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tsc_sync.c

/*
 * check TSC synchronization.
 *
 * Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
 *
 * We check whether all boot CPUs have their TSC's synchronized,
 * print a warning if not and turn off the TSC clock-source.
 *
 * The warp-check is point-to-point between two CPUs, the CPU
 * initiating the bootup is the 'source CPU', the freshly booting
 * CPU is the 'target CPU'.
 *
 * Only two CPUs may participate - they can enter in any order.
 * ( The serial nature of the boot logic and the CPU hotplug lock
 *   protects against more than 2 CPUs entering this code. )
 */
#include <linux/spinlock.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/smp.h>
#include <linux/nmi.h>
#include <asm/tsc.h>

/*
 * Entry/exit counters that make sure that both CPUs
 * run the measurement code at once:
 */
static __cpuinitdata atomic_t start_count;
static __cpuinitdata atomic_t stop_count;

/*
 * We use a raw spinlock in this exceptional case, because
 * we want to have the fastest, inlined, non-debug version
 * of a critical section, to be able to prove TSC time-warps:
 */
static __cpuinitdata raw_spinlock_t sync_lock = __RAW_SPIN_LOCK_UNLOCKED;
static __cpuinitdata cycles_t last_tsc;
static __cpuinitdata cycles_t max_warp;
static __cpuinitdata int nr_warps;

/*
 * TSC-warp measurement loop running on both CPUs:
 */
static __cpuinit void check_tsc_warp(void)
{
      cycles_t start, now, prev, end;
      int i;

      start = get_cycles_sync();
      /*
       * The measurement runs for 20 msecs:
       */
      end = start + tsc_khz * 20ULL;
      now = start;

      for (i = 0; ; i++) {
            /*
             * We take the global lock, measure TSC, save the
             * previous TSC that was measured (possibly on
             * another CPU) and update the previous TSC timestamp.
             */
            __raw_spin_lock(&sync_lock);
            prev = last_tsc;
            now = get_cycles_sync();
            last_tsc = now;
            __raw_spin_unlock(&sync_lock);

            /*
             * Be nice every now and then (and also check whether
             * measurement is done [we also insert a 100 million
             * loops safety exit, so we dont lock up in case the
             * TSC readout is totally broken]):
             */
            if (unlikely(!(i & 7))) {
                  if (now > end || i > 100000000)
                        break;
                  cpu_relax();
                  touch_nmi_watchdog();
            }
            /*
             * Outside the critical section we can now see whether
             * we saw a time-warp of the TSC going backwards:
             */
            if (unlikely(prev > now)) {
                  __raw_spin_lock(&sync_lock);
                  max_warp = max(max_warp, prev - now);
                  nr_warps++;
                  __raw_spin_unlock(&sync_lock);
            }

      }
}

/*
 * Source CPU calls into this - it waits for the freshly booted
 * target CPU to arrive and then starts the measurement:
 */
void __cpuinit check_tsc_sync_source(int cpu)
{
      int cpus = 2;

      /*
       * No need to check if we already know that the TSC is not
       * synchronized:
       */
      if (unsynchronized_tsc())
            return;

      printk(KERN_INFO "checking TSC synchronization [CPU#%d -> CPU#%d]:",
                    smp_processor_id(), cpu);

      /*
       * Reset it - in case this is a second bootup:
       */
      atomic_set(&stop_count, 0);

      /*
       * Wait for the target to arrive:
       */
      while (atomic_read(&start_count) != cpus-1)
            cpu_relax();
      /*
       * Trigger the target to continue into the measurement too:
       */
      atomic_inc(&start_count);

      check_tsc_warp();

      while (atomic_read(&stop_count) != cpus-1)
            cpu_relax();

      /*
       * Reset it - just in case we boot another CPU later:
       */
      atomic_set(&start_count, 0);

      if (nr_warps) {
            printk("\n");
            printk(KERN_WARNING "Measured %Ld cycles TSC warp between CPUs,"
                            " turning off TSC clock.\n", max_warp);
            mark_tsc_unstable("check_tsc_sync_source failed");
            nr_warps = 0;
            max_warp = 0;
            last_tsc = 0;
      } else {
            printk(" passed.\n");
      }

      /*
       * Let the target continue with the bootup:
       */
      atomic_inc(&stop_count);
}

/*
 * Freshly booted CPUs call into this:
 */
void __cpuinit check_tsc_sync_target(void)
{
      int cpus = 2;

      if (unsynchronized_tsc())
            return;

      /*
       * Register this CPU's participation and wait for the
       * source CPU to start the measurement:
       */
      atomic_inc(&start_count);
      while (atomic_read(&start_count) != cpus)
            cpu_relax();

      check_tsc_warp();

      /*
       * Ok, we are done:
       */
      atomic_inc(&stop_count);

      /*
       * Wait for the source CPU to print stuff:
       */
      while (atomic_read(&stop_count) != cpus)
            cpu_relax();
}
#undef NR_LOOPS


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