Commit 93ce99e8 authored Nov 17, 2008 by Venki Pallipadi Committed by Ingo Molnar Nov 18, 2008

x86: add rdtsc barrier to TSC sync check



Impact: fix incorrectly marked unstable TSC clock

Patch (commit 0d12cdd5 "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>

parent 26a3e991

arch/x86/kernel/tsc_sync.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -46,7 +46,9 @@ static __cpuinit void check_tsc_warp(void)
		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 @@ static __cpuinit void check_tsc_warp(void)
		*/
		__raw_spin_lock(&sync_lock);
		prev = last_tsc;
		rdtsc_barrier();
		now = get_cycles();
		rdtsc_barrier();
		last_tsc = now;
		__raw_spin_unlock(&sync_lock);