Root/kernel/sched_clock.c

1/*
2 * sched_clock for unstable cpu clocks
3 *
4 * Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
5 *
6 * Updates and enhancements:
7 * Copyright (C) 2008 Red Hat, Inc. Steven Rostedt <srostedt@redhat.com>
8 *
9 * Based on code by:
10 * Ingo Molnar <mingo@redhat.com>
11 * Guillaume Chazarain <guichaz@gmail.com>
12 *
13 *
14 * What:
15 *
16 * cpu_clock(i) provides a fast (execution time) high resolution
17 * clock with bounded drift between CPUs. The value of cpu_clock(i)
18 * is monotonic for constant i. The timestamp returned is in nanoseconds.
19 *
20 * ######################### BIG FAT WARNING ##########################
21 * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
22 * # go backwards !! #
23 * ####################################################################
24 *
25 * There is no strict promise about the base, although it tends to start
26 * at 0 on boot (but people really shouldn't rely on that).
27 *
28 * cpu_clock(i) -- can be used from any context, including NMI.
29 * sched_clock_cpu(i) -- must be used with local IRQs disabled (implied by NMI)
30 * local_clock() -- is cpu_clock() on the current cpu.
31 *
32 * How:
33 *
34 * The implementation either uses sched_clock() when
35 * !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the
36 * sched_clock() is assumed to provide these properties (mostly it means
37 * the architecture provides a globally synchronized highres time source).
38 *
39 * Otherwise it tries to create a semi stable clock from a mixture of other
40 * clocks, including:
41 *
42 * - GTOD (clock monotomic)
43 * - sched_clock()
44 * - explicit idle events
45 *
46 * We use GTOD as base and use sched_clock() deltas to improve resolution. The
47 * deltas are filtered to provide monotonicity and keeping it within an
48 * expected window.
49 *
50 * Furthermore, explicit sleep and wakeup hooks allow us to account for time
51 * that is otherwise invisible (TSC gets stopped).
52 *
53 *
54 * Notes:
55 *
56 * The !IRQ-safetly of sched_clock() and sched_clock_cpu() comes from things
57 * like cpufreq interrupts that can change the base clock (TSC) multiplier
58 * and cause funny jumps in time -- although the filtering provided by
59 * sched_clock_cpu() should mitigate serious artifacts we cannot rely on it
60 * in general since for !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK we fully rely on
61 * sched_clock().
62 */
63#include <linux/spinlock.h>
64#include <linux/hardirq.h>
65#include <linux/module.h>
66#include <linux/percpu.h>
67#include <linux/ktime.h>
68#include <linux/sched.h>
69
70/*
71 * Scheduler clock - returns current time in nanosec units.
72 * This is default implementation.
73 * Architectures and sub-architectures can override this.
74 */
75unsigned long long __attribute__((weak)) sched_clock(void)
76{
77    return (unsigned long long)(jiffies - INITIAL_JIFFIES)
78                    * (NSEC_PER_SEC / HZ);
79}
80EXPORT_SYMBOL_GPL(sched_clock);
81
82static __read_mostly int sched_clock_running;
83
84#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
85__read_mostly int sched_clock_stable;
86
87struct sched_clock_data {
88    u64 tick_raw;
89    u64 tick_gtod;
90    u64 clock;
91};
92
93static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
94
95static inline struct sched_clock_data *this_scd(void)
96{
97    return &__get_cpu_var(sched_clock_data);
98}
99
100static inline struct sched_clock_data *cpu_sdc(int cpu)
101{
102    return &per_cpu(sched_clock_data, cpu);
103}
104
105void sched_clock_init(void)
106{
107    u64 ktime_now = ktime_to_ns(ktime_get());
108    int cpu;
109
110    for_each_possible_cpu(cpu) {
111        struct sched_clock_data *scd = cpu_sdc(cpu);
112
113        scd->tick_raw = 0;
114        scd->tick_gtod = ktime_now;
115        scd->clock = ktime_now;
116    }
117
118    sched_clock_running = 1;
119}
120
121/*
122 * min, max except they take wrapping into account
123 */
124
125static inline u64 wrap_min(u64 x, u64 y)
126{
127    return (s64)(x - y) < 0 ? x : y;
128}
129
130static inline u64 wrap_max(u64 x, u64 y)
131{
132    return (s64)(x - y) > 0 ? x : y;
133}
134
135/*
136 * update the percpu scd from the raw @now value
137 *
138 * - filter out backward motion
139 * - use the GTOD tick value to create a window to filter crazy TSC values
140 */
141static u64 sched_clock_local(struct sched_clock_data *scd)
142{
143    u64 now, clock, old_clock, min_clock, max_clock;
144    s64 delta;
145
146again:
147    now = sched_clock();
148    delta = now - scd->tick_raw;
149    if (unlikely(delta < 0))
150        delta = 0;
151
152    old_clock = scd->clock;
153
154    /*
155     * scd->clock = clamp(scd->tick_gtod + delta,
156     * max(scd->tick_gtod, scd->clock),
157     * scd->tick_gtod + TICK_NSEC);
158     */
159
160    clock = scd->tick_gtod + delta;
161    min_clock = wrap_max(scd->tick_gtod, old_clock);
162    max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);
163
164    clock = wrap_max(clock, min_clock);
165    clock = wrap_min(clock, max_clock);
166
167    if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
168        goto again;
169
170    return clock;
171}
172
173static u64 sched_clock_remote(struct sched_clock_data *scd)
174{
175    struct sched_clock_data *my_scd = this_scd();
176    u64 this_clock, remote_clock;
177    u64 *ptr, old_val, val;
178
179    sched_clock_local(my_scd);
180again:
181    this_clock = my_scd->clock;
182    remote_clock = scd->clock;
183
184    /*
185     * Use the opportunity that we have both locks
186     * taken to couple the two clocks: we take the
187     * larger time as the latest time for both
188     * runqueues. (this creates monotonic movement)
189     */
190    if (likely((s64)(remote_clock - this_clock) < 0)) {
191        ptr = &scd->clock;
192        old_val = remote_clock;
193        val = this_clock;
194    } else {
195        /*
196         * Should be rare, but possible:
197         */
198        ptr = &my_scd->clock;
199        old_val = this_clock;
200        val = remote_clock;
201    }
202
203    if (cmpxchg64(ptr, old_val, val) != old_val)
204        goto again;
205
206    return val;
207}
208
209/*
210 * Similar to cpu_clock(), but requires local IRQs to be disabled.
211 *
212 * See cpu_clock().
213 */
214u64 sched_clock_cpu(int cpu)
215{
216    struct sched_clock_data *scd;
217    u64 clock;
218
219    WARN_ON_ONCE(!irqs_disabled());
220
221    if (sched_clock_stable)
222        return sched_clock();
223
224    if (unlikely(!sched_clock_running))
225        return 0ull;
226
227    scd = cpu_sdc(cpu);
228
229    if (cpu != smp_processor_id())
230        clock = sched_clock_remote(scd);
231    else
232        clock = sched_clock_local(scd);
233
234    return clock;
235}
236
237void sched_clock_tick(void)
238{
239    struct sched_clock_data *scd;
240    u64 now, now_gtod;
241
242    if (sched_clock_stable)
243        return;
244
245    if (unlikely(!sched_clock_running))
246        return;
247
248    WARN_ON_ONCE(!irqs_disabled());
249
250    scd = this_scd();
251    now_gtod = ktime_to_ns(ktime_get());
252    now = sched_clock();
253
254    scd->tick_raw = now;
255    scd->tick_gtod = now_gtod;
256    sched_clock_local(scd);
257}
258
259/*
260 * We are going deep-idle (irqs are disabled):
261 */
262void sched_clock_idle_sleep_event(void)
263{
264    sched_clock_cpu(smp_processor_id());
265}
266EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
267
268/*
269 * We just idled delta nanoseconds (called with irqs disabled):
270 */
271void sched_clock_idle_wakeup_event(u64 delta_ns)
272{
273    if (timekeeping_suspended)
274        return;
275
276    sched_clock_tick();
277    touch_softlockup_watchdog();
278}
279EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
280
281/*
282 * As outlined at the top, provides a fast, high resolution, nanosecond
283 * time source that is monotonic per cpu argument and has bounded drift
284 * between cpus.
285 *
286 * ######################### BIG FAT WARNING ##########################
287 * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
288 * # go backwards !! #
289 * ####################################################################
290 */
291u64 cpu_clock(int cpu)
292{
293    u64 clock;
294    unsigned long flags;
295
296    local_irq_save(flags);
297    clock = sched_clock_cpu(cpu);
298    local_irq_restore(flags);
299
300    return clock;
301}
302
303/*
304 * Similar to cpu_clock() for the current cpu. Time will only be observed
305 * to be monotonic if care is taken to only compare timestampt taken on the
306 * same CPU.
307 *
308 * See cpu_clock().
309 */
310u64 local_clock(void)
311{
312    u64 clock;
313    unsigned long flags;
314
315    local_irq_save(flags);
316    clock = sched_clock_cpu(smp_processor_id());
317    local_irq_restore(flags);
318
319    return clock;
320}
321
322#else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
323
324void sched_clock_init(void)
325{
326    sched_clock_running = 1;
327}
328
329u64 sched_clock_cpu(int cpu)
330{
331    if (unlikely(!sched_clock_running))
332        return 0;
333
334    return sched_clock();
335}
336
337u64 cpu_clock(int cpu)
338{
339    return sched_clock_cpu(cpu);
340}
341
342u64 local_clock(void)
343{
344    return sched_clock_cpu(0);
345}
346
347#endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
348
349EXPORT_SYMBOL_GPL(cpu_clock);
350EXPORT_SYMBOL_GPL(local_clock);
351

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