arch/x86/kernel/tsc_sync.c

   1 /*
   2  * check TSC synchronization.
   3  *
   4  * Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
   5  *
   6  * We check whether all boot CPUs have their TSC's synchronized,
   7  * print a warning if not and turn off the TSC clock-source.
   8  *
   9  * The warp-check is point-to-point between two CPUs, the CPU
  10  * initiating the bootup is the 'source CPU', the freshly booting
  11  * CPU is the 'target CPU'.
  12  *
  13  * Only two CPUs may participate - they can enter in any order.
  14  * ( The serial nature of the boot logic and the CPU hotplug lock
  15  *   protects against more than 2 CPUs entering this code. )
  16  */
  17 #include <linux/spinlock.h>
  18 #include <linux/kernel.h>
  19 #include <linux/init.h>
  20 #include <linux/smp.h>
  21 #include <linux/nmi.h>
  22 #include <asm/tsc.h>
  23
  24 /*
  25  * Entry/exit counters that make sure that both CPUs
  26  * run the measurement code at once:
  27  */
  28 static __cpuinitdata atomic_t start_count;
  29 static __cpuinitdata atomic_t stop_count;
  30
  31 /*
  32  * We use a raw spinlock in this exceptional case, because
  33  * we want to have the fastest, inlined, non-debug version
  34  * of a critical section, to be able to prove TSC time-warps:
  35  */
  36 static __cpuinitdata raw_spinlock_t sync_lock = __RAW_SPIN_LOCK_UNLOCKED;
  37 static __cpuinitdata cycles_t last_tsc;
  38 static __cpuinitdata cycles_t max_warp;
  39 static __cpuinitdata int nr_warps;
  40
  41 /*
  42  * TSC-warp measurement loop running on both CPUs:
  43  */
  44 static __cpuinit void check_tsc_warp(void)
  45 {
  46         cycles_t start, now, prev, end;
  47         int i;
  48
  49         start = get_cycles();
  50         /*
  51          * The measurement runs for 20 msecs:
  52          */
  53         end = start + tsc_khz * 20ULL;
  54         now = start;
  55
  56         for (i = 0; ; i++) {
  57                 /*
  58                  * We take the global lock, measure TSC, save the
  59                  * previous TSC that was measured (possibly on
  60                  * another CPU) and update the previous TSC timestamp.
  61                  */
  62                 __raw_spin_lock(&sync_lock);
  63                 prev = last_tsc;
  64                 now = get_cycles();
  65                 last_tsc = now;
  66                 __raw_spin_unlock(&sync_lock);
  67
  68                 /*
  69                  * Be nice every now and then (and also check whether
  70                  * measurement is done [we also insert a 10 million
  71                  * loops safety exit, so we dont lock up in case the
  72                  * TSC readout is totally broken]):
  73                  */
  74                 if (unlikely(!(i & 7))) {
  75                         if (now > end || i > 10000000)
  76                                 break;
  77                         cpu_relax();
  78                         touch_nmi_watchdog();
  79                 }
  80                 /*
  81                  * Outside the critical section we can now see whether
  82                  * we saw a time-warp of the TSC going backwards:
  83                  */
  84                 if (unlikely(prev > now)) {
  85                         __raw_spin_lock(&sync_lock);
  86                         max_warp = max(max_warp, prev - now);
  87                         nr_warps++;
  88                         __raw_spin_unlock(&sync_lock);
  89                 }
  90         }
  91         if (!(now-start)) {
  92                 printk("Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
  93                         now-start, end-start);
  94                 WARN_ON(1);
  95         }
  96 }
  97
  98 /*
  99  * Source CPU calls into this - it waits for the freshly booted
 100  * target CPU to arrive and then starts the measurement:
 101  */
 102 void __cpuinit check_tsc_sync_source(int cpu)
 103 {
 104         int cpus = 2;
 105
 106         /*
 107          * No need to check if we already know that the TSC is not
 108          * synchronized:
 109          */
 110         if (unsynchronized_tsc())
 111                 return;
 112
 113         printk(KERN_INFO "checking TSC synchronization [CPU#%d -> CPU#%d]:",
 114                           smp_processor_id(), cpu);
 115
 116         /*
 117          * Reset it - in case this is a second bootup:
 118          */
 119         atomic_set(&stop_count, 0);
 120
 121         /*
 122          * Wait for the target to arrive:
 123          */
 124         while (atomic_read(&start_count) != cpus-1)
 125                 cpu_relax();
 126         /*
 127          * Trigger the target to continue into the measurement too:
 128          */
 129         atomic_inc(&start_count);
 130
 131         check_tsc_warp();
 132
 133         while (atomic_read(&stop_count) != cpus-1)
 134                 cpu_relax();
 135
 136         if (nr_warps) {
 137                 printk("\n");
 138                 printk(KERN_WARNING "Measured %Ld cycles TSC warp between CPUs,"
 139                                     " turning off TSC clock.\n", max_warp);
 140                 mark_tsc_unstable("check_tsc_sync_source failed");
 141         } else {
 142                 printk(" passed.\n");
 143         }
 144
 145         /*
 146          * Reset it - just in case we boot another CPU later:
 147          */
 148         atomic_set(&start_count, 0);
 149         nr_warps = 0;
 150         max_warp = 0;
 151         last_tsc = 0;
 152
 153         /*
 154          * Let the target continue with the bootup:
 155          */
 156         atomic_inc(&stop_count);
 157 }
 158
 159 /*
 160  * Freshly booted CPUs call into this:
 161  */
 162 void __cpuinit check_tsc_sync_target(void)
 163 {
 164         int cpus = 2;
 165
 166         if (unsynchronized_tsc())
 167                 return;
 168
 169         /*
 170          * Register this CPU's participation and wait for the
 171          * source CPU to start the measurement:
 172          */
 173         atomic_inc(&start_count);
 174         while (atomic_read(&start_count) != cpus)
 175                 cpu_relax();
 176
 177         check_tsc_warp();
 178
 179         /*
 180          * Ok, we are done:
 181          */
 182         atomic_inc(&stop_count);
 183
 184         /*
 185          * Wait for the source CPU to print stuff:
 186          */
 187         while (atomic_read(&stop_count) != cpus)
 188                 cpu_relax();
 189 }
 190 #undef NR_LOOPS
 191