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 *
  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
  82__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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