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/smp.h>
  20 #include <linux/nmi.h>
  21 #include <asm/tsc.h>
  22
  23 /*
  24  * Entry/exit counters that make sure that both CPUs
  25  * run the measurement code at once:
  26  */
  27 static atomic_t start_count;
  28 static atomic_t stop_count;
  29
  30 /*
  31  * We use a raw spinlock in this exceptional case, because
  32  * we want to have the fastest, inlined, non-debug version
  33  * of a critical section, to be able to prove TSC time-warps:
  34  */
  35 static arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;
  36
  37 static cycles_t last_tsc;
  38 static cycles_t max_warp;
  39 static int nr_warps;
  40
  41 /*
  42  * TSC-warp measurement loop running on both CPUs:
  43  */
  44 static void check_tsc_warp(unsigned int timeout)
  45 {
  46         cycles_t start, now, prev, end;
  47         int i;
  48
  49         rdtsc_barrier();
  50         start = get_cycles();
  51         rdtsc_barrier();
  52         /*
  53          * The measurement runs for 'timeout' msecs:
  54          */
  55         end = start + (cycles_t) tsc_khz * timeout;
  56         now = start;
  57
  58         for (i = 0; ; i++) {
  59                 /*
  60                  * We take the global lock, measure TSC, save the
  61                  * previous TSC that was measured (possibly on
  62                  * another CPU) and update the previous TSC timestamp.
  63                  */
  64                 arch_spin_lock(&sync_lock);
  65                 prev = last_tsc;
  66                 rdtsc_barrier();
  67                 now = get_cycles();
  68                 rdtsc_barrier();
  69                 last_tsc = now;
  70                 arch_spin_unlock(&sync_lock);
  71
  72                 /*
  73                  * Be nice every now and then (and also check whether
  74                  * measurement is done [we also insert a 10 million
  75                  * loops safety exit, so we dont lock up in case the
  76                  * TSC readout is totally broken]):
  77                  */
  78                 if (unlikely(!(i & 7))) {
  79                         if (now > end || i > 10000000)
  80                                 break;
  81                         cpu_relax();
  82                         touch_nmi_watchdog();
  83                 }
  84                 /*
  85                  * Outside the critical section we can now see whether
  86                  * we saw a time-warp of the TSC going backwards:
  87                  */
  88                 if (unlikely(prev > now)) {
  89                         arch_spin_lock(&sync_lock);
  90                         max_warp = max(max_warp, prev - now);
  91                         nr_warps++;
  92                         arch_spin_unlock(&sync_lock);
  93                 }
  94         }
  95         WARN(!(now-start),
  96                 "Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
  97                         now-start, end-start);
  98 }
  99
 100 /*
 101  * If the target CPU coming online doesn't have any of its core-siblings
 102  * online, a timeout of 20msec will be used for the TSC-warp measurement
 103  * loop. Otherwise a smaller timeout of 2msec will be used, as we have some
 104  * information about this socket already (and this information grows as we
 105  * have more and more logical-siblings in that socket).
 106  *
 107  * Ideally we should be able to skip the TSC sync check on the other
 108  * core-siblings, if the first logical CPU in a socket passed the sync test.
 109  * But as the TSC is per-logical CPU and can potentially be modified wrongly
 110  * by the bios, TSC sync test for smaller duration should be able
 111  * to catch such errors. Also this will catch the condition where all the
 112  * cores in the socket doesn't get reset at the same time.
 113  */
 114 static inline unsigned int loop_timeout(int cpu)
 115 {
 116         return (cpumask_weight(cpu_core_mask(cpu)) > 1) ? 2 : 20;
 117 }
 118
 119 /*
 120  * Source CPU calls into this - it waits for the freshly booted
 121  * target CPU to arrive and then starts the measurement:
 122  */
 123 void check_tsc_sync_source(int cpu)
 124 {
 125         int cpus = 2;
 126
 127         /*
 128          * No need to check if we already know that the TSC is not
 129          * synchronized:
 130          */
 131         if (unsynchronized_tsc())
 132                 return;
 133
 134         if (tsc_clocksource_reliable) {
 135                 if (cpu == (nr_cpu_ids-1) || system_state != SYSTEM_BOOTING)
 136                         pr_info(
 137                         "Skipped synchronization checks as TSC is reliable.\n");
 138                 return;
 139         }
 140
 141         /*
 142          * Reset it - in case this is a second bootup:
 143          */
 144         atomic_set(&stop_count, 0);
 145
 146         /*
 147          * Wait for the target to arrive:
 148          */
 149         while (atomic_read(&start_count) != cpus-1)
 150                 cpu_relax();
 151         /*
 152          * Trigger the target to continue into the measurement too:
 153          */
 154         atomic_inc(&start_count);
 155
 156         check_tsc_warp(loop_timeout(cpu));
 157
 158         while (atomic_read(&stop_count) != cpus-1)
 159                 cpu_relax();
 160
 161         if (nr_warps) {
 162                 pr_warning("TSC synchronization [CPU#%d -> CPU#%d]:\n",
 163                         smp_processor_id(), cpu);
 164                 pr_warning("Measured %Ld cycles TSC warp between CPUs, "
 165                            "turning off TSC clock.\n", max_warp);
 166                 mark_tsc_unstable("check_tsc_sync_source failed");
 167         } else {
 168                 pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
 169                         smp_processor_id(), cpu);
 170         }
 171
 172         /*
 173          * Reset it - just in case we boot another CPU later:
 174          */
 175         atomic_set(&start_count, 0);
 176         nr_warps = 0;
 177         max_warp = 0;
 178         last_tsc = 0;
 179
 180         /*
 181          * Let the target continue with the bootup:
 182          */
 183         atomic_inc(&stop_count);
 184 }
 185
 186 /*
 187  * Freshly booted CPUs call into this:
 188  */
 189 void check_tsc_sync_target(void)
 190 {
 191         int cpus = 2;
 192
 193         if (unsynchronized_tsc() || tsc_clocksource_reliable)
 194                 return;
 195
 196         /*
 197          * Register this CPU's participation and wait for the
 198          * source CPU to start the measurement:
 199          */
 200         atomic_inc(&start_count);
 201         while (atomic_read(&start_count) != cpus)
 202                 cpu_relax();
 203
 204         check_tsc_warp(loop_timeout(smp_processor_id()));
 205
 206         /*
 207          * Ok, we are done:
 208          */
 209         atomic_inc(&stop_count);
 210
 211         /*
 212          * Wait for the source CPU to print stuff:
 213          */
 214         while (atomic_read(&stop_count) != cpus)
 215                 cpu_relax();
 216 }