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 * Create a semi stable clock from a mixture of other events, including:
14 * - gtod
15 * - sched_clock()
16 * - explicit idle events
17 *
18 * We use gtod as base and the unstable clock deltas. The deltas are filtered,
19 * making it monotonic and keeping it within an expected window.
20 *
21 * Furthermore, explicit sleep and wakeup hooks allow us to account for time
22 * that is otherwise invisible (TSC gets stopped).
23 *
24 * The clock: sched_clock_cpu() is monotonic per cpu, and should be somewhat
25 * consistent between cpus (never more than 2 jiffies difference).
26 */
27#include <linux/spinlock.h>
28#include <linux/hardirq.h>
29#include <linux/module.h>
30#include <linux/percpu.h>
31#include <linux/ktime.h>
32#include <linux/sched.h>
33
34/*
35 * Scheduler clock - returns current time in nanosec units.
36 * This is default implementation.
37 * Architectures and sub-architectures can override this.
38 */
39unsigned long long __attribute__((weak)) sched_clock(void)
40{
41    return (unsigned long long)(jiffies - INITIAL_JIFFIES)
42                    * (NSEC_PER_SEC / HZ);
43}
44
45static __read_mostly int sched_clock_running;
46
47#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
48__read_mostly int sched_clock_stable;
49
50struct sched_clock_data {
51    u64 tick_raw;
52    u64 tick_gtod;
53    u64 clock;
54};
55
56static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
57
58static inline struct sched_clock_data *this_scd(void)
59{
60    return &__get_cpu_var(sched_clock_data);
61}
62
63static inline struct sched_clock_data *cpu_sdc(int cpu)
64{
65    return &per_cpu(sched_clock_data, cpu);
66}
67
68void sched_clock_init(void)
69{
70    u64 ktime_now = ktime_to_ns(ktime_get());
71    int cpu;
72
73    for_each_possible_cpu(cpu) {
74        struct sched_clock_data *scd = cpu_sdc(cpu);
75
76        scd->tick_raw = 0;
77        scd->tick_gtod = ktime_now;
78        scd->clock = ktime_now;
79    }
80
81    sched_clock_running = 1;
82}
83
84/*
85 * min, max except they take wrapping into account
86 */
87
88static inline u64 wrap_min(u64 x, u64 y)
89{
90    return (s64)(x - y) < 0 ? x : y;
91}
92
93static inline u64 wrap_max(u64 x, u64 y)
94{
95    return (s64)(x - y) > 0 ? x : y;
96}
97
98/*
99 * update the percpu scd from the raw @now value
100 *
101 * - filter out backward motion
102 * - use the GTOD tick value to create a window to filter crazy TSC values
103 */
104static u64 sched_clock_local(struct sched_clock_data *scd)
105{
106    u64 now, clock, old_clock, min_clock, max_clock;
107    s64 delta;
108
109again:
110    now = sched_clock();
111    delta = now - scd->tick_raw;
112    if (unlikely(delta < 0))
113        delta = 0;
114
115    old_clock = scd->clock;
116
117    /*
118     * scd->clock = clamp(scd->tick_gtod + delta,
119     * max(scd->tick_gtod, scd->clock),
120     * scd->tick_gtod + TICK_NSEC);
121     */
122
123    clock = scd->tick_gtod + delta;
124    min_clock = wrap_max(scd->tick_gtod, old_clock);
125    max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);
126
127    clock = wrap_max(clock, min_clock);
128    clock = wrap_min(clock, max_clock);
129
130    if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
131        goto again;
132
133    return clock;
134}
135
136static u64 sched_clock_remote(struct sched_clock_data *scd)
137{
138    struct sched_clock_data *my_scd = this_scd();
139    u64 this_clock, remote_clock;
140    u64 *ptr, old_val, val;
141
142    sched_clock_local(my_scd);
143again:
144    this_clock = my_scd->clock;
145    remote_clock = scd->clock;
146
147    /*
148     * Use the opportunity that we have both locks
149     * taken to couple the two clocks: we take the
150     * larger time as the latest time for both
151     * runqueues. (this creates monotonic movement)
152     */
153    if (likely((s64)(remote_clock - this_clock) < 0)) {
154        ptr = &scd->clock;
155        old_val = remote_clock;
156        val = this_clock;
157    } else {
158        /*
159         * Should be rare, but possible:
160         */
161        ptr = &my_scd->clock;
162        old_val = this_clock;
163        val = remote_clock;
164    }
165
166    if (cmpxchg64(ptr, old_val, val) != old_val)
167        goto again;
168
169    return val;
170}
171
172u64 sched_clock_cpu(int cpu)
173{
174    struct sched_clock_data *scd;
175    u64 clock;
176
177    WARN_ON_ONCE(!irqs_disabled());
178
179    if (sched_clock_stable)
180        return sched_clock();
181
182    if (unlikely(!sched_clock_running))
183        return 0ull;
184
185    scd = cpu_sdc(cpu);
186
187    if (cpu != smp_processor_id())
188        clock = sched_clock_remote(scd);
189    else
190        clock = sched_clock_local(scd);
191
192    return clock;
193}
194
195void sched_clock_tick(void)
196{
197    struct sched_clock_data *scd;
198    u64 now, now_gtod;
199
200    if (sched_clock_stable)
201        return;
202
203    if (unlikely(!sched_clock_running))
204        return;
205
206    WARN_ON_ONCE(!irqs_disabled());
207
208    scd = this_scd();
209    now_gtod = ktime_to_ns(ktime_get());
210    now = sched_clock();
211
212    scd->tick_raw = now;
213    scd->tick_gtod = now_gtod;
214    sched_clock_local(scd);
215}
216
217/*
218 * We are going deep-idle (irqs are disabled):
219 */
220void sched_clock_idle_sleep_event(void)
221{
222    sched_clock_cpu(smp_processor_id());
223}
224EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
225
226/*
227 * We just idled delta nanoseconds (called with irqs disabled):
228 */
229void sched_clock_idle_wakeup_event(u64 delta_ns)
230{
231    if (timekeeping_suspended)
232        return;
233
234    sched_clock_tick();
235    touch_softlockup_watchdog();
236}
237EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
238
239unsigned long long cpu_clock(int cpu)
240{
241    unsigned long long clock;
242    unsigned long flags;
243
244    local_irq_save(flags);
245    clock = sched_clock_cpu(cpu);
246    local_irq_restore(flags);
247
248    return clock;
249}
250
251#else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
252
253void sched_clock_init(void)
254{
255    sched_clock_running = 1;
256}
257
258u64 sched_clock_cpu(int cpu)
259{
260    if (unlikely(!sched_clock_running))
261        return 0;
262
263    return sched_clock();
264}
265
266
267unsigned long long cpu_clock(int cpu)
268{
269    return sched_clock_cpu(cpu);
270}
271
272#endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
273
274EXPORT_SYMBOL_GPL(cpu_clock);
275

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