x86: add rdtsc barrier to TSC sync check
Impact: fix incorrectly marked unstable TSC clock
Patch (commit 0d12cdd "sched: improve sched_clock() performance") has
a regression on one of the test systems here.
With the patch, I see:
checking TSC synchronization [CPU#0 -> CPU#1]:
Measured 28 cycles TSC warp between CPUs, turning off TSC clock.
Marking TSC unstable due to check_tsc_sync_source failed
Whereas, without the patch syncs pass fine on all CPUs:
checking TSC synchronization [CPU#0 -> CPU#1]: passed.
Due to this, TSC is marked unstable, when it is not actually unstable.
This is because syncs in check_tsc_wrap() goes away due to this commit.
As per the discussion on this thread, correct way to fix this is to add
explicit syncs as below?
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index 9ffb01c..1c0dfbc 100644
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -46,7 +46,9 @@
cycles_t start, now, prev, end;
int i;
+ rdtsc_barrier();
start = get_cycles();
+ rdtsc_barrier();
/*
* The measurement runs for 20 msecs:
*/
@@ -61,7 +63,9 @@
*/
__raw_spin_lock(&sync_lock);
prev = last_tsc;
+ rdtsc_barrier();
now = get_cycles();
+ rdtsc_barrier();
last_tsc = now;
__raw_spin_unlock(&sync_lock);
