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-rw-r--r--arch/x86/kernel/tsc_sync.c83
1 files changed, 78 insertions, 5 deletions
diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index 9ad074c87e72..d7f48a640ff5 100644
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -149,6 +149,7 @@ bool tsc_store_and_check_tsc_adjust(void)
static atomic_t start_count;
static atomic_t stop_count;
static atomic_t skip_test;
+static atomic_t test_runs;
/*
* We use a raw spinlock in this exceptional case, because
@@ -269,6 +270,16 @@ void check_tsc_sync_source(int cpu)
}
/*
+ * Set the maximum number of test runs to
+ * 1 if the CPU does not provide the TSC_ADJUST MSR
+ * 3 if the MSR is available, so the target can try to adjust
+ */
+ if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
+ atomic_set(&test_runs, 1);
+ else
+ atomic_set(&test_runs, 3);
+retry:
+ /*
* Wait for the target to start or to skip the test:
*/
while (atomic_read(&start_count) != cpus - 1) {
@@ -289,7 +300,21 @@ void check_tsc_sync_source(int cpu)
while (atomic_read(&stop_count) != cpus-1)
cpu_relax();
- if (nr_warps) {
+ /*
+ * If the test was successful set the number of runs to zero and
+ * stop. If not, decrement the number of runs an check if we can
+ * retry. In case of random warps no retry is attempted.
+ */
+ if (!nr_warps) {
+ atomic_set(&test_runs, 0);
+
+ pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
+ smp_processor_id(), cpu);
+
+ } else if (atomic_dec_and_test(&test_runs) || random_warps) {
+ /* Force it to 0 if random warps brought us here */
+ atomic_set(&test_runs, 0);
+
pr_warning("TSC synchronization [CPU#%d -> CPU#%d]:\n",
smp_processor_id(), cpu);
pr_warning("Measured %Ld cycles TSC warp between CPUs, "
@@ -297,9 +322,6 @@ void check_tsc_sync_source(int cpu)
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",
- smp_processor_id(), cpu);
}
/*
@@ -315,6 +337,12 @@ void check_tsc_sync_source(int cpu)
* Let the target continue with the bootup:
*/
atomic_inc(&stop_count);
+
+ /*
+ * Retry, if there is a chance to do so.
+ */
+ if (atomic_read(&test_runs) > 0)
+ goto retry;
}
/*
@@ -322,6 +350,9 @@ void check_tsc_sync_source(int cpu)
*/
void check_tsc_sync_target(void)
{
+ struct tsc_adjust *cur = this_cpu_ptr(&tsc_adjust);
+ unsigned int cpu = smp_processor_id();
+ cycles_t cur_max_warp, gbl_max_warp;
int cpus = 2;
/* Also aborts if there is no TSC. */
@@ -337,6 +368,7 @@ void check_tsc_sync_target(void)
return;
}
+retry:
/*
* Register this CPU's participation and wait for the
* source CPU to start the measurement:
@@ -345,7 +377,12 @@ void check_tsc_sync_target(void)
while (atomic_read(&start_count) != cpus)
cpu_relax();
- check_tsc_warp(loop_timeout(smp_processor_id()));
+ cur_max_warp = check_tsc_warp(loop_timeout(cpu));
+
+ /*
+ * Store the maximum observed warp value for a potential retry:
+ */
+ gbl_max_warp = max_warp;
/*
* Ok, we are done:
@@ -362,6 +399,42 @@ void check_tsc_sync_target(void)
* Reset it for the next sync test:
*/
atomic_set(&stop_count, 0);
+
+ /*
+ * Check the number of remaining test runs. If not zero, the test
+ * failed and a retry with adjusted TSC is possible. If zero the
+ * test was either successful or failed terminally.
+ */
+ if (!atomic_read(&test_runs))
+ return;
+
+ /*
+ * If the warp value of this CPU is 0, then the other CPU
+ * observed time going backwards so this TSC was ahead and
+ * needs to move backwards.
+ */
+ if (!cur_max_warp)
+ cur_max_warp = -gbl_max_warp;
+
+ /*
+ * Add the result to the previous adjustment value.
+ *
+ * The adjustement value is slightly off by the overhead of the
+ * sync mechanism (observed values are ~200 TSC cycles), but this
+ * really depends on CPU, node distance and frequency. So
+ * compensating for this is hard to get right. Experiments show
+ * that the warp is not longer detectable when the observed warp
+ * value is used. In the worst case the adjustment needs to go
+ * through a 3rd run for fine tuning.
+ */
+ cur->adjusted += cur_max_warp;
+
+ pr_warn("TSC ADJUST compensate: CPU%u observed %lld warp. Adjust: %lld\n",
+ cpu, cur_max_warp, cur->adjusted);
+
+ wrmsrl(MSR_IA32_TSC_ADJUST, cur->adjusted);
+ goto retry;
+
}
#endif /* CONFIG_SMP */