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  * local_clock()      -- is cpu_clock() on the current cpu.
  30  *
  31  * sched_clock_cpu(i)
  32  *
  33  * How:
  34  *
  35  * The implementation either uses sched_clock() when
  36  * !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the
  37  * sched_clock() is assumed to provide these properties (mostly it means
  38  * the architecture provides a globally synchronized highres time source).
  39  *
  40  * Otherwise it tries to create a semi stable clock from a mixture of other
  41  * clocks, including:
  42  *
  43  *  - GTOD (clock monotomic)
  44  *  - sched_clock()
  45  *  - explicit idle events
  46  *
  47  * We use GTOD as base and use sched_clock() deltas to improve resolution. The
  48  * deltas are filtered to provide monotonicity and keeping it within an
  49  * expected window.
  50  *
  51  * Furthermore, explicit sleep and wakeup hooks allow us to account for time
  52  * that is otherwise invisible (TSC gets stopped).
  53  *
  54  */
  55 #include <linux/spinlock.h>
  56 #include <linux/hardirq.h>
  57 #include <linux/export.h>
  58 #include <linux/percpu.h>
  59 #include <linux/ktime.h>
  60 #include <linux/sched.h>
  61 #include <linux/static_key.h>
  62 #include <linux/workqueue.h>
  63
  64 /*
  65  * Scheduler clock - returns current time in nanosec units.
  66  * This is default implementation.
  67  * Architectures and sub-architectures can override this.
  68  */
  69 unsigned long long __attribute__((weak)) sched_clock(void)
  70 {
  71         return (unsigned long long)(jiffies - INITIAL_JIFFIES)
  72                                         * (NSEC_PER_SEC / HZ);
  73 }
  74 EXPORT_SYMBOL_GPL(sched_clock);
  75
  76 __read_mostly int sched_clock_running;
  77
  78 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
  79 static struct static_key __sched_clock_stable = STATIC_KEY_INIT;
  80
  81 int sched_clock_stable(void)
  82 {
  83         if (static_key_false(&__sched_clock_stable))
  84                 return false;
  85         return true;
  86 }
  87
  88 void set_sched_clock_stable(void)
  89 {
  90         if (!sched_clock_stable())
  91                 static_key_slow_dec(&__sched_clock_stable);
  92 }
  93
  94 static void __clear_sched_clock_stable(struct work_struct *work)
  95 {
  96         /* XXX worry about clock continuity */
  97         if (sched_clock_stable())
  98                 static_key_slow_inc(&__sched_clock_stable);
  99 }
 100
 101 static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable);
 102
 103 void clear_sched_clock_stable(void)
 104 {
 105         if (keventd_up())
 106                 schedule_work(&sched_clock_work);
 107         else
 108                 __clear_sched_clock_stable(&sched_clock_work);
 109 }
 110
 111 struct sched_clock_data {
 112         u64                     tick_raw;
 113         u64                     tick_gtod;
 114         u64                     clock;
 115 };
 116
 117 static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
 118
 119 static inline struct sched_clock_data *this_scd(void)
 120 {
 121         return &__get_cpu_var(sched_clock_data);
 122 }
 123
 124 static inline struct sched_clock_data *cpu_sdc(int cpu)
 125 {
 126         return &per_cpu(sched_clock_data, cpu);
 127 }
 128
 129 void sched_clock_init(void)
 130 {
 131         u64 ktime_now = ktime_to_ns(ktime_get());
 132         int cpu;
 133
 134         for_each_possible_cpu(cpu) {
 135                 struct sched_clock_data *scd = cpu_sdc(cpu);
 136
 137                 scd->tick_raw = 0;
 138                 scd->tick_gtod = ktime_now;
 139                 scd->clock = ktime_now;
 140         }
 141
 142         sched_clock_running = 1;
 143 }
 144
 145 /*
 146  * min, max except they take wrapping into account
 147  */
 148
 149 static inline u64 wrap_min(u64 x, u64 y)
 150 {
 151         return (s64)(x - y) < 0 ? x : y;
 152 }
 153
 154 static inline u64 wrap_max(u64 x, u64 y)
 155 {
 156         return (s64)(x - y) > 0 ? x : y;
 157 }
 158
 159 /*
 160  * update the percpu scd from the raw @now value
 161  *
 162  *  - filter out backward motion
 163  *  - use the GTOD tick value to create a window to filter crazy TSC values
 164  */
 165 static u64 sched_clock_local(struct sched_clock_data *scd)
 166 {
 167         u64 now, clock, old_clock, min_clock, max_clock;
 168         s64 delta;
 169
 170 again:
 171         now = sched_clock();
 172         delta = now - scd->tick_raw;
 173         if (unlikely(delta < 0))
 174                 delta = 0;
 175
 176         old_clock = scd->clock;
 177
 178         /*
 179          * scd->clock = clamp(scd->tick_gtod + delta,
 180          *                    max(scd->tick_gtod, scd->clock),
 181          *                    scd->tick_gtod + TICK_NSEC);
 182          */
 183
 184         clock = scd->tick_gtod + delta;
 185         min_clock = wrap_max(scd->tick_gtod, old_clock);
 186         max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);
 187
 188         clock = wrap_max(clock, min_clock);
 189         clock = wrap_min(clock, max_clock);
 190
 191         if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
 192                 goto again;
 193
 194         return clock;
 195 }
 196
 197 static u64 sched_clock_remote(struct sched_clock_data *scd)
 198 {
 199         struct sched_clock_data *my_scd = this_scd();
 200         u64 this_clock, remote_clock;
 201         u64 *ptr, old_val, val;
 202
 203 #if BITS_PER_LONG != 64
 204 again:
 205         /*
 206          * Careful here: The local and the remote clock values need to
 207          * be read out atomic as we need to compare the values and
 208          * then update either the local or the remote side. So the
 209          * cmpxchg64 below only protects one readout.
 210          *
 211          * We must reread via sched_clock_local() in the retry case on
 212          * 32bit as an NMI could use sched_clock_local() via the
 213          * tracer and hit between the readout of
 214          * the low32bit and the high 32bit portion.
 215          */
 216         this_clock = sched_clock_local(my_scd);
 217         /*
 218          * We must enforce atomic readout on 32bit, otherwise the
 219          * update on the remote cpu can hit inbetween the readout of
 220          * the low32bit and the high 32bit portion.
 221          */
 222         remote_clock = cmpxchg64(&scd->clock, 0, 0);
 223 #else
 224         /*
 225          * On 64bit the read of [my]scd->clock is atomic versus the
 226          * update, so we can avoid the above 32bit dance.
 227          */
 228         sched_clock_local(my_scd);
 229 again:
 230         this_clock = my_scd->clock;
 231         remote_clock = scd->clock;
 232 #endif
 233
 234         /*
 235          * Use the opportunity that we have both locks
 236          * taken to couple the two clocks: we take the
 237          * larger time as the latest time for both
 238          * runqueues. (this creates monotonic movement)
 239          */
 240         if (likely((s64)(remote_clock - this_clock) < 0)) {
 241                 ptr = &scd->clock;
 242                 old_val = remote_clock;
 243                 val = this_clock;
 244         } else {
 245                 /*
 246                  * Should be rare, but possible:
 247                  */
 248                 ptr = &my_scd->clock;
 249                 old_val = this_clock;
 250                 val = remote_clock;
 251         }
 252
 253         if (cmpxchg64(ptr, old_val, val) != old_val)
 254                 goto again;
 255
 256         return val;
 257 }
 258
 259 /*
 260  * Similar to cpu_clock(), but requires local IRQs to be disabled.
 261  *
 262  * See cpu_clock().
 263  */
 264 u64 sched_clock_cpu(int cpu)
 265 {
 266         struct sched_clock_data *scd;
 267         u64 clock;
 268
 269         if (sched_clock_stable())
 270                 return sched_clock();
 271
 272         if (unlikely(!sched_clock_running))
 273                 return 0ull;
 274
 275         preempt_disable();
 276         scd = cpu_sdc(cpu);
 277
 278         if (cpu != smp_processor_id())
 279                 clock = sched_clock_remote(scd);
 280         else
 281                 clock = sched_clock_local(scd);
 282         preempt_enable();
 283
 284         return clock;
 285 }
 286
 287 void sched_clock_tick(void)
 288 {
 289         struct sched_clock_data *scd;
 290         u64 now, now_gtod;
 291
 292         if (sched_clock_stable())
 293                 return;
 294
 295         if (unlikely(!sched_clock_running))
 296                 return;
 297
 298         WARN_ON_ONCE(!irqs_disabled());
 299
 300         scd = this_scd();
 301         now_gtod = ktime_to_ns(ktime_get());
 302         now = sched_clock();
 303
 304         scd->tick_raw = now;
 305         scd->tick_gtod = now_gtod;
 306         sched_clock_local(scd);
 307 }
 308
 309 /*
 310  * We are going deep-idle (irqs are disabled):
 311  */
 312 void sched_clock_idle_sleep_event(void)
 313 {
 314         sched_clock_cpu(smp_processor_id());
 315 }
 316 EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
 317
 318 /*
 319  * We just idled delta nanoseconds (called with irqs disabled):
 320  */
 321 void sched_clock_idle_wakeup_event(u64 delta_ns)
 322 {
 323         if (timekeeping_suspended)
 324                 return;
 325
 326         sched_clock_tick();
 327         touch_softlockup_watchdog();
 328 }
 329 EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
 330
 331 /*
 332  * As outlined at the top, provides a fast, high resolution, nanosecond
 333  * time source that is monotonic per cpu argument and has bounded drift
 334  * between cpus.
 335  *
 336  * ######################### BIG FAT WARNING ##########################
 337  * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
 338  * # go backwards !!                                                  #
 339  * ####################################################################
 340  */
 341 u64 cpu_clock(int cpu)
 342 {
 343         if (static_key_false(&__sched_clock_stable))
 344                 return sched_clock_cpu(cpu);
 345
 346         return sched_clock();
 347 }
 348
 349 /*
 350  * Similar to cpu_clock() for the current cpu. Time will only be observed
 351  * to be monotonic if care is taken to only compare timestampt taken on the
 352  * same CPU.
 353  *
 354  * See cpu_clock().
 355  */
 356 u64 local_clock(void)
 357 {
 358         if (static_key_false(&__sched_clock_stable))
 359                 return sched_clock_cpu(raw_smp_processor_id());
 360
 361         return sched_clock();
 362 }
 363
 364 #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
 365
 366 void sched_clock_init(void)
 367 {
 368         sched_clock_running = 1;
 369 }
 370
 371 u64 sched_clock_cpu(int cpu)
 372 {
 373         if (unlikely(!sched_clock_running))
 374                 return 0;
 375
 376         return sched_clock();
 377 }
 378
 379 u64 cpu_clock(int cpu)
 380 {
 381         return sched_clock();
 382 }
 383
 384 u64 local_clock(void)
 385 {
 386         return sched_clock();
 387 }
 388
 389 #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
 390
 391 EXPORT_SYMBOL_GPL(cpu_clock);
 392 EXPORT_SYMBOL_GPL(local_clock);