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 atomic_t start_count;
  29 static 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 arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;
  37
  38 static cycles_t last_tsc;
  39 static cycles_t max_warp;
  40 static int nr_warps;
  41
  42 /*
  43  * TSC-warp measurement loop running on both CPUs:
  44  */
  45 static void check_tsc_warp(unsigned int timeout)
  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 'timeout' msecs:
  55          */
  56         end = start + (cycles_t) tsc_khz * timeout;
  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                 arch_spin_lock(&sync_lock);
  66                 prev = last_tsc;
  67                 rdtsc_barrier();
  68                 now = get_cycles();
  69                 rdtsc_barrier();
  70                 last_tsc = now;
  71                 arch_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                         arch_spin_lock(&sync_lock);
  91                         max_warp = max(max_warp, prev - now);
  92                         nr_warps++;
  93                         arch_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  * If the target CPU coming online doesn't have any of its core-siblings
 103  * online, a timeout of 20msec will be used for the TSC-warp measurement
 104  * loop. Otherwise a smaller timeout of 2msec will be used, as we have some
 105  * information about this socket already (and this information grows as we
 106  * have more and more logical-siblings in that socket).
 107  *
 108  * Ideally we should be able to skip the TSC sync check on the other
 109  * core-siblings, if the first logical CPU in a socket passed the sync test.
 110  * But as the TSC is per-logical CPU and can potentially be modified wrongly
 111  * by the bios, TSC sync test for smaller duration should be able
 112  * to catch such errors. Also this will catch the condition where all the
 113  * cores in the socket doesn't get reset at the same time.
 114  */
 115 static inline unsigned int loop_timeout(int cpu)
 116 {
 117         return (cpumask_weight(cpu_core_mask(cpu)) > 1) ? 2 : 20;
 118 }
 119
 120 /*
 121  * Source CPU calls into this - it waits for the freshly booted
 122  * target CPU to arrive and then starts the measurement:
 123  */
 124 void check_tsc_sync_source(int cpu)
 125 {
 126         int cpus = 2;
 127
 128         /*
 129          * No need to check if we already know that the TSC is not
 130          * synchronized:
 131          */
 132         if (unsynchronized_tsc())
 133                 return;
 134
 135         if (tsc_clocksource_reliable) {
 136                 if (cpu == (nr_cpu_ids-1) || system_state != SYSTEM_BOOTING)
 137                         pr_info(
 138                         "Skipped synchronization checks as TSC is reliable.\n");
 139                 return;
 140         }
 141
 142         /*
 143          * Reset it - in case this is a second bootup:
 144          */
 145         atomic_set(&stop_count, 0);
 146
 147         /*
 148          * Wait for the target to arrive:
 149          */
 150         while (atomic_read(&start_count) != cpus-1)
 151                 cpu_relax();
 152         /*
 153          * Trigger the target to continue into the measurement too:
 154          */
 155         atomic_inc(&start_count);
 156
 157         check_tsc_warp(loop_timeout(cpu));
 158
 159         while (atomic_read(&stop_count) != cpus-1)
 160                 cpu_relax();
 161
 162         if (nr_warps) {
 163                 pr_warning("TSC synchronization [CPU#%d -> CPU#%d]:\n",
 164                         smp_processor_id(), cpu);
 165                 pr_warning("Measured %Ld cycles TSC warp between CPUs, "
 166                            "turning off TSC clock.\n", max_warp);
 167                 mark_tsc_unstable("check_tsc_sync_source failed");
 168         } else {
 169                 pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
 170                         smp_processor_id(), cpu);
 171         }
 172
 173         /*
 174          * Reset it - just in case we boot another CPU later:
 175          */
 176         atomic_set(&start_count, 0);
 177         nr_warps = 0;
 178         max_warp = 0;
 179         last_tsc = 0;
 180
 181         /*
 182          * Let the target continue with the bootup:
 183          */
 184         atomic_inc(&stop_count);
 185 }
 186
 187 /*
 188  * Freshly booted CPUs call into this:
 189  */
 190 void check_tsc_sync_target(void)
 191 {
 192         int cpus = 2;
 193
 194         if (unsynchronized_tsc() || tsc_clocksource_reliable)
 195                 return;
 196
 197         /*
 198          * Register this CPU's participation and wait for the
 199          * source CPU to start the measurement:
 200          */
 201         atomic_inc(&start_count);
 202         while (atomic_read(&start_count) != cpus)
 203                 cpu_relax();
 204
 205         check_tsc_warp(loop_timeout(smp_processor_id()));
 206
 207         /*
 208          * Ok, we are done:
 209          */
 210         atomic_inc(&stop_count);
 211
 212         /*
 213          * Wait for the source CPU to print stuff:
 214          */
 215         while (atomic_read(&stop_count) != cpus)
 216                 cpu_relax();
 217 }