linux/kernel/sched_clock.c
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   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        /*
  52         * Raw spinlock - this is a special case: this might be called
  53         * from within instrumentation code so we dont want to do any
  54         * instrumentation ourselves.
  55         */
  56        raw_spinlock_t          lock;
  57
  58        u64                     tick_raw;
  59        u64                     tick_gtod;
  60        u64                     clock;
  61};
  62
  63static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
  64
  65static inline struct sched_clock_data *this_scd(void)
  66{
  67        return &__get_cpu_var(sched_clock_data);
  68}
  69
  70static inline struct sched_clock_data *cpu_sdc(int cpu)
  71{
  72        return &per_cpu(sched_clock_data, cpu);
  73}
  74
  75void sched_clock_init(void)
  76{
  77        u64 ktime_now = ktime_to_ns(ktime_get());
  78        int cpu;
  79
  80        for_each_possible_cpu(cpu) {
  81                struct sched_clock_data *scd = cpu_sdc(cpu);
  82
  83                scd->lock = (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
  84                scd->tick_raw = 0;
  85                scd->tick_gtod = ktime_now;
  86                scd->clock = ktime_now;
  87        }
  88
  89        sched_clock_running = 1;
  90}
  91
  92/*
  93 * min, max except they take wrapping into account
  94 */
  95
  96static inline u64 wrap_min(u64 x, u64 y)
  97{
  98        return (s64)(x - y) < 0 ? x : y;
  99}
 100
 101static inline u64 wrap_max(u64 x, u64 y)
 102{
 103        return (s64)(x - y) > 0 ? x : y;
 104}
 105
 106/*
 107 * update the percpu scd from the raw @now value
 108 *
 109 *  - filter out backward motion
 110 *  - use the GTOD tick value to create a window to filter crazy TSC values
 111 */
 112static u64 __update_sched_clock(struct sched_clock_data *scd, u64 now)
 113{
 114        s64 delta = now - scd->tick_raw;
 115        u64 clock, min_clock, max_clock;
 116
 117        if (unlikely(delta < 0))
 118                delta = 0;
 119
 120        /*
 121         * scd->clock = clamp(scd->tick_gtod + delta,
 122         *                    max(scd->tick_gtod, scd->clock),
 123         *                    scd->tick_gtod + TICK_NSEC);
 124         */
 125
 126        clock = scd->tick_gtod + delta;
 127        min_clock = wrap_max(scd->tick_gtod, scd->clock);
 128        max_clock = wrap_max(scd->clock, scd->tick_gtod + TICK_NSEC);
 129
 130        clock = wrap_max(clock, min_clock);
 131        clock = wrap_min(clock, max_clock);
 132
 133        scd->clock = clock;
 134
 135        return scd->clock;
 136}
 137
 138static void lock_double_clock(struct sched_clock_data *data1,
 139                                struct sched_clock_data *data2)
 140{
 141        if (data1 < data2) {
 142                __raw_spin_lock(&data1->lock);
 143                __raw_spin_lock(&data2->lock);
 144        } else {
 145                __raw_spin_lock(&data2->lock);
 146                __raw_spin_lock(&data1->lock);
 147        }
 148}
 149
 150u64 sched_clock_cpu(int cpu)
 151{
 152        u64 now, clock, this_clock, remote_clock;
 153        struct sched_clock_data *scd;
 154
 155        if (sched_clock_stable)
 156                return sched_clock();
 157
 158        scd = cpu_sdc(cpu);
 159
 160        /*
 161         * Normally this is not called in NMI context - but if it is,
 162         * trying to do any locking here is totally lethal.
 163         */
 164        if (unlikely(in_nmi()))
 165                return scd->clock;
 166
 167        if (unlikely(!sched_clock_running))
 168                return 0ull;
 169
 170        WARN_ON_ONCE(!irqs_disabled());
 171        now = sched_clock();
 172
 173        if (cpu != raw_smp_processor_id()) {
 174                struct sched_clock_data *my_scd = this_scd();
 175
 176                lock_double_clock(scd, my_scd);
 177
 178                this_clock = __update_sched_clock(my_scd, now);
 179                remote_clock = scd->clock;
 180
 181                /*
 182                 * Use the opportunity that we have both locks
 183                 * taken to couple the two clocks: we take the
 184                 * larger time as the latest time for both
 185                 * runqueues. (this creates monotonic movement)
 186                 */
 187                if (likely((s64)(remote_clock - this_clock) < 0)) {
 188                        clock = this_clock;
 189                        scd->clock = clock;
 190                } else {
 191                        /*
 192                         * Should be rare, but possible:
 193                         */
 194                        clock = remote_clock;
 195                        my_scd->clock = remote_clock;
 196                }
 197
 198                __raw_spin_unlock(&my_scd->lock);
 199        } else {
 200                __raw_spin_lock(&scd->lock);
 201                clock = __update_sched_clock(scd, now);
 202        }
 203
 204        __raw_spin_unlock(&scd->lock);
 205
 206        return clock;
 207}
 208
 209void sched_clock_tick(void)
 210{
 211        struct sched_clock_data *scd;
 212        u64 now, now_gtod;
 213
 214        if (sched_clock_stable)
 215                return;
 216
 217        if (unlikely(!sched_clock_running))
 218                return;
 219
 220        WARN_ON_ONCE(!irqs_disabled());
 221
 222        scd = this_scd();
 223        now_gtod = ktime_to_ns(ktime_get());
 224        now = sched_clock();
 225
 226        __raw_spin_lock(&scd->lock);
 227        scd->tick_raw = now;
 228        scd->tick_gtod = now_gtod;
 229        __update_sched_clock(scd, now);
 230        __raw_spin_unlock(&scd->lock);
 231}
 232
 233/*
 234 * We are going deep-idle (irqs are disabled):
 235 */
 236void sched_clock_idle_sleep_event(void)
 237{
 238        sched_clock_cpu(smp_processor_id());
 239}
 240EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
 241
 242/*
 243 * We just idled delta nanoseconds (called with irqs disabled):
 244 */
 245void sched_clock_idle_wakeup_event(u64 delta_ns)
 246{
 247        if (timekeeping_suspended)
 248                return;
 249
 250        sched_clock_tick();
 251        touch_softlockup_watchdog();
 252}
 253EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
 254
 255#else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
 256
 257void sched_clock_init(void)
 258{
 259        sched_clock_running = 1;
 260}
 261
 262u64 sched_clock_cpu(int cpu)
 263{
 264        if (unlikely(!sched_clock_running))
 265                return 0;
 266
 267        return sched_clock();
 268}
 269
 270#endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
 271
 272unsigned long long cpu_clock(int cpu)
 273{
 274        unsigned long long clock;
 275        unsigned long flags;
 276
 277        local_irq_save(flags);
 278        clock = sched_clock_cpu(cpu);
 279        local_irq_restore(flags);
 280
 281        return clock;
 282}
 283EXPORT_SYMBOL_GPL(cpu_clock);
 284