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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>
This commit is contained in:
Venki Pallipadi 2008-11-17 14:43:58 -08:00 committed by Ingo Molnar
parent 26a3e99160
commit 93ce99e849
1 changed files with 4 additions and 0 deletions
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4
arch/x86/kernel/tsc_sync.c
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@ -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);