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  */
  39 unsigned long long __attribute__((weak)) sched_clock(void)
  40 {
  41         return (unsigned long long)(jiffies - INITIAL_JIFFIES)
  42                                         * (NSEC_PER_SEC / HZ);
  43 }
  44
  45 static __read_mostly int sched_clock_running;
  46
  47 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
  48 __read_mostly int sched_clock_stable;
  49
  50 struct sched_clock_data {
  51         u64                     tick_raw;
  52         u64                     tick_gtod;
  53         u64                     clock;
  54 };
  55
  56 static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
  57
  58 static inline struct sched_clock_data *this_scd(void)
  59 {
  60         return &__get_cpu_var(sched_clock_data);
  61 }
  62
  63 static inline struct sched_clock_data *cpu_sdc(int cpu)
  64 {
  65         return &per_cpu(sched_clock_data, cpu);
  66 }
  67
  68 void 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
  88 static inline u64 wrap_min(u64 x, u64 y)
  89 {
  90         return (s64)(x - y) < 0 ? x : y;
  91 }
  92
  93 static 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  */
 104 static u64 sched_clock_local(struct sched_clock_data *scd)
 105 {
 106         u64 now, clock, old_clock, min_clock, max_clock;
 107         s64 delta;
 108
 109 again:
 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 (cmpxchg(&scd->clock, old_clock, clock) != old_clock)
 131                 goto again;
 132
 133         return clock;
 134 }
 135
 136 static 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);
 143 again:
 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 (cmpxchg(ptr, old_val, val) != old_val)
 167                 goto again;
 168
 169         return val;
 170 }
 171
 172 u64 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
 195 void 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  */
 220 void sched_clock_idle_sleep_event(void)
 221 {
 222         sched_clock_cpu(smp_processor_id());
 223 }
 224 EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
 225
 226 /*
 227  * We just idled delta nanoseconds (called with irqs disabled):
 228  */
 229 void 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 }
 237 EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
 238
 239 #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
 240
 241 void sched_clock_init(void)
 242 {
 243         sched_clock_running = 1;
 244 }
 245
 246 u64 sched_clock_cpu(int cpu)
 247 {
 248         if (unlikely(!sched_clock_running))
 249                 return 0;
 250
 251         return sched_clock();
 252 }
 253
 254 #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
 255
 256 unsigned long long cpu_clock(int cpu)
 257 {
 258         unsigned long long clock;
 259         unsigned long flags;
 260
 261         local_irq_save(flags);
 262         clock = sched_clock_cpu(cpu);
 263         local_irq_restore(flags);
 264
 265         return clock;
 266 }
 267 EXPORT_SYMBOL_GPL(cpu_clock);