xref: /openbmc/linux/kernel/time/sched_clock.c (revision a44d924c)
1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * Generic sched_clock() support, to extend low level hardware time
4  * counters to full 64-bit ns values.
5  */
6 #include <linux/clocksource.h>
7 #include <linux/init.h>
8 #include <linux/jiffies.h>
9 #include <linux/ktime.h>
10 #include <linux/kernel.h>
11 #include <linux/moduleparam.h>
12 #include <linux/sched.h>
13 #include <linux/sched/clock.h>
14 #include <linux/syscore_ops.h>
15 #include <linux/hrtimer.h>
16 #include <linux/sched_clock.h>
17 #include <linux/seqlock.h>
18 #include <linux/bitops.h>
19 
20 /**
21  * struct clock_read_data - data required to read from sched_clock()
22  *
23  * @epoch_ns:		sched_clock() value at last update
24  * @epoch_cyc:		Clock cycle value at last update.
25  * @sched_clock_mask:   Bitmask for two's complement subtraction of non 64bit
26  *			clocks.
27  * @read_sched_clock:	Current clock source (or dummy source when suspended).
28  * @mult:		Multipler for scaled math conversion.
29  * @shift:		Shift value for scaled math conversion.
30  *
31  * Care must be taken when updating this structure; it is read by
32  * some very hot code paths. It occupies <=40 bytes and, when combined
33  * with the seqcount used to synchronize access, comfortably fits into
34  * a 64 byte cache line.
35  */
36 struct clock_read_data {
37 	u64 epoch_ns;
38 	u64 epoch_cyc;
39 	u64 sched_clock_mask;
40 	u64 (*read_sched_clock)(void);
41 	u32 mult;
42 	u32 shift;
43 };
44 
45 /**
46  * struct clock_data - all data needed for sched_clock() (including
47  *                     registration of a new clock source)
48  *
49  * @seq:		Sequence counter for protecting updates. The lowest
50  *			bit is the index for @read_data.
51  * @read_data:		Data required to read from sched_clock.
52  * @wrap_kt:		Duration for which clock can run before wrapping.
53  * @rate:		Tick rate of the registered clock.
54  * @actual_read_sched_clock: Registered hardware level clock read function.
55  *
56  * The ordering of this structure has been chosen to optimize cache
57  * performance. In particular 'seq' and 'read_data[0]' (combined) should fit
58  * into a single 64-byte cache line.
59  */
60 struct clock_data {
61 	seqcount_t		seq;
62 	struct clock_read_data	read_data[2];
63 	ktime_t			wrap_kt;
64 	unsigned long		rate;
65 
66 	u64 (*actual_read_sched_clock)(void);
67 };
68 
69 static struct hrtimer sched_clock_timer;
70 static int irqtime = -1;
71 
72 core_param(irqtime, irqtime, int, 0400);
73 
74 static u64 notrace jiffy_sched_clock_read(void)
75 {
76 	/*
77 	 * We don't need to use get_jiffies_64 on 32-bit arches here
78 	 * because we register with BITS_PER_LONG
79 	 */
80 	return (u64)(jiffies - INITIAL_JIFFIES);
81 }
82 
83 static struct clock_data cd ____cacheline_aligned = {
84 	.read_data[0] = { .mult = NSEC_PER_SEC / HZ,
85 			  .read_sched_clock = jiffy_sched_clock_read, },
86 	.actual_read_sched_clock = jiffy_sched_clock_read,
87 };
88 
89 static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
90 {
91 	return (cyc * mult) >> shift;
92 }
93 
94 unsigned long long notrace sched_clock(void)
95 {
96 	u64 cyc, res;
97 	unsigned long seq;
98 	struct clock_read_data *rd;
99 
100 	do {
101 		seq = raw_read_seqcount(&cd.seq);
102 		rd = cd.read_data + (seq & 1);
103 
104 		cyc = (rd->read_sched_clock() - rd->epoch_cyc) &
105 		      rd->sched_clock_mask;
106 		res = rd->epoch_ns + cyc_to_ns(cyc, rd->mult, rd->shift);
107 	} while (read_seqcount_retry(&cd.seq, seq));
108 
109 	return res;
110 }
111 
112 /*
113  * Updating the data required to read the clock.
114  *
115  * sched_clock() will never observe mis-matched data even if called from
116  * an NMI. We do this by maintaining an odd/even copy of the data and
117  * steering sched_clock() to one or the other using a sequence counter.
118  * In order to preserve the data cache profile of sched_clock() as much
119  * as possible the system reverts back to the even copy when the update
120  * completes; the odd copy is used *only* during an update.
121  */
122 static void update_clock_read_data(struct clock_read_data *rd)
123 {
124 	/* update the backup (odd) copy with the new data */
125 	cd.read_data[1] = *rd;
126 
127 	/* steer readers towards the odd copy */
128 	raw_write_seqcount_latch(&cd.seq);
129 
130 	/* now its safe for us to update the normal (even) copy */
131 	cd.read_data[0] = *rd;
132 
133 	/* switch readers back to the even copy */
134 	raw_write_seqcount_latch(&cd.seq);
135 }
136 
137 /*
138  * Atomically update the sched_clock() epoch.
139  */
140 static void update_sched_clock(void)
141 {
142 	u64 cyc;
143 	u64 ns;
144 	struct clock_read_data rd;
145 
146 	rd = cd.read_data[0];
147 
148 	cyc = cd.actual_read_sched_clock();
149 	ns = rd.epoch_ns + cyc_to_ns((cyc - rd.epoch_cyc) & rd.sched_clock_mask, rd.mult, rd.shift);
150 
151 	rd.epoch_ns = ns;
152 	rd.epoch_cyc = cyc;
153 
154 	update_clock_read_data(&rd);
155 }
156 
157 static enum hrtimer_restart sched_clock_poll(struct hrtimer *hrt)
158 {
159 	update_sched_clock();
160 	hrtimer_forward_now(hrt, cd.wrap_kt);
161 
162 	return HRTIMER_RESTART;
163 }
164 
165 void __init
166 sched_clock_register(u64 (*read)(void), int bits, unsigned long rate)
167 {
168 	u64 res, wrap, new_mask, new_epoch, cyc, ns;
169 	u32 new_mult, new_shift;
170 	unsigned long r;
171 	char r_unit;
172 	struct clock_read_data rd;
173 
174 	if (cd.rate > rate)
175 		return;
176 
177 	WARN_ON(!irqs_disabled());
178 
179 	/* Calculate the mult/shift to convert counter ticks to ns. */
180 	clocks_calc_mult_shift(&new_mult, &new_shift, rate, NSEC_PER_SEC, 3600);
181 
182 	new_mask = CLOCKSOURCE_MASK(bits);
183 	cd.rate = rate;
184 
185 	/* Calculate how many nanosecs until we risk wrapping */
186 	wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask, NULL);
187 	cd.wrap_kt = ns_to_ktime(wrap);
188 
189 	rd = cd.read_data[0];
190 
191 	/* Update epoch for new counter and update 'epoch_ns' from old counter*/
192 	new_epoch = read();
193 	cyc = cd.actual_read_sched_clock();
194 	ns = rd.epoch_ns + cyc_to_ns((cyc - rd.epoch_cyc) & rd.sched_clock_mask, rd.mult, rd.shift);
195 	cd.actual_read_sched_clock = read;
196 
197 	rd.read_sched_clock	= read;
198 	rd.sched_clock_mask	= new_mask;
199 	rd.mult			= new_mult;
200 	rd.shift		= new_shift;
201 	rd.epoch_cyc		= new_epoch;
202 	rd.epoch_ns		= ns;
203 
204 	update_clock_read_data(&rd);
205 
206 	if (sched_clock_timer.function != NULL) {
207 		/* update timeout for clock wrap */
208 		hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
209 	}
210 
211 	r = rate;
212 	if (r >= 4000000) {
213 		r /= 1000000;
214 		r_unit = 'M';
215 	} else {
216 		if (r >= 1000) {
217 			r /= 1000;
218 			r_unit = 'k';
219 		} else {
220 			r_unit = ' ';
221 		}
222 	}
223 
224 	/* Calculate the ns resolution of this counter */
225 	res = cyc_to_ns(1ULL, new_mult, new_shift);
226 
227 	pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lluns\n",
228 		bits, r, r_unit, res, wrap);
229 
230 	/* Enable IRQ time accounting if we have a fast enough sched_clock() */
231 	if (irqtime > 0 || (irqtime == -1 && rate >= 1000000))
232 		enable_sched_clock_irqtime();
233 
234 	pr_debug("Registered %pF as sched_clock source\n", read);
235 }
236 
237 void __init generic_sched_clock_init(void)
238 {
239 	/*
240 	 * If no sched_clock() function has been provided at that point,
241 	 * make it the final one one.
242 	 */
243 	if (cd.actual_read_sched_clock == jiffy_sched_clock_read)
244 		sched_clock_register(jiffy_sched_clock_read, BITS_PER_LONG, HZ);
245 
246 	update_sched_clock();
247 
248 	/*
249 	 * Start the timer to keep sched_clock() properly updated and
250 	 * sets the initial epoch.
251 	 */
252 	hrtimer_init(&sched_clock_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
253 	sched_clock_timer.function = sched_clock_poll;
254 	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
255 }
256 
257 /*
258  * Clock read function for use when the clock is suspended.
259  *
260  * This function makes it appear to sched_clock() as if the clock
261  * stopped counting at its last update.
262  *
263  * This function must only be called from the critical
264  * section in sched_clock(). It relies on the read_seqcount_retry()
265  * at the end of the critical section to be sure we observe the
266  * correct copy of 'epoch_cyc'.
267  */
268 static u64 notrace suspended_sched_clock_read(void)
269 {
270 	unsigned long seq = raw_read_seqcount(&cd.seq);
271 
272 	return cd.read_data[seq & 1].epoch_cyc;
273 }
274 
275 static int sched_clock_suspend(void)
276 {
277 	struct clock_read_data *rd = &cd.read_data[0];
278 
279 	update_sched_clock();
280 	hrtimer_cancel(&sched_clock_timer);
281 	rd->read_sched_clock = suspended_sched_clock_read;
282 
283 	return 0;
284 }
285 
286 static void sched_clock_resume(void)
287 {
288 	struct clock_read_data *rd = &cd.read_data[0];
289 
290 	rd->epoch_cyc = cd.actual_read_sched_clock();
291 	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
292 	rd->read_sched_clock = cd.actual_read_sched_clock;
293 }
294 
295 static struct syscore_ops sched_clock_ops = {
296 	.suspend	= sched_clock_suspend,
297 	.resume		= sched_clock_resume,
298 };
299 
300 static int __init sched_clock_syscore_init(void)
301 {
302 	register_syscore_ops(&sched_clock_ops);
303 
304 	return 0;
305 }
306 device_initcall(sched_clock_syscore_init);
307