summaryrefslogtreecommitdiffstats
path: root/kernel/time/sched_clock.c
blob: fa3f800d7d763feac3fcdebc99939f2ef749a23b (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
// SPDX-License-Identifier: GPL-2.0
/*
 * Generic sched_clock() support, to extend low level hardware time
 * counters to full 64-bit ns values.
 */
#include <linux/clocksource.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/ktime.h>
#include <linux/kernel.h>
#include <linux/moduleparam.h>
#include <linux/sched.h>
#include <linux/sched/clock.h>
#include <linux/syscore_ops.h>
#include <linux/hrtimer.h>
#include <linux/sched_clock.h>
#include <linux/seqlock.h>
#include <linux/bitops.h>

#include "timekeeping.h"

/**
 * struct clock_read_data - data required to read from sched_clock()
 *
 * @epoch_ns:		sched_clock() value at last update
 * @epoch_cyc:		Clock cycle value at last update.
 * @sched_clock_mask:   Bitmask for two's complement subtraction of non 64bit
 *			clocks.
 * @read_sched_clock:	Current clock source (or dummy source when suspended).
 * @mult:		Multipler for scaled math conversion.
 * @shift:		Shift value for scaled math conversion.
 *
 * Care must be taken when updating this structure; it is read by
 * some very hot code paths. It occupies <=40 bytes and, when combined
 * with the seqcount used to synchronize access, comfortably fits into
 * a 64 byte cache line.
 */
struct clock_read_data {
	u64 epoch_ns;
	u64 epoch_cyc;
	u64 sched_clock_mask;
	u64 (*read_sched_clock)(void);
	u32 mult;
	u32 shift;
};

/**
 * struct clock_data - all data needed for sched_clock() (including
 *                     registration of a new clock source)
 *
 * @seq:		Sequence counter for protecting updates. The lowest
 *			bit is the index for @read_data.
 * @read_data:		Data required to read from sched_clock.
 * @wrap_kt:		Duration for which clock can run before wrapping.
 * @rate:		Tick rate of the registered clock.
 * @actual_read_sched_clock: Registered hardware level clock read function.
 *
 * The ordering of this structure has been chosen to optimize cache
 * performance. In particular 'seq' and 'read_data[0]' (combined) should fit
 * into a single 64-byte cache line.
 */
struct clock_data {
	seqcount_t		seq;
	struct clock_read_data	read_data[2];
	ktime_t			wrap_kt;
	unsigned long		rate;

	u64 (*actual_read_sched_clock)(void);
};

static struct hrtimer sched_clock_timer;
static int irqtime = -1;

core_param(irqtime, irqtime, int, 0400);

static u64 notrace jiffy_sched_clock_read(void)
{
	/*
	 * We don't need to use get_jiffies_64 on 32-bit arches here
	 * because we register with BITS_PER_LONG
	 */
	return (u64)(jiffies - INITIAL_JIFFIES);
}

static struct clock_data cd ____cacheline_aligned = {
	.read_data[0] = { .mult = NSEC_PER_SEC / HZ,
			  .read_sched_clock = jiffy_sched_clock_read, },
	.actual_read_sched_clock = jiffy_sched_clock_read,
};

static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
{
	return (cyc * mult) >> shift;
}

unsigned long long notrace sched_clock(void)
{
	u64 cyc, res;
	unsigned int seq;
	struct clock_read_data *rd;

	do {
		seq = raw_read_seqcount(&cd.seq);
		rd = cd.read_data + (seq & 1);

		cyc = (rd->read_sched_clock() - rd->epoch_cyc) &
		      rd->sched_clock_mask;
		res = rd->epoch_ns + cyc_to_ns(cyc, rd->mult, rd->shift);
	} while (read_seqcount_retry(&cd.seq, seq));

	return res;
}

/*
 * Updating the data required to read the clock.
 *
 * sched_clock() will never observe mis-matched data even if called from
 * an NMI. We do this by maintaining an odd/even copy of the data and
 * steering sched_clock() to one or the other using a sequence counter.
 * In order to preserve the data cache profile of sched_clock() as much
 * as possible the system reverts back to the even copy when the update
 * completes; the odd copy is used *only* during an update.
 */
static void update_clock_read_data(struct clock_read_data *rd)
{
	/* update the backup (odd) copy with the new data */
	cd.read_data[1] = *rd;

	/* steer readers towards the odd copy */
	raw_write_seqcount_latch(&cd.seq);

	/* now its safe for us to update the normal (even) copy */
	cd.read_data[0] = *rd;

	/* switch readers back to the even copy */
	raw_write_seqcount_latch(&cd.seq);
}

/*
 * Atomically update the sched_clock() epoch.
 */
static void update_sched_clock(void)
{
	u64 cyc;
	u64 ns;
	struct clock_read_data rd;

	rd = cd.read_data[0];

	cyc = cd.actual_read_sched_clock();
	ns = rd.epoch_ns + cyc_to_ns((cyc - rd.epoch_cyc) & rd.sched_clock_mask, rd.mult, rd.shift);

	rd.epoch_ns = ns;
	rd.epoch_cyc = cyc;

	update_clock_read_data(&rd);
}

static enum hrtimer_restart sched_clock_poll(struct hrtimer *hrt)
{
	update_sched_clock();
	hrtimer_forward_now(hrt, cd.wrap_kt);

	return HRTIMER_RESTART;
}

void __init
sched_clock_register(u64 (*read)(void), int bits, unsigned long rate)
{
	u64 res, wrap, new_mask, new_epoch, cyc, ns;
	u32 new_mult, new_shift;
	unsigned long r, flags;
	char r_unit;
	struct clock_read_data rd;

	if (cd.rate > rate)
		return;

	/* Cannot register a sched_clock with interrupts on */
	local_irq_save(flags);

	/* Calculate the mult/shift to convert counter ticks to ns. */
	clocks_calc_mult_shift(&new_mult, &new_shift, rate, NSEC_PER_SEC, 3600);

	new_mask = CLOCKSOURCE_MASK(bits);
	cd.rate = rate;

	/* Calculate how many nanosecs until we risk wrapping */
	wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask, NULL);
	cd.wrap_kt = ns_to_ktime(wrap);

	rd = cd.read_data[0];

	/* Update epoch for new counter and update 'epoch_ns' from old counter*/
	new_epoch = read();
	cyc = cd.actual_read_sched_clock();
	ns = rd.epoch_ns + cyc_to_ns((cyc - rd.epoch_cyc) & rd.sched_clock_mask, rd.mult, rd.shift);
	cd.actual_read_sched_clock = read;

	rd.read_sched_clock	= read;
	rd.sched_clock_mask	= new_mask;
	rd.mult			= new_mult;
	rd.shift		= new_shift;
	rd.epoch_cyc		= new_epoch;
	rd.epoch_ns		= ns;

	update_clock_read_data(&rd);

	if (sched_clock_timer.function != NULL) {
		/* update timeout for clock wrap */
		hrtimer_start(&sched_clock_timer, cd.wrap_kt,
			      HRTIMER_MODE_REL_HARD);
	}

	r = rate;
	if (r >= 4000000) {
		r /= 1000000;
		r_unit = 'M';
	} else {
		if (r >= 1000) {
			r /= 1000;
			r_unit = 'k';
		} else {
			r_unit = ' ';
		}
	}

	/* Calculate the ns resolution of this counter */
	res = cyc_to_ns(1ULL, new_mult, new_shift);

	pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lluns\n",
		bits, r, r_unit, res, wrap);

	/* Enable IRQ time accounting if we have a fast enough sched_clock() */
	if (irqtime > 0 || (irqtime == -1 && rate >= 1000000))
		enable_sched_clock_irqtime();

	local_irq_restore(flags);

	pr_debug("Registered %pS as sched_clock source\n", read);
}

void __init generic_sched_clock_init(void)
{
	/*
	 * If no sched_clock() function has been provided at that point,
	 * make it the final one one.
	 */
	if (cd.actual_read_sched_clock == jiffy_sched_clock_read)
		sched_clock_register(jiffy_sched_clock_read, BITS_PER_LONG, HZ);

	update_sched_clock();

	/*
	 * Start the timer to keep sched_clock() properly updated and
	 * sets the initial epoch.
	 */
	hrtimer_init(&sched_clock_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
	sched_clock_timer.function = sched_clock_poll;
	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL_HARD);
}

/*
 * Clock read function for use when the clock is suspended.
 *
 * This function makes it appear to sched_clock() as if the clock
 * stopped counting at its last update.
 *
 * This function must only be called from the critical
 * section in sched_clock(). It relies on the read_seqcount_retry()
 * at the end of the critical section to be sure we observe the
 * correct copy of 'epoch_cyc'.
 */
static u64 notrace suspended_sched_clock_read(void)
{
	unsigned int seq = raw_read_seqcount(&cd.seq);

	return cd.read_data[seq & 1].epoch_cyc;
}

int sched_clock_suspend(void)
{
	struct clock_read_data *rd = &cd.read_data[0];

	update_sched_clock();
	hrtimer_cancel(&sched_clock_timer);
	rd->read_sched_clock = suspended_sched_clock_read;

	return 0;
}

void sched_clock_resume(void)
{
	struct clock_read_data *rd = &cd.read_data[0];

	rd->epoch_cyc = cd.actual_read_sched_clock();
	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL_HARD);
	rd->read_sched_clock = cd.actual_read_sched_clock;
}

static struct syscore_ops sched_clock_ops = {
	.suspend	= sched_clock_suspend,
	.resume		= sched_clock_resume,
};

static int __init sched_clock_syscore_init(void)
{
	register_syscore_ops(&sched_clock_ops);

	return 0;
}
device_initcall(sched_clock_syscore_init);