1 files changed, 12 insertions, 1 deletions

diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index 78083bf23ed1..40f8edd55151 100644
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -37,6 +37,7 @@ static arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;
 static cycles_t last_tsc;
 static cycles_t max_warp;
 static int nr_warps;
+static int random_warps;
 
 /*
  * TSC-warp measurement loop running on both CPUs.  This is not called
@@ -45,7 +46,7 @@ static int nr_warps;
 static void check_tsc_warp(unsigned int timeout)
 {
         cycles_t start, now, prev, end;
-        int i;
+        int i, cur_warps = 0;
 
         start = rdtsc_ordered();
         /*
@@ -85,7 +86,14 @@ static void check_tsc_warp(unsigned int timeout)
                 if (unlikely(prev > now)) {
                         arch_spin_lock(&sync_lock);
                         max_warp = max(max_warp, prev - now);
+                        /*
+                         * Check whether this bounces back and forth. Only
+                         * one CPU should observe time going backwards.
+                         */
+                        if (cur_warps != nr_warps)
+                                random_warps++;
                         nr_warps++;
+                        cur_warps = nr_warps;
                         arch_spin_unlock(&sync_lock);
                 }
         }
@@ -160,6 +168,8 @@ void check_tsc_sync_source(int cpu)
                         smp_processor_id(), cpu);
                 pr_warning("Measured %Ld cycles TSC warp between CPUs, "
                            "turning off TSC clock.\n", max_warp);
+                if (random_warps)
+                        pr_warning("TSC warped randomly between CPUs\n");
                 mark_tsc_unstable("check_tsc_sync_source failed");
         } else {
                 pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
@@ -170,6 +180,7 @@ void check_tsc_sync_source(int cpu)
          * Reset it - just in case we boot another CPU later:
          */
         atomic_set(&start_count, 0);
+        random_warps = 0;
         nr_warps = 0;
         max_warp = 0;
         last_tsc = 0;