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
  38 static __cpuinitdata cycles_t last_tsc;
  39 static __cpuinitdata cycles_t max_warp;
  40 static __cpuinitdata int nr_warps;
  41
  42 /*
  43  * TSC-warp measurement loop running on both CPUs:
  44  */
  45 static __cpuinit void check_tsc_warp(void)
  46 {
  47         cycles_t start, now, prev, end;
  48         int i;
  49
  50         rdtsc_barrier();
  51         start = get_cycles();
  52         rdtsc_barrier();
  53         /*
  54          * The measurement runs for 20 msecs:
  55          */
  56         end = start + tsc_khz * 20ULL;
  57         now = start;
  58
  59         for (i = 0; ; i++) {
  60                 /*
  61                  * We take the global lock, measure TSC, save the
  62                  * previous TSC that was measured (possibly on
  63                  * another CPU) and update the previous TSC timestamp.
  64                  */
  65                 __raw_spin_lock(&sync_lock);
  66                 prev = last_tsc;
  67                 rdtsc_barrier();
  68                 now = get_cycles();
  69                 rdtsc_barrier();
  70                 last_tsc = now;
  71                 __raw_spin_unlock(&sync_lock);
  72
  73                 /*
  74                  * Be nice every now and then (and also check whether
  75                  * measurement is done [we also insert a 10 million
  76                  * loops safety exit, so we dont lock up in case the
  77                  * TSC readout is totally broken]):
  78                  */
  79                 if (unlikely(!(i & 7))) {
  80                         if (now > end || i > 10000000)
  81                                 break;
  82                         cpu_relax();
  83                         touch_nmi_watchdog();
  84                 }
  85                 /*
  86                  * Outside the critical section we can now see whether
  87                  * we saw a time-warp of the TSC going backwards:
  88                  */
  89                 if (unlikely(prev > now)) {
  90                         __raw_spin_lock(&sync_lock);
  91                         max_warp = max(max_warp, prev - now);
  92                         nr_warps++;
  93                         __raw_spin_unlock(&sync_lock);
  94                 }
  95         }
  96         WARN(!(now-start),
  97                 "Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
  98                         now-start, end-start);
  99 }
 100
 101 /*
 102  * Source CPU calls into this - it waits for the freshly booted
 103  * target CPU to arrive and then starts the measurement:
 104  */
 105 void __cpuinit check_tsc_sync_source(int cpu)
 106 {
 107         int cpus = 2;
 108
 109         /*
 110          * No need to check if we already know that the TSC is not
 111          * synchronized:
 112          */
 113         if (unsynchronized_tsc())
 114                 return;
 115
 116         if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) {
 117                 printk_once(KERN_INFO "Skipping synchronization checks as TSC is reliable.\n");
 118                 return;
 119         }
 120
 121         pr_info("checking TSC synchronization [CPU#%d -> CPU#%d]:",
 122                 smp_processor_id(), cpu);
 123
 124         /*
 125          * Reset it - in case this is a second bootup:
 126          */
 127         atomic_set(&stop_count, 0);
 128
 129         /*
 130          * Wait for the target to arrive:
 131          */
 132         while (atomic_read(&start_count) != cpus-1)
 133                 cpu_relax();
 134         /*
 135          * Trigger the target to continue into the measurement too:
 136          */
 137         atomic_inc(&start_count);
 138
 139         check_tsc_warp();
 140
 141         while (atomic_read(&stop_count) != cpus-1)
 142                 cpu_relax();
 143
 144         if (nr_warps) {
 145                 printk("\n");
 146                 pr_warning("Measured %Ld cycles TSC warp between CPUs, "
 147                            "turning off TSC clock.\n", max_warp);
 148                 mark_tsc_unstable("check_tsc_sync_source failed");
 149         } else {
 150                 printk(" passed.\n");
 151         }
 152
 153         /*
 154          * Reset it - just in case we boot another CPU later:
 155          */
 156         atomic_set(&start_count, 0);
 157         nr_warps = 0;
 158         max_warp = 0;
 159         last_tsc = 0;
 160
 161         /*
 162          * Let the target continue with the bootup:
 163          */
 164         atomic_inc(&stop_count);
 165 }
 166
 167 /*
 168  * Freshly booted CPUs call into this:
 169  */
 170 void __cpuinit check_tsc_sync_target(void)
 171 {
 172         int cpus = 2;
 173
 174         if (unsynchronized_tsc() || boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
 175                 return;
 176
 177         /*
 178          * Register this CPU's participation and wait for the
 179          * source CPU to start the measurement:
 180          */
 181         atomic_inc(&start_count);
 182         while (atomic_read(&start_count) != cpus)
 183                 cpu_relax();
 184
 185         check_tsc_warp();
 186
 187         /*
 188          * Ok, we are done:
 189          */
 190         atomic_inc(&stop_count);
 191
 192         /*
 193          * Wait for the source CPU to print stuff:
 194          */
 195         while (atomic_read(&stop_count) != cpus)
 196                 cpu_relax();
 197 }