = hwlat_read,
518         .write          = hwlat_window_write,
519 };
520 
521 /**
522  * init_tracefs - A function to initialize the tracefs interface files
523  *
524  * This function creates entries in tracefs for "hwlat_detector".
525  * It creates the hwlat_detector directory in the tracing directory,
526  * and within that directory is the count, width and window files to
527  * change and view those values.
528  */
529 static int init_tracefs(void)
530 {
531         struct dentry *d_tracer;
532         struct dentry *top_dir;
533 
534         d_tracer = tracing_init_dentry();
535         if (IS_ERR(d_tracer))
536                 return -ENOMEM;
537 
538         top_dir = tracefs_create_dir("hwlat_detector", d_tracer);
539         if (!top_dir)
540                 return -ENOMEM;
541 
542         hwlat_sample_window = tracefs_create_file("window", 0640,
543                                                   top_dir,
544                                                   &hwlat_data.sample_window,
545                                                   &window_fops);
546         if (!hwlat_sample_window)
547                 goto err;
548 
549         hwlat_sample_width = tracefs_create_file("width", 0644,
550                                                  top_dir,
551                                                  &hwlat_data.sample_width,
552                                                  &width_fops);
553         if (!hwlat_sample_width)
554                 goto err;
555 
556         return 0;
557 
558  err:
559         tracefs_remove_recursive(top_dir);
560         return -ENOMEM;
561 }
562 
563 static void hwlat_tracer_start(struct trace_array *tr)
564 {
565         int err;
566 
567         err = start_kthread(tr);
568         if (err)
569                 pr_err(BANNER "Cannot start hwlat kthread\n");
570 }
571 
572 static void hwlat_tracer_stop(struct trace_array *tr)
573 {
574         stop_kthread();
575 }
576 
577 static bool hwlat_busy;
578 
579 static int hwlat_tracer_init(struct trace_array *tr)
580 {
581         /* Only allow one instance to enable this */
582         if (hwlat_busy)
583                 return -EBUSY;
584 
585         hwlat_trace = tr;
586 
587         disable_migrate = false;
588         hwlat_data.count = 0;
589         tr->max_latency = 0;
590         save_tracing_thresh = tracing_thresh;
591 
592         /* tracing_thresh is in nsecs, we speak in usecs */
593         if (!tracing_thresh)
594                 tracing_thresh = last_tracing_thresh;
595 
596         if (tracer_tracing_is_on(tr))
597                 hwlat_tracer_start(tr);
598 
599         hwlat_busy = true;
600 
601         return 0;
602 }
603 
604 static void hwlat_tracer_reset(struct trace_array *tr)
605 {
606         stop_kthread();
607 
608         /* the tracing threshold is static between runs */
609         last_tracing_thresh = tracing_thresh;
610 
611         tracing_thresh = save_tracing_thresh;
612         hwlat_busy = false;
613 }
614 
615 static struct tracer hwlat_tracer __read_mostly =
616 {
617         .name           = "hwlat",
618         .init           = hwlat_tracer_init,
619         .reset          = hwlat_tracer_reset,
620         .start          = hwlat_tracer_start,
621         .stop           = hwlat_tracer_stop,
622         .allow_instances = true,
623 };
624 
625 __init static int init_hwlat_tracer(void)
626 {
627         int ret;
628 
629         mutex_init(&hwlat_data.lock);
630 
631         ret = register_tracer(&hwlat_tracer);
632         if (ret)
633                 return ret;
634 
635         init_tracefs();
636 
637         return 0;
638 }
639 late_initcall(init_hwlat_tracer);
640 

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