If time warps can be observed then they should only ever be observed on one
CPU. If they are observed on both CPUs then the system is completely hosed.
Add a check for this condition and notify if it happens.
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
---
arch/x86/kernel/tsc_sync.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -37,6 +37,7 @@ static arch_spinlock_t sync_lock = __ARC
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
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;
