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