kernel/time/sched_clock.c

   1 /*
   2  * sched_clock.c: support for extending counters to full 64-bit ns counter
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License version 2 as
   6  * published by the Free Software Foundation.
   7  */
   8 #include <linux/clocksource.h>
   9 #include <linux/init.h>
  10 #include <linux/jiffies.h>
  11 #include <linux/kernel.h>
  12 #include <linux/moduleparam.h>
  13 #include <linux/sched.h>
  14 #include <linux/syscore_ops.h>
  15 #include <linux/timer.h>
  16 #include <linux/sched_clock.h>
  17
  18 struct clock_data {
  19         u64 epoch_ns;
  20         u32 epoch_cyc;
  21         u32 epoch_cyc_copy;
  22         unsigned long rate;
  23         u32 mult;
  24         u32 shift;
  25         bool suspended;
  26 };
  27
  28 static void sched_clock_poll(unsigned long wrap_ticks);
  29 static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0);
  30 static int irqtime = -1;
  31
  32 core_param(irqtime, irqtime, int, 0400);
  33
  34 static struct clock_data cd = {
  35         .mult   = NSEC_PER_SEC / HZ,
  36 };
  37
  38 static u32 __read_mostly sched_clock_mask = 0xffffffff;
  39
  40 static u32 notrace jiffy_sched_clock_read(void)
  41 {
  42         return (u32)(jiffies - INITIAL_JIFFIES);
  43 }
  44
  45 static u32 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;
  46
  47 static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
  48 {
  49         return (cyc * mult) >> shift;
  50 }
  51
  52 static unsigned long long notrace sched_clock_32(void)
  53 {
  54         u64 epoch_ns;
  55         u32 epoch_cyc;
  56         u32 cyc;
  57
  58         if (cd.suspended)
  59                 return cd.epoch_ns;
  60
  61         /*
  62          * Load the epoch_cyc and epoch_ns atomically.  We do this by
  63          * ensuring that we always write epoch_cyc, epoch_ns and
  64          * epoch_cyc_copy in strict order, and read them in strict order.
  65          * If epoch_cyc and epoch_cyc_copy are not equal, then we're in
  66          * the middle of an update, and we should repeat the load.
  67          */
  68         do {
  69                 epoch_cyc = cd.epoch_cyc;
  70                 smp_rmb();
  71                 epoch_ns = cd.epoch_ns;
  72                 smp_rmb();
  73         } while (epoch_cyc != cd.epoch_cyc_copy);
  74
  75         cyc = read_sched_clock();
  76         cyc = (cyc - epoch_cyc) & sched_clock_mask;
  77         return epoch_ns + cyc_to_ns(cyc, cd.mult, cd.shift);
  78 }
  79
  80 /*
  81  * Atomically update the sched_clock epoch.
  82  */
  83 static void notrace update_sched_clock(void)
  84 {
  85         unsigned long flags;
  86         u32 cyc;
  87         u64 ns;
  88
  89         cyc = read_sched_clock();
  90         ns = cd.epoch_ns +
  91                 cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
  92                           cd.mult, cd.shift);
  93         /*
  94          * Write epoch_cyc and epoch_ns in a way that the update is
  95          * detectable in cyc_to_fixed_sched_clock().
  96          */
  97         raw_local_irq_save(flags);
  98         cd.epoch_cyc_copy = cyc;
  99         smp_wmb();
 100         cd.epoch_ns = ns;
 101         smp_wmb();
 102         cd.epoch_cyc = cyc;
 103         raw_local_irq_restore(flags);
 104 }
 105
 106 static void sched_clock_poll(unsigned long wrap_ticks)
 107 {
 108         mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks));
 109         update_sched_clock();
 110 }
 111
 112 void __init setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate)
 113 {
 114         unsigned long r, w;
 115         u64 res, wrap;
 116         char r_unit;
 117
 118         if (cd.rate > rate)
 119                 return;
 120
 121         BUG_ON(bits > 32);
 122         WARN_ON(!irqs_disabled());
 123         read_sched_clock = read;
 124         sched_clock_mask = (1ULL << bits) - 1;
 125         cd.rate = rate;
 126
 127         /* calculate the mult/shift to convert counter ticks to ns. */
 128         clocks_calc_mult_shift(&cd.mult, &cd.shift, rate, NSEC_PER_SEC, 0);
 129
 130         r = rate;
 131         if (r >= 4000000) {
 132                 r /= 1000000;
 133                 r_unit = 'M';
 134         } else if (r >= 1000) {
 135                 r /= 1000;
 136                 r_unit = 'k';
 137         } else
 138                 r_unit = ' ';
 139
 140         /* calculate how many ns until we wrap */
 141         wrap = cyc_to_ns((1ULL << bits) - 1, cd.mult, cd.shift);
 142         do_div(wrap, NSEC_PER_MSEC);
 143         w = wrap;
 144
 145         /* calculate the ns resolution of this counter */
 146         res = cyc_to_ns(1ULL, cd.mult, cd.shift);
 147         pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lums\n",
 148                 bits, r, r_unit, res, w);
 149
 150         /*
 151          * Start the timer to keep sched_clock() properly updated and
 152          * sets the initial epoch.
 153          */
 154         sched_clock_timer.data = msecs_to_jiffies(w - (w / 10));
 155         update_sched_clock();
 156
 157         /*
 158          * Ensure that sched_clock() starts off at 0ns
 159          */
 160         cd.epoch_ns = 0;
 161
 162         /* Enable IRQ time accounting if we have a fast enough sched_clock */
 163         if (irqtime > 0 || (irqtime == -1 && rate >= 1000000))
 164                 enable_sched_clock_irqtime();
 165
 166         pr_debug("Registered %pF as sched_clock source\n", read);
 167 }
 168
 169 unsigned long long __read_mostly (*sched_clock_func)(void) = sched_clock_32;
 170
 171 unsigned long long notrace sched_clock(void)
 172 {
 173         return sched_clock_func();
 174 }
 175
 176 void __init sched_clock_postinit(void)
 177 {
 178         /*
 179          * If no sched_clock function has been provided at that point,
 180          * make it the final one one.
 181          */
 182         if (read_sched_clock == jiffy_sched_clock_read)
 183                 setup_sched_clock(jiffy_sched_clock_read, 32, HZ);
 184
 185         sched_clock_poll(sched_clock_timer.data);
 186 }
 187
 188 static int sched_clock_suspend(void)
 189 {
 190         sched_clock_poll(sched_clock_timer.data);
 191         cd.suspended = true;
 192         return 0;
 193 }
 194
 195 static void sched_clock_resume(void)
 196 {
 197         cd.epoch_cyc = read_sched_clock();
 198         cd.epoch_cyc_copy = cd.epoch_cyc;
 199         cd.suspended = false;
 200 }
 201
 202 static struct syscore_ops sched_clock_ops = {
 203         .suspend = sched_clock_suspend,
 204         .resume = sched_clock_resume,
 205 };
 206
 207 static int __init sched_clock_syscore_init(void)
 208 {
 209         register_syscore_ops(&sched_clock_ops);
 210         return 0;
 211 }
 212 device_initcall(sched_clock_syscore_init);