summaryrefslogtreecommitdiffstats
path: root/kernel/trace/trace_hwlat.c
blob: d440ddd5fd8b29adf605075004fc5edd9a7ee17c (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
// 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);