From 93ce99e849433ede4ce8b410b749dc0cad1100b2 Mon Sep 17 00:00:00 2001 From: Venki Pallipadi Date: Mon, 17 Nov 2008 14:43:58 -0800 Subject: [PATCH] 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 Signed-off-by: Ingo Molnar --- arch/x86/kernel/tsc_sync.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c index 9ffb01c31c40..1c0dfbca87c1 100644 --- a/arch/x86/kernel/tsc_sync.c +++ b/arch/x86/kernel/tsc_sync.c @@ -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); -- 2.30.2