arch/x86_64/kernel/tsc_sync.c

   1 /*
   2  * arch/x86_64/kernel/tsc_sync.c: 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_sync();
  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_sync();
  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 100 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 > 100000000)
  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         }
  92 }
  93
  94 /*
  95  * Source CPU calls into this - it waits for the freshly booted
  96  * target CPU to arrive and then starts the measurement:
  97  */
  98 void __cpuinit check_tsc_sync_source(int cpu)
  99 {
 100         int cpus = 2;
 101
 102         /*
 103          * No need to check if we already know that the TSC is not
 104          * synchronized:
 105          */
 106         if (unsynchronized_tsc())
 107                 return;
 108
 109         printk(KERN_INFO "checking TSC synchronization [CPU#%d -> CPU#%d]:",
 110                           smp_processor_id(), cpu);
 111
 112         /*
 113          * Reset it - in case this is a second bootup:
 114          */
 115         atomic_set(&stop_count, 0);
 116
 117         /*
 118          * Wait for the target to arrive:
 119          */
 120         while (atomic_read(&start_count) != cpus-1)
 121                 cpu_relax();
 122         /*
 123          * Trigger the target to continue into the measurement too:
 124          */
 125         atomic_inc(&start_count);
 126
 127         check_tsc_warp();
 128
 129         while (atomic_read(&stop_count) != cpus-1)
 130                 cpu_relax();
 131
 132         /*
 133          * Reset it - just in case we boot another CPU later:
 134          */
 135         atomic_set(&start_count, 0);
 136
 137         if (nr_warps) {
 138                 printk("\n");
 139                 printk(KERN_WARNING "Measured %Ld cycles TSC warp between CPUs,"
 140                                     " turning off TSC clock.\n", max_warp);
 141                 mark_tsc_unstable("check_tsc_sync_source failed");
 142                 nr_warps = 0;
 143                 max_warp = 0;
 144                 last_tsc = 0;
 145         } else {
 146                 printk(" passed.\n");
 147         }
 148
 149         /*
 150          * Let the target continue with the bootup:
 151          */
 152         atomic_inc(&stop_count);
 153 }
 154
 155 /*
 156  * Freshly booted CPUs call into this:
 157  */
 158 void __cpuinit check_tsc_sync_target(void)
 159 {
 160         int cpus = 2;
 161
 162         if (unsynchronized_tsc())
 163                 return;
 164
 165         /*
 166          * Register this CPU's participation and wait for the
 167          * source CPU to start the measurement:
 168          */
 169         atomic_inc(&start_count);
 170         while (atomic_read(&start_count) != cpus)
 171                 cpu_relax();
 172
 173         check_tsc_warp();
 174
 175         /*
 176          * Ok, we are done:
 177          */
 178         atomic_inc(&stop_count);
 179
 180         /*
 181          * Wait for the source CPU to print stuff:
 182          */
 183         while (atomic_read(&stop_count) != cpus)
 184                 cpu_relax();
 185 }
 186 #undef NR_LOOPS
 187